Glossary of underwater diving terminology

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This is a glossary of technical terms, jargon, diver slang and acronyms used in underwater diving. The definitions listed are in the context of underwater diving. There may be other meanings in other contexts.

Underwater diving can be described as a human activity – intentional, purposive, conscious and subjectively meaningful sequence of actions. Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to the order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand.

Many of the terms are in general use by English speaking divers from many parts of the world, both amateur and professional, and using any of the modes of diving. Others are more specialised, variable by location, mode, or professional environment. There are instances where a term may have more than one meaning depending on context, and others where several terms refer to the same concept, or there are variations in spelling. A few are loan-words from other languages.



Main article: 6061 aluminium alloy

Aluminium alloy used for manufacture of new diving cylinders exclusively since mid 1988 as it is not susceptible to sustained load cracking.[1][2]

See: Aluminium alloy#Wrought alloys

Aluminium alloy 6351 is subject to sustained load cracking and requires periodical eddy current testing to identify crack development at an early stage.[1] Not used for new cylinder manufacture since 1988, but many cylinders of this alloy are still in service.[2]

See: adjustable buoyancy life jacket

absolute pressure

See: Pressure measurement#Absolute, gauge and differential pressures - zero reference

Total static pressure at the reference point: Pressure relative to vacuum.

See: auto-closure device

A-clamp adaptor

See: yoke adaptor

A-clamp fitting
A-clamp valve

See: yoke fitting, and CGA 850

active addition

See: Rebreather#Active addition semi-closed circuit

System for semi-closed circuit rebreather feed gas addition in which gas is added to the breathing circuit by a mechanism, regardless of current volume, and excess gas is vented to keep the loop volume within limits. Compare with passive addition
activated carbon
activated charcoal

Main article: Activated carbon

A filter medium used to remove oil, water, and odours from breathing gas.

See: Australian Diver Accreditation Scheme


See: Association of Diving Contractors International

adjustable buoyancy life jacket

Also: "ABLJ", "Fenzy" , or "horse collar buoyancy compensator"

See: Buoyancy compensator (diving)#Adjustable buoyancy life jacket

An early style of combination buoyancy compensator and inflatable life jacket worn on the chest and round the neck.[3]
1.  See: automatic diluent valve
2.  See: automatic dump valve
Part of a launch and recovery system. A gantry or davit for launching and recovering bells, stages, anchors, or large ROVs.[4][5] Usually deployed by hydraulic rams which swing the frame over the deck or overboard as required. The load is hoisted and lowered by cables from the top of the frame.

See: arterial gas embolism

aggressive decompression

See: Decompression practice#Conservatism

Decompression schedule tending to shorter overall decompression time for a given pre-ascent dive profile, accepting increased risk of decompression sickness to reduce the overall ascent time.[6]
air dome
1.  A section of cave which traps air or other gas at the top. This gas is not directly connected to the atmosphere.[7]
2.  The upper part of an open diving bell which can be filled with air for divers to breathe.
air integration

See: Dive computer#Additional functionality and features

Feature of some dive computers to measure cylinder pressure using an integral pressure gauge connected by HP hose to the regulator first stage, or receive data from one or more remote pressure transducers fitted to the HP port of a regulator, and display the pressure on the screen. Other features may also be available using the data.[8]

Main article: Airlift (dredging device)

A device based on a steeply rising pipe, used by divers to suck small objects, sand and mud from the sea bed and to transport the resulting debris upwards and away from its source. Air is injected into the lower end of the pipe and the rising bubbles entrain water and cause an upward flow which draws the material from the bottom along.
air line

See: Surface-supplied diving#Airline

Simple low pressure hose carrying breathing air from a surface supply to a diver.
airline diving

Also: "hooka", "hookah"

See: Surface-supplied diving#Airline

Surface-supplied diving where the breathing air is supplied to the diver by a simple hose. The diver usually breathes through a mouth held demand valve.
air top
air top-up

see: Gas blending for scuba diving#Air top-up

1.  Completing a planned breathing gas mix by topping up the cylinder with compressed air to a calculated pressure.[9]
2.  Topping up a partly used breathing gas mix with compressed air, providing a different mixture which is analysed after the fill.[9]
algal bloom

Main article: Algal bloom

A rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Visibility can severely deteriorate over a period of hours to days.
Articulated Loading Platform, a type of single buoy mooring consisting of a buoyant upper structure with a lattice leg linked by an articulating joint to a mooring.[10]
alternative air source

Main article: Alternative air source

A secondary supply of air or other breathing gas used by a diver in an emergency. See also emergency gas supply (EGS)
alternobaric vertigo

Main article: Alternobaric vertigo

Dizziness caused by different internal pressure in the middle ears.
altitude diving

Main article: Altitude diving

Diving at a location where the water surface is at an altitude which requires modification of decompression schedules.[11] (more than about 300 m (980 ft) above sea level.
ambient pressure

Main article: Ambient pressure

Pressure of the local surroundings.

Main article: Hypoxia (medical)

Tissues completely lacking in oxygen.
Environment or gas completely lacking in oxygen.
Association of Offshore Diving Contractors, one of the predecessors to the International Marine Contractors Association (IMCA).[12]

Main article: Aphasia

An impairment of language ability which may range from having difficulty remembering words to being completely unable to speak, read, or write
1.  Suspension of breathing, breath-hold.
2.  Freediving.
appropriate breathing mix
appropriate breathing gas

See: Scuba gas planning#Choosing a suitable breathing gas mixture, Saturation diving#Atmosphere control

A breathing gas mixture that is suitable in composition and temperature, and of adequate pressure, having regard to the system and equipment used in the diving operations, the work undertaken in those operations, and the conditions and depth at which they are to be carried out.[13]

Main article: Aqua-lung

Self contained open circuit demand underwater breathing equipment consisting of a diving cylinder and diving regulator.
arterial bubble model

See: History of decompression research and development#The arterial bubble decompression model

Decompression model in which the filtering capacity of the lung is assumed to have a threshold radius of the size of a red blood cell and sufficiently small decompression bubbles can pass to the arterial side, especially during the initial phase of ascent.
arterial gas embolism

Main article: Arterial gas embolism

Blockage of an artery by a gas bubble. A possible consequence of lung overpressure injury and decompression sickness.
articulated loading platform

See: ALP

AR vest
A waistcoat (vest) style harness of heavy cloth with strong adjustable webbing straps so that the diver can not slide out under any reasonably foreseeable circumstance.[14]

See: Ascending and descending (diving)

Part of the dive profile where the diver is moving upwards towards the surface. An ascent may be interrupted by stops (q.v.), when the diver maintains a functionally constant depth for the purpose of decompression, and pulls (q.v.), during which periods there is consistently upwards movement (minor variations in the scale of a few seconds are generally ignored).
ascent rate

See: Decompression practice#Ascent rate

The rate at which depth is reduced at the end of a dive. An important component of decompression.
Association of SCUBA Service Engineers and Technicians.[15]
Association of Diving Contractors International


Main article: Association of Diving Contractors International

A non-profit organization to promote commercial diving, and establish and encourage observance of safe standards for commercial diving.
atmospheres absolute

See: Standard atmosphere (unit)

Unit of absolute pressure equivalent to standard atmospheric pressure.
atmospheric diving suit

Main article: Atmospheric diving suit

A small one-man articulated submersible of anthropomorphic form which resembles a suit of armour, with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere.
attack swimmer

See: combat swimmer


See: diver's attendant

Australian Diver Accreditation Scheme

Main article: Australian Diver Accreditation Scheme

An international commercial and occupational diver certification scheme based in Australia.
auto-closure device
automatic closure device

Also: "ACD"

See: Diving regulator#Automatic closure device

A mechanism for automatically closing off the inlet opening of a regulator first stage when it is disconnected from a cylinder to seal it against ingress of contamination.[16][17]
automatic diluent valve

See: Automatic Diluent Valve

A demand valve set into the breathing loop of a rebreather to inject diluent gas into the loop when the loop volume falls and there is not enough gas for inhalation.
automatic dump valve
Adjustable spring-loaded overpressure valve with manual override used on a constant volume dry suit to automatically discharge excess suit inflation gas as it expands during ascent, thereby maintaining a nearly constant volume and buoyancy of the suit.
Autonomous diver

Main article: Autonomous diver

EN 14153-2 / ISO 24801-2 standard competence for recreational scuba diver. The level 2 "Autonomous diver" has sufficient knowledge, skill and experience to make dives, in open water, which do not require in-water decompression stops, to a recommended maximum depth of 20 m with other scuba divers of the same level, only when appropriate support is available at the surface, and under conditions that are equal or better than the conditions in which they were trained, without supervision of a scuba instructor, unless they have additional training or are accompanied by a dive leader.[18]
available gas

See: Scuba gas planning#Available gas

Breathing gas available from a cylinder after taking into account ambient pressure and the pressure needed for correct function of the delivery system at a useful flow rate. This may also take into account a reserve allocation.


back gas
Breathing gas carried by a scuba diver in back mounted cylinders. Generally the primary breathing gas for the bottom or longest sector of a dive.[7]
back kick
backward kick

See: Back kick (finning)

A finning technique for moving backwards. Not an easy, powerful or elegant kick, but useful in many situations. The fins are angled outwards in opposite directions with the legs straight, then swept upwards and towards the diver by bending the knees in the power stroke. The knees may move downwards a bit at the same time by bending at the hips for stability. The return stroke feathers the fins by pointing them backwards in line with the body axis, to reduce forward thrust until the legs are straight again
back mount
The practice of carrying a scuba set on the back of the diver, supported by a harness, backplate or stabilisor jacket BCD. Compare with sidemount
A rigid or semi-rigid structure similar in function to a backplate, usually made of moulded plastic, but sometimes of metal, used either as a stiffener and reinforcement for a jacket style buoyancy compensator, or as the basis of a scuba harness independent of a buoyancy compensator. The backpack supports and stabilises the scuba cylinder on the diver's back.

Main article: Backplate and wing

A plate, normally made from metal, which rests against the diver’s back, and to which the primary scuba cylinders are attached. Held to the body by harness straps over the shoulders and round the waist. Sometimes also crotch straps and chest straps. Usually used with a back inflation buoyancy compensator.
backup light
Dive light carried as a spare to be used in case of failure of the primary light.[7]
backup regulator

Also: "secondary regulator"

A second complete scuba regulator connected to a cylinder or manifolded twin set.[7] Compare with octopus regulator.
backward roll
backward roll entry
back roll entry

See: Scuba skills#Entries

Water entry method in which the seated diver rolls backwards off the side of the boat, allowing the scuba cylinders to strike the water first.[19]: 125 
bailout block
A gas switching block specifically intended for connection of a bailout set to the main gas supply (which may be scuba or surface supply) which allows the diver to switch from main gas supply to emergency gas supply while continuously using the same mouthpiece, regulator second stage, full face mask or helmet. A bailout block is generally used on open circuit breathing apparatus, the equivalent function on a rebreather is provided by a bailout valve (BOV). The bailout block may be mounted on the side of a diving helmet or full-face mask, or may be mounted in a convenient place on the diver's harness, and includes a bailout valve, used to select the gas source, and one or more non-return valves to ensure that the emergency gas supply is directed only to the diver.
bailout bottle
bailout cylinder

Main article: Bailout cylinder

A scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure.[13]
bailout set
bailout system

Also: "EGS", "emergency gas supply"

Main article: Bailout cylinder

An independent breathing gas supply carried by a diver for use in case of failure of the main gas supply. Usually consists of a bailout cylinder with a first stage regulator, and either a second stage regulator or connected to a bailout block or bailout valve (q.v.) A submersible pressure gauge is also usually provided.[20]
bailout valve
1.  Also: "integrated open-circuit regulator" An open circuit demand valve built into a rebreather mouthpiece, or other part of the breathing loop, which can be isolated while the diver is using the rebreather to recycle breathing gas, and opened at the same time as isolating the breathing loop when the diver bails out to open circuit.
2.  A valve which opens the gas supply from the bailout cylinder of a surface supplied diver, used in case of surface gas failure, usually mounted on the side of a diving helmet or full-face mask, or on a manifold block on the diver's harness.
Baited remote underwater video

Main article: Baited remote underwater video

A system for assessing fish populations using video cameras to record fish attracted to a bait canister.
balanced regulator

See: Mechanism of diving regulators#Balancing

Regulator designed to provide a consistent demand effort not affected by cylinder gas pressure or depth.[7]
band mask

See: Surface-supplied diving#Band mask

A heavy duty full-face mask with many of the characteristics of a lightweight demand helmet. In structure it is the front section of a lightweight helmet from above the faceplate to below the demand valve and exhaust ports, including the bailout block and communications connections on the sides. This rigid frame is attached to a neoprene hood by a metal clamping band, hence the name.

See: cylinder bands


Main article: bar (unit)

Metric unit of pressure commonly used in diving, equal to 100 kiloPascal, and nearly equal to standard atmospheric pressure.

Main article: Barodontalgia

Tooth pain caused by pressure change.

Main article: Barotrauma

Injury caused by pressure difference.
1.  Also: "tank factor". Numeric value computed for a cylinder or manifolded set that relates volume and pressure in the imperial system of units. Computed by dividing nominal capacity (cubic feet) by cylinder working pressure (psi) to express cubic feet of volume per psi of fill pressure (sometimes multiplied by 100 to give cubic feet per 100 psi). Used to convert cylinder pressure to free gas volume.[7]
2.  A line that is a base for measurement or for construction; see datum (calculations or comparisons)
3.  A data set which is a point of reference (engineering or science) for later data.

See: diving stage

BAT wing
Buoyancy and trim wing. A back mounted buoyancy compensator cell used with sidemount harness. The buoyancy volume is mostly over the lower back.[21]

See: buoyancy compensator

BCD blowup
Uncontrolled buoyant ascent caused by inability to release gas from the buoyancy compensator faster than it expands due to pressure reduction of ascent.
beach master

See also: dive marshall

A person on the beach who records when divers enter and exit the water. Typically used during recreational scuba training to keep track of the students, watch the gear, and provide assistance when required.
1.  See: diving bell
2.  Short tug on a lifeline, used in pairs, for signalling purposes.
bell bounce dive
Surface oriented diving operation in which the divers are transported in and deployed from a closed bell, and are either decompressed in the bell at the surface or transferred under pressure to a deck decompression chamber for decompression.
bell cursor

See: Diving bell#Deployment of a modern diving bell

Mechanism or structure for guiding and constraining the motion of a bell when in the close vicinity of the deployment platform to improve handling in bad weather.[4]
bell diving
1.  Any diving operation in which the divers travel in or work from a diving bell
2.  Diving operations in which divers are transported in and deployed from a closed bell, either as a surface oriented (bell bounce) or saturation dive.[20]
bell harness
A safety harness made of strong webbing, which is fastened around a diver over the exposure suit, and allows the diver to be lifted without risk of falling out of the harness.[22]

See: Diving bell#Deployment of a modern diving bell

Standby diver deployed in the diving bell[4]
bellows ratio
The swept volume ratio between the inner and outer bellows of the counterlung of a passive addition semi-closed rebreather.[23]
bell run
The part of a bell dive operation from bell lock-off to bell lock-on (from and back to the life support system)[4]
bell stage
A framework extending below a closed bell which keeps the base of the pressure vessel off the bottom sufficiently to provide clearance for the divers to use the bottom hatch when the bell is resting on the bottom or on the clump weight.
bell umbilical

See: Umbilical cable#Diver

The combined supply and return hoses and cables for life-support, power and communications between a diving bell and the support platform
belt slide
belt slider

Also: "triglide", "weight slider", "weight stop"

Hardware item with two parallel slots which is fitted to harness or weightbelt webbing to prevent other components such as D-rings and weights from sliding along the webbing.[7]
benign water
benign conditions

See: Benign water

Sometimes also referred to as confined water. Environments of low risk, where it is extremely unlikely or impossible for the diver to get lost or entrapped, or be exposed to hazards other than the basic underwater environment.
bent D-ring
A D-ring which has been bent about 45° near the straight section on both sides, forcing it to project slightly from the harness when pushed to one side, allowing easier attachment of clips.
Decompression sickness: Injury caused by bubble formation in the body tissues after hyperbaric exposure.

See: built in breathing system

Billy ring
Three D rings welded together along their straight sides so that one is perpendicular to the other two. This uses the two flat rings to maintain the third in an upright position when mounted on harness webbing, allowing it to be more easily accessed to clip on stage cylinders. Named after Captain Billy Deans.[7]
BK hook
B.K. hook
BK safety hook
A type of safety lifting hook which is held closed when under load by a lever system where the weight of the load holds the bill of the hook against the safety latch.[24]
black-water diving

see also: blue-water diving

Diving in mid-water at night where the bottom is out of diving range.
blending stick
Mixing tube in which gases are continuously mixed prior to intake by a compressor, usually at atmospheric pressure.[9] Usually refers to manufacture of nitrox from air with added oxygen, but also used for trimix. Gas mixture is usually continuously analysed at the exit of the blending stick to monitor composition.
blind traverse
Passing through a cave from one entrance to a different exit which the diver has not used before.[7]
block adaptor
Screw-in adaptor fitting which is fitted to a 200/240 bar DIN pillar valve to allow connection of a yoke regulator or filling whip.
Procedure of pressurising a diving chamber or saturation habitat.

Main article: Blowout (well drilling)

Uncontrolled release of crude oil and/or natural gas from a well after pressure control systems have failed.
blowout preventer

Main article: Blowout preventer

A large, specialized valve used to seal, control and monitor oil and gas wells. Generally operated remotely by hydraulic ram.
blowout preventer stack
An assembly of several blowout preventer rams which may be used to shut off a well.
blue hole

Main article: Blue hole

A sink hole in a lake or the sea that is often the entrance to a cave. Blue holes in the sea are subject to tides so that their flows regularly reverse.[7]
blue-water diving

See also: Underwater diving#Diving environment

Underwater diving in mid-water where the bottom is not visible and is out of diving range.[25]
bolt snap

Main article: bolt snap

A metal connector comprising a hook with a spring-loaded axial sliding rod which must be manually retracted to allow the hook to be clipped onto something or removed. May be single- or double-ended, and if single-ended is usually fitted with a swivel ring opposite the jaws.
Commercial diver slang for high pressure gas storage cylinder of around 50 litres water capacity, also known as a "J".
bondage wing

See: bungee wing

Derogatory term for bungee wing based on fetishist terminology.[26]
The upper part of a copper helmet which encloses the diver's head and is sealed to the lower part or corselet (q.v.)
booster pump

Main article: Booster pump

Machine used to increase pressure of a gas. Usually refers to the case where inlet pressure is above ambient pressure already.

See: blowout preventer

BOP stack

See: blowout preventer stack

bottom gas
The gas breathed by the diver at the deepest part of the dive. Compare with travel gas and decompression gas.
bottom time
Time used in calculating decompression obligation from decompression tables. For most tables this is defined as the elapsed time from starting the descent to starting the final ascent to the surface, excluding ascent and decompression time.[27]
bottom timer
Device used to measure and record the total time spent underwater during a dive. They do not generally only record bottom time (q.v.).
bottom out
A rebreather counterlung becoming completely deflated during inhalation.[28]
bounce dive
bounce diving
1.  Also: "surface oriented dive" : In commercial diving, bounce diving is the alternative to saturation diving. Any dive where the diver is decompressed directly after the dive.[13]
2.  In recreational diving, a bounce dive is a descent to maximum depth and then a direct ascent back to the surface with minimal bottom time, in a dive profile resembling a spike.

See: bailout valve


Main article: Bowline

Knot used to form a secure, non-slipping loop at the end of a line.
Boyle's law

Main article: Boyle's law

Relationship between pressure and volume at constant temperature in an ideal gas.
breakdown room
An area in a cave where a large amount of material has fallen from the overhead.[7]
The point at which an object being lifted which is partly embedded in the bottom sediments overcomes the adhesion of the sediments and the force required to lift it drops rapidly to the apparent weight of the object.
The failure of a rebreather scrubber to remove all the carbon dioxide in the gas passing through the absorbent material.[28]

See: corselet

breathing gas
breathing mixture

Main article: Breathing gas

Gas supplied to the diver to breathe, either directly to the diver or to the hyperbaric environment of the diving bell, dive chamber or saturation habitat.[20][27] Colloquially just "gas" or "mix".
breathing hose
Large bore hose carrying the breathing gas in a rebreather breathing loop[28] or a twin-hose demand valve.
breathing loop
The gas flow path in a rebreather comprising the diver's lungs, the mouthpiece, valves, hoses, counterlungs and scrubber through which gas is rebreathed.
A small plate with a slot for a webbing belt and two side by side holes for clipping on equipment, generally used similarly to a D-ring in combination with a belt slider as an alternative to a butterfly slider
Broco cutter
A type of thermal lance initiated by an electric arc, in common use for underwater cutting work.

See: Baited remote underwater video


See: Ear clearing#Methods

Béance tubaire volontaire: Voluntary opening of the Eustachian tubes to equalise the middle ear.
bubble models

See: Decompression theory#Bubble models

Decompression models based on the assumption that bubbles will form during non-symptomatic decompression.
buccal pumping

See: glossopharangeal insufflation

buddy bottle
Bailout cylinder carried by a scuba diver, particularly when diving solo.[7]
buddy breathing
Sharing breathing gas from one demand valve by two or more divers, generally after an out-of-gas emergency.
buddy check

Main article: Buddy check

A procedure carried out by scuba divers using the buddy system where each diver checks that the other's diving equipment is configured, fitted, and functioning correctly just before entering the water to dive.
buddy diving

Main article: Buddy diving

A safety procedure where two or three divers monitor each other constantly during a dive and provide assistance or rescue when needed.
buddy lights
Warning lights visible to the diver's buddy that indicate rebreather system status.[28]
buddy line
A short line between two divers, used to maintain contact during a dive, generally in poor visibility, or other conditions where the divers might become separated and not be able to quickly locate each other.[27]
buddy system

Main article: Buddy diving

A procedure where two divers look out for the safety of each other, and give assistance if the other gets into difficulty.
Bühlmann algorithm
Bühlmann tables

Main article: Bühlmann decompression algorithm

Diving tables and decompression algorithm on which the tables are based, and some dive computers are programmed, based on the dissolved gas decompression model derived and tested by Dr Albert A. Bühlmann.
built in breathing system

Also: "BIBS"

A demand breathing gas supply system with external exhaust used to provide chamber occupants with breathing gases other than the gas used to pressurise the chamber. Used for treatment gases and emergency breathing gas if the chamber is contaminated.[11]
A set of gas cylinders fastened together for transportation and manifolded for use as a unit,[29] also cylinder bundle.
bungee (sidemount)

See also: ring bungee

Length of shock cord used to restrain the top end of side mount cylinders and keep them tucked in at the diver's shoulder while swimming.[21] Usually clipped to a shoulder D-ring of the harness and looped around the cylinder valve. May be attached to the back of the harness between the shoulder blades, or run continuous from one shoulder D-ring, around the back under the arms to the other shoulder D-ring.
bungee wing
Back inflation buoyancy compensator with shock cord lacing or loops which exert a force on the bladder to oppose expansion during inflation and accelerate deflation.

Main article: Buoyancy

1.  Upward force on an object immersed in a fluid due to pressure exerted over the immersed surface.
2.  Resultant upward force of buoyancy and weight of an object immersed in a fluid.
buoyancy check
Procedure to test and adjust weights carried by an underwater diver. The diver wears all the personal equipment to be used for the planned dive, with the scuba tank(s) nearly empty, and the buoyancy compensator empty, in shallow water of the same density as expected on the dive, and adds or removes weights until neutrally buoyant. After the buoyancy check it is usual to distribute the weights for safety, trim and convenience.
buoyancy compensator

Also: "ABLJ", "BC", "BCD", "buoyancy compensator device", "horse collar", "stabilisor jacket", "stab jacket" or "wing"

Main article: Buoyancy compensator (diving)

An airtight bladder worn by a diver which can be filled with air and vented to adjust and control the buoyancy of the diver.
buoyancy control
The skill of maintaining the appropriate buoyancy at any time during a dive.
burn tester
Device for measuring the actual capacity of a battery relative to its nominal capacity, and the associated functional time for the device that the battery is powering.[7]
burn time
The effective use time of a battery powered device. Mainly used in reference to dive lights and scooters.[7]
burst disk

Main article: Rupture disk

A non-reclosing pressure relief device used to protect a diving cylinder from overpressurization.
butterfly clip
A type of bolt snap with a tapered guide gate opening formed by a protrusion on both the piston and the fixed sides of the gate.[7]
butterfly slider
butterfly D-ring
A plate with two D-shaped cutouts on opposite sides of two to four parallel longitudinal slots for webbing. Used at the top back of the crotch strap in place of a butt-plate (q.v.) on minimalist sidemount harnesses as a clip-on point for equipment.[21]
A rigid or fairly stiff flexible lower extension to a backplate or other scuba harness supporting butt-plate rails, used for clipping off the lower end of sidemount cylinders to the harness.[21]


cage diving
Diving in a cage designed to protect the diver from potentially aggressive large marine animals, usually sharks
calibration gas
Gas of known composition used to calibrate gas sensors.[28]

See: catenary anchor leg mooring

cam band
A strap, usually of webbing, with a cam action tensioning buckle, generally used to secure a diving cylinder to a backplate, stabilisor jacket BCD or other form of diving harness.

See: Lifting bag#Closed lift bags (camels)

A closed lifting bag, for use at or near the surface, which retains air in rough conditions.
canister light

See: Dive light#Canister light

Dive light comprising a light head connected to a battery canister by a cable.[26]
canoe diving

Main article: Canoe diving

Scuba diving from canoes, used when the dive site is beyond convenient swimming distances.
carbon dioxide poisoning

Main article: Carbon dioxide poisoning

The toxic effects of carbon dioxide, due to incomplete elimination of carbon dioxide resulting from skip breathing, excessive work of breathing, scrubber failure in a rebreather system, or inadequate ventilation in a diving chamber or free flow helmet. Occasionally caused by contaminated gas supply.
carbon monoxide poisoning

Main article: Carbon monoxide poisoning

The toxic effects of carbon monoxide , usually due to contaminated breathing gas supply.
cardio-pulmonary resuscitation

Main article:— Cardio-pulmonary resuscitation

An emergency procedure which is performed in an effort to manually preserve intact brain function until further measures are taken to restore spontaneous blood circulation and breathing in a person in cardiac arrest

See: critical air supply

cascade filling

Main article: Cascade storage system

Decanting from several storage cylinders in succession, generally using a procedure to maximise charge pressures. Often used in partial pressure gas blending.
catenary anchor leg mooring

Also: CALM

Single point mooring type named for the catenary curve of the anchor cables that hold the buoy in position. Also referred to as single buoy mooring, monobuoy or loading buoy.[4]
caustic cocktail

See: Rebreather diving#Scrubber failure

A mixture of water and carbon dioxide absorbent caused by flooding the scrubber of a diving rebreather, and which may reach the diver's mouth through the breathing loop. The alkalinity depends on the absorbent used, and may injure the diver if aspirated.

Main article: Cave

A naturally occurring cavity in bedrock, or an underwater passage not illuminated by natural daylight, large enough to be entered by a human. Statute 810.13 of the Florida legislature defines a cave as: any void, cavity, recess, or system of interconnecting passages which naturally occurs beneath the surface of the earth or within a cliff or ledge, including natural subsurface water and drainage systems but not including any mine, tunnel, aqueduct, or other manmade excavation, and which is large enough to permit a person to enter. The word "cave" includes any cavern, natural pit, or sinkhole which is an extension of an entrance to a cave.[7]
cave arrow

See: Line marker

Directional line markers which point the way to an exit.
cave fill
Filling a scuba cylinder to a pressure significantly above the rated safe working pressure (charging pressure).[7] Illegal in some jurisdictions, and increases risk of catastrophic failure.
cave line
1.  Application: A distance line laid in a cave for navigation.
2.  Material: Small diameter braided synthetic cordage used for distance lines in cave diving .
cave reel

See: Distance line

A reel specifically made for cave diving, used to lay and recover large lengths of cave line which is used as a temporary guide line to find the exit or a permanent guide line.
1.  Two or more interconnected underground rooms or passages in bedrock, each large enough to be entered by a human.[7]
2.  The initial space in an underwater cave system that is illuminated by natural daylight.[7]
3.  A naturally occurring cavity in bedrock or an underwater passage, large enough to be entered by a human, which is illuminated by natural daylight, or in which it is possible from all points to see the exit by natural daylight.[7]
cavern dive
Defined as a penetration dive under rock where visibility is greater than 40 feet, maximum penetration does not exceed 130 feet, maximum depth does not exceed 70 feet, the diver is always within the area illuminated by ambient sunlight, and does not pass through any restrictions.[7]

See: certification card


See: closed circuit rebreather


See: closed circuit underwater breathing apparatus

Solid overhead or decompression restriction to a safe direct vertical ascent to the surface

Main article: Cenote

A sinkhole in Mexico. Generally with vertical or overhanging walls or shafts with water that open into a cave system.[7]
certification card
A plastic card issued to a diver by a diver certification agency as evidence of having completed the diver training and experience required for the level of certification.

See: clusterfuck


See: Compressed Gas Association

CGA 850
Compressed Gas Association valve standard CGA 850 is the standard for the scuba cylinder valve outlet for yoke connectors.[30]
chain chaser
Ring or hook shaped components used for installing and recovering conventional mooring systems. The chaser is hooked around the chain and pulled in the direction of the anchor until it slides onto the anchor shank and is stopped by the crown. The chaser is then used to break the anchor out by pulling directly upwards.[4]
chamber dive
Simulated dive in a hyperbaric chamber pressurised to equivalent pressure to the nominal depth of the dive.[27]
chamber operator
Person competent to operate a diving chamber[11]
Formation of void spaces in the granular absorbent in a scrubber which allow gas to bypass close contact with the absorbent material, allowing carbon dioxide to pass through the scrubber.[28]
charging pressure

See also: working pressure

Pressure stamped on a container for a permanent gas to indicate the maximum gauge pressure measured at, or corrected to the reference temperature (usually 15°C or 20°C) that may be applied at the time of filling.[31]
Charles's law

Main article: Charles's law

Relation of volume to temperature at constant pressure of an ideal gas.
charlie foxtrot

See: clusterfuck

chicken vest
Sleeveless neoprene wetsuit vest with attached hood.[32][33]
A section of a cave that is vertical or near vertical and like a shaft.[7]
Chinese lantern
Connection between pipeline end manifold (PLEM) and single point mooring buoy (SPM) using two to four separate curved, flexible, underwater hose strings.[4]
choked flow

See: constant mass flow

choker (sidemount)

See: Sidemount diving#Cylinders

A strap around the neck of a sidemount cylinder used to hold the bolt snap closer to the neck so that the head of the cylinder stays closer to the diver's armpit. The choker can be a small webbing strap with a sliding buckle for adjustment, so it can be tightened to bring the clip closer to the neck or slacked off while in use.

See: Decompression sickness#Classification

A symptom of decompression sickness manifested by shortness of breath, caused by a large number of venous gas bubbles in the lung capillaries which interfere with gas exchange.[26]
Christmas tree

Also: "tree"

Main article: Christmas tree (oil well)

An assembly of valves, spools, and fittings installed on top of the wellhead and used primarily to control the flow, usually oil or gas, out of the well.[4]
Christmas tree ladder
A boarding ladder which has a single central rail with rigid cantilevered rungs to each side, allowing use while wearing swimfins.[34]
Usually Christo-lube MCG111, an oxygen compatible lubricant suitable for use in breathing apparatus in oxygen service.[7]
Cave dive route in which there is a one-way segment. The circuit can be simple or complex depending on the number of jumps involved.[7]

See: inshore diving

closed bell

Also: "dry bell"

See Diving bell#Structure of a typical closed bell

A closed or dry bell is a pressure vessel for human occupancy which is lowered into the sea to the workplace, equalised in pressure to the environment, and opened to allow the divers in and out. Divers may be decompressed in the bell or transferred under pressure to a hyperbaric chamber at the surface.
closed circuit rebreather
closed-circuit rebreather

See: Rebreather diving#Closed circuit rebreather

Underwater breathing apparatus in which exhaled gas is scrubbed to remove carbon dioxide, and the oxygen is replenished to maintain a specific partial pressure, before returning it to the diver as breathing gas. See also ECCR and MCCR.
closed circuit underwater breathing apparatus
Military alternative term for closed circuit rebreather.
clump weight
A weight used to keep the guide wires of a diving bell aligned and tensioned.

Also: "CF", "charlie foxtrot"

Occasion when the situation diverges notably and usually uncontrollably from the plan, sometimes involving immediate hazard to life and limb, and often involving poor judgement.[26]
cobra guard

See: rams head

code of practice
A systematic set of professional standards or written guidelines and rules of procedures to be followed by members of a profession, trade, occupation or organization. A code of practice may be compiled and agreed on by members of a particular profession or written guidelines issued by an official body or a professional association to its members to help them comply with its ethical standards. A code of practice does not normally have the force of law, but is often required or compulsory practice for members of an organisation.
coded welder
Welder who is trained and qualified and assessed as competent for a specified type of welding under specified conditions.
combat swimmer

Main article: Combat swimmer

Person trained in scuba diving or swimming underwater in a military capacity which can include combat
command signal
A signal from a diver in a team that requires a response from the other diver. There are three: "Are you OK", "Hold", and "Surface" (terminate the dive).[7]
commercial diving

Main article: Commercial diving

Working under pressure: Occupational activity where gas is breathed while immersed at pressure in excess of atmospheric pressure, usually underwater.

Main article: Completion (oil and gas wells)

The process of making a well, that has been drilled, ready for production.
competent person
Person who is able to perform a task or operation safely and according to required procedures. Recognition of competence may require formal assessment or accreditation, or registration with government body.[31]
compressed air
Air at a pressure greater than ambient.
Compressed Gas Association

Main article: Compressed Gas Association

An American trade association for the industrial and medical gas supply industries.[35] The CGA publishes standards and practices that codify industry practices. In cases where government regulation is not specific, CGA documents are considered authoritative. CGA V-1 Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections covers diving cylinder valve outlets.[36] Safety devices like burst disk overpressure protection are specified by the CGA Standard S1.1.
compressed neoprene
Foam neoprene that has been compressed to reduce volume. Used for dry suits and hot water suits which are less affected by depth induced buoyancy changes.
The process of increasing the ambient pressure on a diver by descending into the water or pressurising the chamber.[20]
compression arthralgia

Main article: Compression arthralgia

Pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression

Main article: Compressor

Machine which pressurises gas. Generally intake gas is at ambient pressure, outlet gas at higher pressure. High pressure breathing air compressor output pressure is usually 200 to 330 bar. Machines which compress gas at higher intake pressures are called boosters
compressor log
Book or file containing records of compressor operation, filling of cylinders and maintenance records.
compressor operator
Person who operates a compressor, either to fill cylinders, or to provide breathing air to surface supplied divers.

Main article: Condensation

Liquid resulting from phase change from gas due to cooling, pressure increase, or both.
Tunnel or passage in a cave system.[7]
confined water
Water that is enclosed and bounded sufficiently for safe training purposes. Generally implies that conditions are not affected by geographic or weather conditions, and that divers cannot get lost.[37]
conservative (decompression)

See: Conservatism (diving)

Decompression profile tending to minimise risk of decompression sickness at the cost of more decompression time for a given pre-ascent dive profile.[6]
constant depth blackout
constant pressure blackout

Also: "isobaric blackout"

A Freediving blackout which occurs while the diver maintains a near constant shallow depth, where reduction of oxygen partial pressure due to ascent is not a factor. Usually induced by pre-dive hyperventilation. Also referred to as shallow water blackout, which is an ambiguous term
constant mass flow

Also: "choked flow"

Main article: Choked flow

Sonic flow through an orifice – the maximum possible flow rate for the gas through the orifice for a given absolute upstream pressure.[28]
constant volume dry suit
Dry suit with an automatic dump valve.

See: restriction

containment grip

See: whip sock

contents gauge

See: submersible pressure gauge

continuous decompression

See: Decompression practice#Continuous decompression

Decompression without stops. Instead of a fairly rapid ascent rate to the first stop, followed by a period at static depth during the stop, the ascent is slower, but without officially stopping. Ascent rate may vary with depth, usually slowing as the depth reduces.
contra-indications to diving
Conditions (usually medical) that indicate that a person should not dive.
control compartment
The tissue compartment that dictates the ascent profile of a given dive because it is theoretically the highest risk compartment for DCS.[6]
controlled buoyant lift

Main article: Controlled buoyant lift

A rescue technique used by scuba divers to raise an incapacitated diver to the surface from depth.
controlled emergency swimming ascent

Also: "CESA"

Main article: Controlled emergency swimming ascent

An emergency technique for surfacing, usually when no breathable gas is available at depth. The diver fins upward while gently exhaling to keep expanding air in the lungs from causing lung expansion injuries.[38]

Main article: Line marker

Personal non-directional line markers that mark specific locations, or the direction of one's own exit at line intersections.
copper hat
copper helmet

See: Standard diving dress

A diving helmet of traditional design and construction, usually made from spun or beaten copper, with brass or bronze fittings. There are usually two main sub-assemblies; the bonnet is the roughly spherical part which covers the head and is provided with viewports, valves and various other fittings, and the corselet, which rests on the upper torso of the diver, and to which the bonnet is connected when in use, and which may be sealed to the suit and ballasted to compensate for the buoyancy of the airspace inside the helmet.

Also: "breastplate"

See: Standard diving dress#General description

Lower part of a standard copper helmet and some other heavy helmets, which clamps to the diving suit, rests on the diver's shoulders, and to which the helmet upper part, or bonnet, is clamped, screwed or bolted.

See: Rebreather#Counterlungs

Flexible bag or bellows in a rebreather which compensates for the changes in volume of gas in the loop during the breathing cycle.

See: cardio-pulmonary resuscitation

cracking pressure
Pressure required to open a spring-loaded valve. Often applied to the difference in pressure over the diaphragm of a demand valve required to open the valve to start flow. This may differ significantly from the pressure difference required to keep the valve open once flow has been initiated, and the pressure required to keep the valve open may vary with flow rate.
critical air supply

See: critical pressure

The amount of breathing air required to safely exit a penetration dive. When the available air supply reaches the calculated critical pressure the dive has reached a planned turning point.[7]
critical difference hypothesis

See: Decompression theory#Critical difference hypothesis

Hypothesis that bubble formation during decompression will not occur if a critical pressure difference between tissue inert gas tension and ambient pressure is not exceeded.
critical flicker-fusion frequency

Also: CFFF

The frequency at which a flickering light is perceived as continuous, a measurement used to evaluate visual temporal processing as an indicator of alertness and arousal in humans, including use as an experimental indicator of inert gas narcosis in divers, using a simple uncomplicated, non-invasive and objective methodology.[39]
critical pressure

See: Scuba gas planning#Critical pressures

Cylinder gas pressure which determines a safe limit to an underwater activity on scuba, such as start of ascent or turnaround during a penetration.
critical ratio hypothesis

See: Decompression theory#Critical ratio hypothesis

Hypothesis that bubble formation during decompression will not occur if a critical ratio of tissue inert gas tension and ambient pressure is not exceeded.
critical volume hypothesis

See: Decompression theory#The critical volume approach

Hypothesis that symptoms of decompression sickness will not be evident if a critical volume of tissue gas bubbles is not exceeded.
crotch strap
Harness strap that passes from the lower part of the harness at the back, through between the diver's legs, to the waistbelt in front of the harness, effectively securing the harness from sliding up along the torso. In commercial diving safety harness this is often in two parallel parts and allows the diver to be lifted by the harness without risk of falling out.[7]
crushed neoprene
Proprietary material for dry suits manufactured by DUI in a process where the foam neoprene suit material is degassed by exposure to high hydrostatic pressure to reduce the volume after assembly. There is less buoyancy variation with depth as the material is less compressible after the treatment.[40]
cuff dump
A spring loaded over-pressure dump valve mounted on a dry suit sleeve near the cuff, usually relatively small and non-adjustable, which dumps excess gas from the suit if it is raised sufficiently.
current limited

See: Electro-galvanic oxygen sensor#Failure modes

Common failure mode of an oxygen sensor cell in which an increase of oxygen partial pressure above the limiting level does not produce an increase in output current. Such a failure would prevent the control system from recognising an excessive partial pressure of oxygen.[28]
current shear
Variation in flow velocity along an axis othogonal to the direction of flow. Current shear may be vertical, where shallow water flows at a different velocity to deeper water, horizontal, where flow velocity varies across the width of the current, or mixed. Wind induced current displays vertical shear and directional variation with depth in an Ekman spiral.

See: Diphenyl oxalate

Trademark name Cyalume is a solid ester whose oxidation products are responsible for the chemiluminescence in a glowstick.

Main article: Cyanosis

The appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen.
1.  Diving cylinder: Refillable compressed gas container of water capacity between 0,5 L and 150 L used to supply gas for underwater diving.[31]
2.  That part of a reciprocating compressor or booster in which the piston is moved to compress the gas. The internal space is cylindrical with a circular section. The external surface is usually finned for air cooling.
cylinder boot

See: Diving cylinder#Cylinder boot

Rubber or plastic cover for the base of a scuba cylinder to protect it from abrasion, and in the case of domed end cylinders, to allow it to stand upright.
cylinder bundle

See: bundle

cylinder neck

See: Diving cylinder#Cylinder neck

The part of the cylinder end which is shaped as a narrow concentric cylinder, and internally threaded to fit a cylinder valve.
cylinder shoulder

See: Diving cylinder#The pressure vessel

The domed top of the cylinder between the parallel section and the neck with the cylinder valve.
cylinder valve

See: Diving cylinder#The cylinder valve

Valve fitted to a compressed gas cylinder to control gas flow into and out of the cylinder. Also pillar valve.


Dalton's law

See: Dalton's law

Gas law describing the relation of component pressures of gases in a mixture to the total pressure.[7]

See: Divers Alert Network

(Derogatory) Any part of a diver's equipment that dangles in a position that might impact the bottom or get caught on the surroundings.[7]
Decompression Computation and Analysis Program: Decompression planning software by Bill Hamilton.[7]

Also: "manual CCR" (mCCR)

See: Diving rebreather#Control of the breathing gas mix

Diver-controlled closed-circuit rebreather. A closed circuit rebreather which requires the diver to monitor oxygen levels and manually inject oxygen or diluent as needed to maintain an appropriate partial pressure in the loop.

Main article: DRDC Toronto

Defence and Civil Institute of Environmental Medicine (Toronto, Canada).
DCIEM tables

See: History of decompression research and development#DCIEM model and tables

Decompression tables based on the Kidd-Stubbs model, developed and published by the Defence and Civil Institute of Environmental Medicine.

See: decompression illness


See: decompression sickness


See: deck decompression chamber

dead man anchor

See: Lifting bag#Breakout

A heavy weight used to control the rise of a lifting bag after breakout, or to capsize it to prevent a runaway lift
dead space
dead volume

See: Dead space (physiology)

1.  The volume of a breathing apparatus which holds exhaled air, which is subsequently inhaled directly. without passing through a scrubber to remove carbon dioxide and without oxygen addition.
2.  The volume of inhaled air, which does not take part in gas exchange either because it remains in the conducting airways or in alveoli that are poorly perfused.

See: Cascade filling system

To transfer gas between cylinders by differential pressure. No energy is input, flow will stop when pressures are equalised.
deck decompression chamber

See: Deck decompression chamber

A twin-lock hyperbaric chamber suitable for surface decompression and emergency recompression. Large enough to hold at least two occupants, one of them lying down.

Main article: Decompression (diving)

Reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure, and the process of allowing dissolved inert gases to be eliminated from the body tissues during this reduction in pressure.[20]
decompression algorithm

See: Decompression practice#Decompression algorithms

Specified step-by step procedures used to calculate the decompression stops needed for a given dive profile. The algorithm can be used to generate decompression schedules for a particular dive profile, decompression tables for more general use, or be implemented in dive computer software to perform real-time analysis of decompression status of the diver.
decompression bar

See: decompression trapeze

decompression buoy


decompression chamber

See: Diving chamber#Decompression chamber

Hyperbaric chamber used for decompressing divers and emergency therapeutic recompression.[11]
decompression computer

See: dive computer

decompression gas

See: Decompression practice#Accelerated decompression

Gas breathed during decompression. Commonly implies a composition chosen to accelerate decompression by using an increased oxygen content.[11]
decompression illness

Main article: Decompression illness

Illness caused by decompression. Includes decompression sickness and arterial gas embolism due to lung overexpansion injury.
decompression obligation

See: Decompression practice#Decompression

Calculated theoretical requirement to make decompression stops during ascent based on the dive profile, breathing gases, and the decompression model in use.
decompression schedule

See: Decompression practice#Decompression schedule

A specific ascent rate and series of increasingly shallower decompression stops that a diver uses to allow inert gases to be eliminated from the body tissues during ascent after a specific hyperbaric exposure, to reduce the risk of decompression sickness.
decompression sickness

Main article: Decompression sickness

Also: "the bends", "caisson disease", "DCS", or "divers' disease"

A condition arising from dissolved inert gases coming out of solution during decompression as bubbles in the tissues, organs and blood vessels of the body causing symptoms ranging from rashes to death.
decompression status
Theoretical tissue inert gas loading after a given dive history according to the chosen decompression model. It is a measure of whether, and how much, decompression is recommended by the algorithm.
decompression stop

See: Decompression practice#Decompression stops

A pause during the ascent phase of a dive that a diver spends at a constant relatively shallow depth to allow safe release of inert gases from the body tissues to avoid decompression sickness.[11]
decompression stress
An indicator of risk for decompression sickness associated with the excess (supersaturation) of inert gas dissolved in the various tissues throughout the body, driving bubble formation and growth.
decompression tables

See: Decompression practice#Decompression tables

Printed cards or booklets that allow divers to determine a decompression schedule for a particular dive profile and breathing gas.[11]
decompression trapeze

Also: "decompression bar"

See: Decompression practice#Decompression trapezes

A horizontal bar or bars suspended at the depth of intended decompression stops by buoys, used to make decompression stops more comfortable and more secure and provide the divers' surface cover with a visual reference for the divers' position.
deep stops

See: Decompression theory#Thermodynamic model and deep stops

Decompression stops which are deeper than the deepest stops required by decompression algorithms using dissolved phase models.
deep water blackout
1.  Freediving: An ambiguous alternative term for blackout of ascent following a deep breath-hold dive, in which loss of consciousness occurs as the surface is approached, or at the surface, caused by cerebral hypoxia arising from the rapid drop in the partial pressure of oxygen in the lungs during ascent as the ambient pressure drops and the gas in the lungs expands to surface volume.[41]
2.  Scuba and surface-supplied diving: Loss of consciousness in deep air diving at depths below 50 m with no clear primary cause, associated with nitrogen narcosis, a neurological impairment with anaesthetic effects caused by high partial pressure of nitrogen dissolved in nerve tissue, and possibly acute oxygen toxicity. The term is not in widespread use at present.[42]

Main article: Dehydration

A condition where the water content of the body is reduced.
delta P
Environments where a pressure difference causes flow. Usually refers to cases where the flow is likely to entrain and pull the diver into an enclosed space or through an opening.
demand helmet

See; Surface-supplied diving#Lightweight demand helmets

Diving helmet which provides breathing gas supply flow only when the diver inhales, triggered by the pressure drop.
demand valve

See: Diving regulator#Demand valve

Mechanism for providing the user with breathing gas flow only when required. The valve may be triggered by a reduction of pressure across the actuator diaphragm to below ambient due to inhalation, or by pressing a manual override (Purge button).
depth gauge

See: Depth gauge

A pressure gauge calibrated to measure depth as a function of ambient pressure.
descending line

See also: jackstay, downline and shotline

A substantial heavily weighted line attached to a secure point at the surface, such as a boat or buoy, which can be used by a diver to control position and depth during descent an ascent.[43]
developed pressure

See: Diving cylinder#Developed pressure

The pressure of the compressed gas in a cylinder at a temperature other than the nominal temperature at which charging pressure is specified. Usually refers to pressure when fully charged at a variation from the reference temperature.[31]

See: Differential Global Positioning System

An enhancement to Global Positioning System that provides improved location accuracy. DGPS uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.
Diamond Reef System

Main article: Diamond Reef System

A diving skills program that uses a set of standardized portable obstacles to train and evaluate buoyancy skills and educate scuba divers on how to interact with coral reefs.
differential pressure hazard
delta P hazard
Situation where water flows from a region of higher pressure to one of lower pressure, and where obstructing the flow will induce a large force on the obstruction. A type of diving hazard.[13]
diffusion limited

Main article: Decompression theory#Diffusion limited tissues and the "Tissue slab", and series models

Decompression hypothesis that the uptake and elimination of inert gas is limited by diffusion rates in the tissues. Compare with perfusion limited.

Also: "dil"

See Rebreather#Gas sources

Gas mixture used to dilute the oxygen in the loop of a closed circuit rebreather to a partial pressure suited to the depth.
diluent flush

Also: "flush", "dil flush", "loop flush"

See Rebreather diving#Diluent flush

Replacing the gas within the breathing loop of a rebreather by injecting diluent gas while venting the previous gas mix. Usually done to get a breathable mixture of known composition in the loop to check calibration of the oxygen cells.[28]
DIN fitting

Also: "DIN valve", "DIN regulator", "DIN thread"

Usually refers to G5/8" x 14 tpi[44] parallel thread fittings used to connect a cylinder valve to a filling connection or regulator first stage. Available in 200 bar and 300 bar versions which should only be inter-connectable in safe combinations.
DIN plug

See: Diving cylinder#Connection to the regulator

Screw in adaptor which can be used with many recent 200/240 bar DIN cylinder valves to allow connection of Yoke regulators or filling whips.

See: Doing It Right

display integrated vibratory alarm

Also: "DIVA"

Display integrated vibrating alarm – A head-up display module which produces a warning vibration to draw the attention of the diver, generally mounted on the diver's mask or the rebreather mouthpiece.[28]
distance line

Also: "penetration line" or "guideline"

Main article Distance line

A line used by scuba divers as a means of returning to a safe starting point in conditions of low visibility, water currents or where pilotage is difficult.

See: display integrated vibratory alarm

(legal) A human activity in which a person enters water or any other liquid, or a chamber in which they are subject to a pressure greater than 100 millibars above atmospheric pressure and, while in such an environment, they breathe air or other gas at a pressure greater than atmospheric pressure.[13][45]
dive computer

Also: "decompression computer"

Main article: Dive computer

A device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness.[46]
dive factor
Factor used in gas consumption estimates which allows for increased breathing rate due to conditions other than depth. Values range from 1.1 for relaxed, stress free conditions to more than 3.0 for heavy work.[47]
dive flag

Main article: Dive flag

Flag used to indicate that there are divers in the water. There are two versions: the international code letter flag 'Alpha', ICS Alpha.svg, and the red flag with white diagonal bar, ICS Diver.svg.
dive ladder
diver ladder

See: Human factors in diving equipment design#Diver ladders

A ladder suitable for divers dressed for the specific dive to climb up and down between the surface deployment area and the water.
Dive Leader
EN 14153-3 / ISO 24801-3 standard competence for recreational scuba diver. A level 3 "Dive Leader" has sufficient knowledge, skill and experience to plan, organise and conduct their dives and lead other recreational scuba divers in open water, to conduct any specialised recreational scuba diving activities for which they have received appropriate training, plan and execute emergency procedures appropriate for the diving environment and activities. If diving and environmental conditions are significantly different from those previously experienced, they require an appropriate orientation with regard to local environmental conditions, and must have appropriate specialised training and experience to lead on dives which have more demanding operational parameters.[48]
dive location
(Professional) The vessel, structure, or base from which occupational dives are conducted and supported. More specifically, the point from which the actual dive is controlled.[13]
dive marshall
Recreational and club diving equivalent of a diving supervisor. The person who organizes and plans a group dive outing for recreational divers, assesses risk, logs divers into and out of the water and is available at the site to manage incident response.[49]

Main article: Divemaster

A professional level recreational diver who leads a group of less experienced or visiting divers underwater.
dive platform
1.  Recreational: Fixed or movable platform on a dive boat which divers use to more easily enter and leave the water.
2.  Commercial: Structure on which the diving spread is established. Usually a vessel or an offshore or waterside structure.
dive profile

Main article: Dive profile

The variation of depth with elapsed time during a dive, often depicted as a graph.
dive site

Main article: Dive site

The place at which the dive occurs. Also, more broadly, a place at which diving occurs, has occurred, or is planned to occur, and the general locality, with extent depending on context. Professional divers may also refer to a work site, or search area if relevant.
dive skins
A lycra suit worn by a diver in warm water or under a wet suit.
dive station
The site from which a professional diving operation is directly controlled.[13]
dive tables

See: decompression tables

dive time
The total elapsed time spent underwater during a dive.
dive timer

See: Dive timer

An automatically operated electronic timer which records the elapsed time from the start of a dive.
diver lift

See: Dive boat#Diver lifts

A mechanism used to lower one or more divers into the water to a shallow depth, and to lift them out again.
diver training manual

See also: Training manual, Diving manual

A publication containing instructional material for diver training. This may relate to a specific certification or a range of certifications, and is usually published either by a certification agency or a diving school for their own use, but may also be published and sold for general consumption.
diver transfer chamber
Hyperbaric chamber connecting other component chambers of a saturation life support system which may be at different pressures.[11]
diver umbilical
diver's umbilical

See: Umbilical cable#Diver

See also: umbilical cable

Bundle of life-support hoses, communication cable, pneumofathomer hose and strength member between the surface control point and a surface-supplied diver.
Divers Alert Network

Also: "DAN"

Main article Divers Alert Network

A non-profit organization for assisting divers in medical and travel emergencies, advising divers on medical matters, and medical research on recreational diving safety.
diver's attendant

Also: "dive tender" and "line attendant"[27]

A person who assists the working diver to prepare for a dive, get in and out of the water, and to undress from the diving equipment, and who tends the lifeline or umbilical while the diver is in the water.[27]
diver's slate

Also: wrist slate

See also: wet notes

Piece of opaque white or pale coloured rigid plastic sheet with matte finish that is easily marked with a graphite pencil, used for taking notes, making sketches and written communication underwater.
dive/surface valve

See: Rebreather#Dive/Surface valve

Valve on the mouthpiece of a rebreather which can be switched between the loop and ambient air at the surface. It seals the breathing loop on the surface setting to prevent flooding the loop and is used for this purpose if the mouthpiece is removed from the mouth in the water. Compare with bailout valve (BOV).
Underwater activity and related recompression facility operations where personnel are subjected to elevated ambient pressure.[13]
diving basket
A diver deployment device similar to a diving stage, normally designed with an open cage.[13]
diving bell

Main article: Diving bell

A rigid chamber suspended from a cable and used to transport divers to depth and back to the surface.
diving chamber
1.  A simple form of submersible vessel to take divers underwater and to provide a temporary base and retrieval system in the depths (diving bell).
2.  A land or ship-based hyperbaric chamber to artificially reproduce the hyperbaric conditions under the sea.
diving contractor
Legal persona responsible for commercial diving operations.[45]
diving depth
Generally the maximum depth to which the diver is exposed during a dive.[20]
diving heavy

See also: overweighting

Practice of carrying significantly more ballast weight than necessary to neutralise buoyancy. Common in professional diving operations where the diver needs to remain in firm contact with the bottom to work effectively, and is tethered by a lifeline or umbilical to a control point at a place of safety, which is managed by a tender.
diving manual
1.  See: operations manual
2.  See: diver training manual
Diving Medical Advisory Council

Also: "DMAC"

Main article Diving Medical Advisory Council

An independent body of diving medical specialists from Northern Europe which provides advice about medical and certain safety aspects of commercial diving.[50]
diving medical technician

Also: "DMT"

A paramedic specialising in diving related conditions, and medically fit to dive in a hyperbaric chamber.
diving method
diving mode

See: Diving mode

The combination of diving equipment, breathing medium and compression/decompression used for a diving operation, e.g. breathhold, open circuit scuba, nitrox rebreather, surface-supplied air, heliox saturation, etc.[13]
diving operation

Main article: Diving operation

A portion of a diving project that can be safely supervised by one person, which can be a single dive or a number of dives.[13]
diving project
The overall diving job by a diving contractor under a specific contract or plan, regardless of duration.[13][45]
diving regulator

See: regulator

diving response
diving reflex

Main article: Diving reflex

The involuntary physiological response to immersion which exists in all air-breathing vertebrates. It is a series of autonomic responses to apnea which are strengthened by facial cooling and hypoxia. It consists of peripheral vasoconstriction and associated hypertension, vagally induced bradycardia and reduction of cardiac output. This appears to preferentially supply oxygen to the brain. Another aspect is splenic contraction which increases haemoglobin content of the blood.[51]
diving safety officer

Main article: Diving safety officer

The person who administers a United States university's research diving safety program.[52]
diving signals

Main article: Diver communications

Hand sign and light sign system used by scuba divers to communicate when underwater.
diving stage

Also: "basket"

See: Decompression practice#Diving stages and wet bells

A platform on which a diver stands which is hoisted into the water, lowered to the workplace at the bottom, and then hoisted up again to return the diver to the surface and lift them out of the water. The diving stage is particularly effective for controlling rate of descent and ascent.[27]
diving superintendent
Person with overall responsibility for commercial diving operations at a large installation.[4]
diving supervisor

Main article: Diving supervisor

Person in charge of, and responsible for safety of a commercial diving operation. Usually trained, assessed as competent, certified and registered. Formally appointed by the diving contractor.[20]
diving support vessel

Also: "DSV"

Main article: Diving support vessel

A ship or boat used as a base for diving operations, particularly if designed or fitted out for that purpose.
diving system
The complete set of equipment necessary to support a diving operation.[20] The equipment and facilities required to execute the planned diving activity, including compression, decompression, rescue and recovery.[13]

See: Diving Medical Advisory Council


See: Diving medical technician

Doing It Right

Also: "DIR"

See: Doing It Right

A holistic approach to scuba diving, which encompasses several essential elements, including fundamental diving skills, teamwork, physical fitness, strictly defined standard procedures, and the use of standardised, streamlined and minimalistic equipment configurations.[26]
dome port
A domed window of optical quality glass or plastic which covers the front of an underwater camera or video housing.
donating the octopus

See: donating the secondary

donating the primary
Rescue technique where the donor of breathing gas provides it via the primary second stage – the one from which the donor was breathing – as it is known to be working and providing the correct gas. The donor then switches to their backup DV, often stowed under the chin by a bungee necklace with a breakaway connection.
donating the secondary
Rescue technique where the donor of breathing gas provides it via the secondary, or octopus, second stage, and continues to breathe off the primary.
donkey dick
Slang term for the corrugated buoyancy compensator inflation and deflation hose.[7]
Doppler bubble detection

See: Decompression theory#Doppler ultrasonic bubble detection

Ultrasonic signals reflected from bubble surfaces to identify and quantify gas bubbles present in venous blood.
Dorf arrow

Also: "line arrow"

Triangular plastic line marker with two slots which is mounted on a cave guide line to indicate the direction of the exit.[7]
down line
A rope leading from the surface down to the underwater workplace which allows a commercial diver to travel directly to and from the job site and to control rate of descent and ascent in the same way as using a shotline. Also sometimes called a jackstay.[53] A downline used for open ocean diving is much the same as a shotline (q.v.), but does not reach all the way to the bottom. An open-ocean downline is weighted at the bottom, and attached to a substantial float at the surface, which may be tethered to the boat. It may be marked at intervals by knots or loops, and may be attached to decompression trapeze system. In some cases a sea anchor may be used to limit wind drift, particularly if attached to a boat with significant windage.[54]
In the direction of flow. Displaced from the reference point in the direction of flow
downstream valve

See: Diving regulator#Upstream vs downstream

Valve in which the closure is downstream of the orifice. Pressure in the line tends to assist in opening the valve. When spring-loaded a downstream valve may open automatically if the pressure difference is excessive, thus functioning as a pressure relief valve
down time
Period when planned activities can not be done due to unforeseen or uncontrollable circumstances.[4]
DP alert
Status of the dynamic positioning system regarding positional accuracy and reliability. Green indicates normal operation, yellow indicates degraded operation and red indicates emergency.[4]
DP footprint
Dynamic positioning footprint: The area around the nominal position to which a dynamically positioned vessel is constrained by the DP system.[4]
1.  See: diver propulsion vehicle
2.  See: dynamically positioned vessel
A ring shaped like a capital D, usually of stainless steel or plastic, stitched or buckled to a diver's harness and used as an attachment point for lifeline, cylinders or other equipment.[26]
Dräger tube
Draeger tube
Indicator tube used for testing breathing gas quality.
drift diving

Main article: drift diving

Any dive where the diver is transported significantly by drifting with currents during the dive.[26]
Ship built or converted for offshore well drilling, using dynamic positioning to maintain position in deep water.[4]
drop cylinder

See: stage cylinder

drop weight
Weight used during descent and ascent, but left on the bottom at the guideline during the deep part of the dive when it is not needed due to suit compression.[7]

Main article: Drowning

The process of experiencing respiratory impairment from submersion/immersion in liquid.
dry bag

Main article: Dry bag

Bag which seals in a watertight manner. Used for keeping clothes and other equipment dry in a wet environment.
dry bell

See: closed bell

dry filling

See: Diving cylinder#Temperature change during filling

Filling scuba cylinders without the use of a water bath for cooling.[55]
dry suit

Main article: Dry suit

A watertight suit worn to keep the diver dry and to provide protection from the environment. Thermal insulation may be provided by the suit or garments worn under the suit.
dry suit blowup

See: Dry suit#Over-inflation

Uncontrolled ascent due to over-inflation of a dry suit.
1.  See: dive/surface valve
2.  See: diving support vessel

Also: "delayed surface marker buoy" and "decompression buoy"

An inflatable marker buoy deployed from underwater to indicate the position of a diver and to control ascent rate. Can also be used to mark a position or signal an emergency.
duckbill valve

Main article: Duckbill valve

A non-return valve, manufactured from rubber or synthetic elastomer, and shaped somewhat like the beak of a duck. Used as exhaust valve in some twin-hose diving regulators
dump line
A rope attached to the dump valve on a lift bag which is used by a diver to operate the dump valve to release air for control and adjustment of the bag buoyancy.[13]
dump valve
Valve used to release excess gas from a dry suit, buoyancy compensator, rebreather, or lift bag. Usually manually operated in buoyancy compensators and lift bags, and automatic in dry suits and rebreathers. May also function as an overpressure valve.[7]
dwell time

See: Dwell time (filtration)

Time that the breathing gas passing through a filter stack or absorbent canister remains in contact with the active filtration medium where the absorption of impurities can occur.[56]
dynamically positioned vessel

See: Dynamic positioning

Vessel which maintains position and heading using thrusters and positional feedback [26]
dynamic positioning

Main article: Dynamic positioning

Method of keeping a floating platform in position without anchoring, using thrusters and positional feedback.
dynamic setpoint

Also: "floating setpoint"

A target value for oxygen partial pressure in a rebreather loop which varies as a function of depth. Generally a setpoint that changes to optimize gas use, no stop time and other dive variables.[28]

Main article: Dysbarism

Medical conditions resulting from changes in ambient pressure.



See equivalent air depth


See Enriched Air Nitrox

ear beer
A home made mixture of alcohol and acetic acid in water used as a drying agent and disinfectant to rinse the ears after diving, to prevent ear infections.
ear clearing

Main article Ear clearing

Equalising the pressure in the middle and external ear by opening the Eustachian tubes. Several techniques are used.[57]

See: Diving rebreather#Control of the breathing gas mix

Electronic closed circuit rebreather. Sometimes ECCCR for electronically controlled closed circuit rebreather, which is the same thing.
eddy current test

See: Eddy-current testing

Method of non-destructive testing using electromagnetic induction to detect flaws in conductive materials. It is used to detect cracks in parallel neck threads of aluminium cylinders. Also called Visual Plus inspection. Required for cylinders of AA6351 alloy.
European Diving Technology Committee

See: emergency gas supply

E-L algorithm

See: VVAL18

electro-galvanic oxygen sensor

Main article: electro-galvanic oxygen sensor

An Electro-chemical fuel cell which produces a voltage proportional to the partial pressure of oxygen.
emergency gas supply

See also: bailout cylinder

Main article: Bailout bottle, also Surface-supplied diving#Bailout gas supply

Breathing gas supply to a diver that is intended for use in a failure of primary, and where applicable, secondary, breathing gas supply systems. More than one emergency gas supply may be available, of which at least one is usually carried by the diver (scuba).
emergency position indicating radio beacon
emergency position indicating radiobeacon

Also: "emergency beacon" or "EPIRB"

Main article: Emergency position-indicating radiobeacon station

Tracking transmitters which aid in the detection and location of boats and people in distress at sea.
emergency swimming ascent

Also: "ESA", "Controlled emergency swimming ascent", "CESA"

See: Emergency swimming ascent

Emergency procedure where the diver makes an ascent at approximately neutral buoyancy from depth after a breathing gas supply failure.
Using a diving suit which completely isolates the diver from direct contact with the environment.[58]

See: Equivalent Narcotic Depth

Enriched Air Nitrox

See: nitrox


See: emergency position indicating radio beacon

Balance pressure of a gas filled space with the ambient pressure, by adding or venting gas, to prevent barotrauma or pressure damage.
equivalent air depth

Also: "EAD"

Main article: Equivalent air depth

Depth at which partial pressure of nitrogen in a nitrox mixture at a given depth is equal to the partial pressure of nitrogen in air. Used for approximating the decompression requirements of nitrox mixtures by finding the depth at which air would require the same decompression. Based on exposure to equal partial pressures of nitrogen at maximum depth.[11]
equivalent narcotic depth
equivalent nitrogen depth

Also: "END"

Main article: Equivalent narcotic depth

A way of expressing the narcotic effect of a breathing gas mixture at depth by comparison with the depth at which air would have a similar effect. Used to choose nitrogen content of a Trimix breathing gas for a planned dive profile to limit nitrogen narcosis. Based on limiting the partial pressure of nitrogen during the dive by diluting the breathing gas with helium.
Electronic semi-closed circuit rebreather, where an electronic control system monitors the partial pressure of oxygen and adds gas to maintain a floating setpoint to optimise gas use and compensate for variations in diver exertion.[28]
European Diving Technology Committee

Main article: European Diving Technology Committee

International committee of European representatives promoting good standards for diving and co-ordinating, where possible, differing standards with the aim of making European professional diving safer.[20][59]
exceptional exposure
A dive in which the risk of decompresssion sickness, oxygen toxicity, and/or exposure to the elements is substantially greater than on a normal working dive.[60]
excursion dive
Saturation diving where the divers work at a depth deeper or shallower than the saturation depth, after which they are returned to the original saturation pressure.[20]
excursion tables
Tables for use in saturation diving that specify the limit upward and downward of excursions from the nominal depth, and provide a zone in which the diver can move freely without regard to the number of excursions or their duration without incurring a decompression penalty.[13]
excursion umbilical

See: Umbilical cable#Diver

The combined supply and return hoses and cables for life-line, life-support, heating, power and communications between a diving bell and the diver
exhaust valve
A valve controlling the venting of gas from any higher pressure source such as a diving chamber, diver’s helmet, dry suit, buoyancy system, volume tank, lift bag etc. For some of these applications, also known as a dump valve.[13]
extraction ratio

Also: "ventilation/oxygen extraction ratio"

Ratio between minute ventilation and oxygen uptake, the volume rate of gas breathed to the amount of oxygen taken up in the bloodstream. A typical surface extraction ratio of 20 would mean that for every 20 litres of gas breathed, 1 litre of oxygen would be absorbed in the lungs.


Viewport of a full-face mask or helmet. The transparent window through which the diver can see the surroundings.
failure modes and effects analysis

Also: "FMEA"

See: Failure mode and effects analysis

A methodology used to identify potential failure modes, determine their effects and identify actions to mitigate the potential failures.[13]
failure modes, effects and criticality analysis

Also: "FMECA"

See: Failure mode, effects and criticality analysis

An extension to FMEA of a criticality analysis, which combines the probability of failure modes with the severity of their consequences to identify relative risk of each mode, allowing remedial effort to be directed where it is likely to produce the greatest effect.[13]
farmer john
Wet suit that covers the torso and legs only; it resembles a bib overall or salopettes.
fatigue cracking

Main article: Fatigue (material)

Cracking in a material resulting from multiple stress cycles below the ultimate or yield strength. Usually refers to large number of cycles.
feather breathing

See: Sidemount diving#Skills

Technique for emergency breathing from a free-flowing demand valve where the diver manually controls air flow by opening and closing the cylinder valve.
feet sea water

See: feet sea water

Unit of pressure equal to 1/33 atm. Not a linear measure of depth. Generally defined as the pressure exerted by a foot depth of seawater having a specific gravity of 1.027 and is approximately equal to 0.445 pounds per square inch.[13]

See: full-face mask


See: Feet fresh water

Feet fresh water. Unit of pressure equal to 1/34 atm. Not a linear measure of depth.
filling ratio
Ratio of the mass of gas in a cylinder to the internal volume of the cylinder (water capacity), usually expressed in kilograms per litre, or pounds per cubic foot.[29]
filling whip

See: Diving air compressor#Filling diving cylinders and Diving cylinder#Filling

High pressure flexible hose used to connect a cylinder to the storage cylinder, filling panel, booster or compressor, through which high pressure gas flows to fill the cylinder.

Main article: Filtration

Process for removing impurities from a fluid. Particulates are commonly removed by passing the fluid through porous material with pore size small enough to trap the particles (e.g. micron filters). Liquids and gases are commonly absorbed or adsorbed by the surface of the filter medium (e.g. activated carbon, molecular sieve, silica gel), or may be chemically combined with the medium (e.g. Sodalime) or catalytically converted (e.g. Hopcalite) into a less objectionable substance.
fin keepers
fin retainers

Also: "fin holders", "fin fasteners", "fin grips", "fin keeps", "fin guards", "flipper fixers", "Y-straps", "ankle straps", "accessory safety straps", "fix fins", "grip fins" and, in recognition of their French origin, "fixe-palmes".

See: Swimfin#Attachment

Y-shaped elastic rubber straps worn over the arch, the heel and the instep of each foot to help prevent swim-fins from falling off the diver's feet. Mainly used with full-foot-pocket fins.
first stage

See: Diving regulator#First stage

Diving regulator component which reduces gas pressure from storage pressure in the cylinder to interstage pressure for supply to the second stage and for suit and buoyancy compensator inflation.
fixed diving system
A diving system (q.v.) installed permanently on a vessel or structure.[13]
floating setpoint

See: dynamic setpoint

flood-up valve

Also: bell flooding valve[61]

A valve in a diving bell which allows air to escape and internal water level to rise. This can be useful to assist the bellman in recovering an incapacitated diver through the bottom hatch.[62][63]
Movement of water through a cave or ducting system. Similar in meaning to current in open water.[7]
fluorocarbon elastomers

Main: Fluoroelastomer

Synthetic elastomers (rubber) with good performance in high partial pressures of oxygen. Preferred material for o-rings in diving regulators for oxygen service.

See: diluent flush

flutter kick

See: Finning techniques#Flutter kick

finning style where the fins are alternately moved up and down by movements of the full leg.[7] Thrust is developed on both up and down strokes. Vortices shed move both upwards and downwards. See also modified flutter kick.

See: failure modes and effects analysis


See: failure modes, effects and criticality analysis


Main article: Anti-fog

Condensation of water vapour on the inside surface of a mask or helmet faceplate, reducing visibility.
forward roll entry

See: Scuba skills#Entries

Water entry technique used by scuba divers from a boat or platform too high or unsuitable for backward roll entry. The diver bends forward at the hips and waist and falls forward into the water, making a partial somersault and breaking the water with the cylinder, back and shoulders. Not suitable for heights more than about 2 m, and can be problematic if the diver is carrying several heavy items like stage cylinders or large cameras.
four-wire system
Voice communications using separate wire pairs for each direction.
Transportable assemblies of gas cylinders connected by a manifold and securely mounted to a structural framework.[4] See also quad and kelly.
free air

See also: free gas

Air at normal atmospheric pressure.

Main article: Free-diving

Underwater diving that does not involve the use of external breathing apparatus, but relies on a diver's ability to hold their breath until resurfacing. See also breath-hold diving, and apnea (q.v.)
1.  Constant flow rate air supply
2.  See: Mechanism of diving regulators#Free-flow
Malfunction of a demand regulator where the valve sticks in the open position, causing a continuous flow.
free-flow helmet

See: Diving helmet#Free-flow helmets

A helmet where the breathing air supply is supplied at an adjustable, but approximately constant rate regardless of the diver's instantaneous breathing rate.
free-flow valve
Valve on the side of a demand helmet or full-face mask which opens a continuous flow of breathing gas into the helmet interior, usually directed over the interior of the viewport, hence alternative term defogging valve, as it is often used to blow condensation from the inside of the viewport.
free gas
Gas at normal atmospheric pressure. Usually refers to the volume of some amount of compressed gas when allowed to expand to atmospheric pressure at constant temperature
free gas volume
Equivalent volume of a compressed gas if expanded to standard atmospheric pressure at constant temperature.[64]
Frenzel maneuver

Main article: Ear clearing

Technique for equalising the middle ear by pinching the nose closed and moving the back of the tongue upwards.
frog kick

Main article: Frog kick

Finning technique where thrust is developed by sweeping the fins horizontally toward each other with the fins twisted into a nearly vertical plane, with the soles facing each other, followed by a recovery stroke which develops negligible thrust where the fin blades are feathered. The legs are fairly straight during the power stroke. See also: modified frog kick.

Main article: Frogman

A scuba diver, particularly a military diver on an undercover mission.

See: feet sea water

full duplex

See: Duplex (telecommunications)#Full duplex

Voice communication system where both users can transmit and receive at the same time. Compare with half duplex
full-face mask

See: Full-face diving mask

Diving mask covering the eyes, nose and mouth, and which provides the diver with breathing gas



See also: jump

The space between two cave guidelines. Usually between a main guideline and the start tie-off of a branch line.
gap spool

See: Distance line#Spools

A relatively short length of cave line on a spool used to bridge a gap between lines when making an excursion from the main guideline to a branch guideline. The line is left in place during the excursion, and usually retrieved on the way out

Also: "gator" (US)

Textile legging wrapped around the calf and ankle area over a dry suit to restrict the amount of air that can get into the lower leg area. Also can reduce drag of the suit in this area by smoothing over creases and folds.
gas blender
1.  Person who mixes breathing gases for diving, filling diving cylinders with gas mixes such as nitrox or trimix.
2.  Certification of competence to mix breathing gases for diving.[65]
gas blending

Main article: Gas blending for scuba diving

Mixing breathing gases for diving, filling diving cylinders with gas mixes such as nitrox or trimix.
gas block

See: gas switching block

gas embolism

Main article: Gas embolism

Blockage of a blood vessel by a bubble of gas.
gas extender
Carbon dioxide scrubber used to allow partial recirculation of surface-supplied or scuba breathing gas to reduce waste. A form of semi-closed circuit rebreather.
gaseous impurities
Contaminants in the compressed breathing air or gas mixture which are in gaseous form. Compare with particulate and condensate impurities.
gas fraction

Main article: Gas composition

The fraction by molecular count, volume or pressure (they all come to the same thing) of a specific gas in a mixture of gases.
gas matching

See: Scuba gas planning#Gas matching

The calculation of reserve and turn pressures for divers using different cylinder volumes on the same dive, allowing each diver to ensure that sufficient gas is retained at all times to allow for foreseeable contingencies based on each diver's cylinder volumes, and both divers' individual gas consumption rates.[66]
gas narcosis

See: inert gas narcosis

gas panel

See: Surface-supplied diving#Gas panel

The control equipment for providing breathing gas to surface supplied divers via umbilicals. Primary and reserve gas is supplied to the panel through shutoff valves from low pressure compressors or high pressure storage cylinders.
gas reversal point
The depth during an ascent or decompression when the intake of dissolved gas is exceeded by outgassing.
gas sensor
A component that produces a signal in the presence of a specific gas, often in proportion to the concentration of the gas.
gas switching

See: Gas switching

The procedure of changing from one breathing gas mixture to another during a dive. This may be done to avoid oxygen toxicity, hypoxia, or nitrogen narcosis, to accelerate decompression, or to avoid running out of breathing gas. Generally applied to open circuit breathing equipment, where a physical change-over of gas source is made. In closed circuit systems the gas composition is continuously controlled to follow the chosen set-point.
gas switching block
A valve set used for switching from one breathing gas to another during a dive. The most common application is a bailout block, but also used to switch gases for decompression on a full face mask, and to switch between deep and shallow gases on a semi-closed rebreather.
gauge mode

See: Dive computer#Additional functionality and features, Dive computer#Management of violations

Operating mode for a personal dive computer where the decompression calculation is disabled, and the unit operated only as a timer and depth gauge. Typically used when diving with gas mixtures not supported by the algorithm, in which case decompression tables are used to monitor and control the decompression schedule. Some dive computers will automatically switch to gauge mode if the diver violates a depth limit, leaving the diver without decompression information.
gauge pressure

Main article: Pressure measurement#Absolute, gauge and differential pressures - zero reference

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure
gauge snubber
A needle valve or small bore orifice between the pressure supply and the gauge which damps pressure fluctuations.
Gay-Lussac's law

Main article: Gay-Lussac's law

Relation between temperature and pressure in an ideal gas for a constant volume.
general gas equation
general gas law
1.  Relation between the variables pressure, volume and temperature for a given mass of a given mixture of an ideal gas.
2.  Thermodynamic equation of state for gases for the variables volume, pressure, temperature and number and atomic weight of molecules.
giant stride entry

See: stride entry

glossopharangeal insufflation

Also: "buccal pumping", "lung packing"

A method used by freedivers for filling the lungs with more air than maximal inspiration to normal total lung capacity (TLC). After a full inhalation, the diver fills the mouth with air, while the glottis remains closed, then opens the glottis and forces this air into the lung using the cheeks and tongue to reduce the mouth volume. This may be repeated several times.[67]
A single-use, translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence
A general purpose adjective to denote a particularly difficult section of a cave, which may be low, tight, silty, etc. or a combination.[7]
go into decompression
Incur a decompression obligation. Generally refers to having a theoretical tissue inert gas concentration that requires the diver to make staged decompression stops during ascent to avoid an unacceptable risk of symptomatic decompression sickness according to the decompression model, algorithm, tables or dive computer in use.
golden rule
The convention in cave diving that anyone can turn the dive at any time for any reason.[7]
gold line
The permanent main guideline in a cave system, that usually starts well inside the cave. Often yellow or gold in colour.[7]
Goodman handle
A handle used to carry the primary dive light head comprising a rigid slot through which the fingers and palm of the hand are extended, so that the light rests on the back of the hand, facing the direction of the extended fingers.

Main article: Global Positioning System

A satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites
gradient factor

See: Decompression theory#Gradient factors

A way of modifying the M-values of a decompression algorithm to more conservative values in proportion to depth. Often used to bias the algorithm towards deeper stops by using a smaller value for the deeper value.
guide line

See: distance line



Main article: Underwater habitat

An underwater structure inside which divers can carry out dry welding or which is fitted out with life support facilities.[13]

See: Decompression theory#Critical ratio hypothesis

Decompression models based on the principles described by John Scott Haldane.
An alternative reserve calculation strategy to the rule of thirds for breathing stage cylinders. The cylinder is breathed down to half of its starting pressure plus 200 psi, so that a stage cylinder filled to 3000 psi would be breathed down to 1700 psi before being dropped.[7] This is a less conservative strategy for pressures greater than 1200 psi.
half duplex

See: Duplex (telecommunications)#Half duplex

Voice communications system where users take turns to send and receive.
half mask

Main article: Diving mask

Diver's mask which covers the eyes and nose but not the mouth.
half times

See: tissue half times


Main article: Halocline

A strong variation in salinity over a small depth range within a body of water. Often visible as a blurred or shimmering region due to uneven refractive index.[26]
hand-off cylinder
A diving cylinder, complete with regulator, which can be handed off to another diver in an emergency, so that the two divers are not obliged to remain in close proximity during the exit and ascent. Transfer of a hand-off cylinder should not compromise either diver's buoyancy to the extent that they cannot make a normal, controlled ascent at neutral buoyancy.
To remain stationary at a specific depth and location, particularly when decompressing.[7]
hard hat diving
Surface supplied diving, generally in professional diving, either wearing a modern diving helmet or the old-style standard diving dress with copper helmet.
hardwire communications
Voice communications using a cable for transmission.
Straps and webbing with associated buckles, D-rings and other accessories used to support the breathing apparatus and secure it to the diver. The harness often has other functions such as supporting weighting and buoyancy control systems and for recovery of the diver from the water. In professional diving the harness is used as a strong point to attach the lifeline or umbilical to the diver.[13]
Commercial diver term for diving helmet.
Hazard identification study: A systematic qualitative assessment of potential hazards and threats to health, safety, equipment, property, environment, production, or reputation. May be followed by a risk assessment.
hazmat diving

Main article: Hazmat diving

Diving in a known hazardous materials environment. The environment may be contaminated by hazardous materials, the diving medium may be inherently a hazardous material, or the environment in which the diving medium is situated may include hazardous materials with a significant risk of exposure to these materials to members of the diving team. Special precautions, equipment and procedures are associated with hazmat diving.
head-up display

Also: "HUD"

See: Head-up display

A visual display mounted where it is normally in the diver's field of vision.[28]
Health and Safety Executive

Also: "HSE"

Main article: Health and Safety Executive

UK government department responsible for occupational diver safety in UK.
heavy gear

See: standard diving dress


Main article Trimix (breathing gas)

Trimix blends made by topping up helium with air.[15]
helicopter turn
Manoeuver in which a horizontally trimmed diver uses small fin movements to rotate on the spot on a vertical axis.

Main article: Heliox

Mixtures of helium and oxygen for use as a breathing gas.[65][15]

Main article: Helium

An inert gas which is used as a component of breathing gas mixtures for deep diving.
helium analyzer

Main article: Helium analyzer

An instrument used to identify the presence and concentration of helium in a mixture of gases
helium unscrambler

See: speech unscrambler

helmet diving

See: hard hat diving

Henry's law
Description of the changes in solubility of a given gas in a given liquid as pressure varies.
HID light
High intensity discharge light: Used in cave diving light heads (q.v.).
Hazard Identification and Risk Analysis: A risk management procedure for identifying hazards and assessing the risk associated with them and ways to reduce the risk to an acceptable level.
Hogarthian configuration
A scuba combination of backplate, wing, one-piece harness with crotch-strap, regulator arrangement including long-hose primary with a necklaced secondary demand valve, and, if used with twin cylinders, an isolation manifold. Named after William Hogarth Main, a developer and proponent of the system.[26]
Hog looped
A scuba configuration where the primary demand valve has a long hose which is routed under the right arm, usually tucked under a light battery canister on the waist belt of the harness, and around behind the neck to reach the mouth from the right hand side. Part of the Hogarthian configuration (q.v.).[26]
hold-back line
hold-back rigging
Rigging provided to restrain excessive buoyancy of a lifting bag when it is attached to the load and inflated.[13]
Amateur blended mixed gas.[26]
Close-fitting thermal head protection, usually neoprene foam, but also latex on some dry suits.
hook breathing

See: recovery breathing


See: airline diving


Main article: Hopcalite

Catalyst sometimes used in breathing air compressor filters to oxidise carbon monoxide to carbon dioxide. Hopcalite is a mixture of manganese oxide, copper oxide and a small amount of silver oxide.
horse collar

See: adjustable buoyancy life jacket[26]

hose dash size
Number indicating the bore of a hose in multiples of 1/16" preceded by a dash. for example a –04 hose, typically used for a pneumofathometer hose would have a nominal bore of 14 inch (6.4 mm).
hose organiser
Equipment to position and stabilise a hose, particularly in the context of sidemount or sling mounted scuba cylinders, where the regulator air hoses are stored against the cylinder when not in use.
hot mix
A breathing gas mixture with a high percentage of oxygen for the depth at which it is used. Using a gas mixture with a high partial pressure of oxygen.
hot water suit

See: Diving suit#Hot water suits

A loose fitting wetsuit supplied with heated water through a hose in the diver's umbilical, which links the diver to the surface support. The diver controls the flow rate of the water, allowing them to vary the warmth of the suit.
High pressure, generally gas pressures in excess of 30 bar. In most of the world a high pressure diving cylinder is a 300 bar cylinder, but in the US it may refer to anything over 3000 psi working pressure. In a diving context gas working pressures do not frequently exceed 300 bar, but pressures in hydraulic systems and high pressure water jetting equipment may be considerably higher.
high-pressure nervous syndrome

Main article: High-pressure nervous syndrome

A neurological and physiological diving disorder that occurs when a diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture.
1.  See: hyperbaric rescue capsule
2.  See: hyperbaric rescue chamber

See: hyperbaric rescue unit


See: Health and Safety Executive


See: Head-up display


See: Scuba cylinder valve#Dual outlet valves

Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to Y-valve but in configuration where the second valve is parallel to the primary, though the secondary valve can sometimes be swivelled.
Slang/abbreviation for hydrostatic test (q.v.)[26]
hydrophobic membrane
A membrane that is freely permeable to gas but does not allow the passage of water at low pressure differentials.[28]
hydrostatic pressure

Main article: Hydrostatic pressure

Pressure due to the weight of the water column above a point at depth.
hydrostatic test

Main article: Hydrostatic test

Non-destructive test to revalidate pressure vessels which uses water as a test medium. The vessel is pressurised to the test pressure (q.v.) and measured for permanent set.

See: Hydreliox

Deep diving breathing gas mixture of hydrogen, helium and oxygen.

Main article: Hydrox (breathing gas)

Deep diving breathing gas mixture of hydrogen and oxygen.
hyperbaric evacuation
Evacuation of divers under pressure from a saturation chamber to a hyperbaric rescue craft via a pressurised transfer system.[13]
hyperbaric evacuation system
System for evacuating divers under pressure from a saturation system in an emergency.[20] The whole system set up to provide hyperbaric evacuation, including planning, procedures, equipment used for evacuation, reception facility, contingency plans, possible safe havens and anything else requited for a successful hyperbaric evacuation.[13]
hyperbaric lifeboat
A lifeboat with a hyperbaric chamber and life support system built into it for evacuating saturation divers in an emergency.
hyperbaric rescue capsule
A self contained buoyant hyperbaric chamber intended for emergency evacuation of saturation divers under pressure from a platform which has become so dangerous that it is considered safer to put the divers into the sea in the HRC to be picked up by a rescue vessel for transfer to another hyperbaric system for decompression.
hyperbaric rescue chamber
Normally a pressure vessel adapted to function as a means of hyperbaric evacuation with buoyancy chambers and lifting points, but not fitted in a lifeboat hull.[13]
hyperbaric evacuation unit
hyperbaric rescue unit

Also: "HEU", "HRU"

Generic term for the unit provided to evacuate divers from a saturation system. It may be an HRC or a SPHL or some other suitable pressure vessel.[13]

Main article: Hypercapnia

Excessive level of carbon dioxide in the body.

See: Gas blending for scuba diving#

Additional filter to produce air of oxygen compatible quality, usually for partial pressure gas blending. This may be built into the filling system or temporarily connected when required.
hyperoxic linearity
Property of an oxygen sensor to produce linear output at partial pressures above the highest calibration point.[28]
1.  A deliberate deep breathing to reduce blood carbon dioxide level to extend the duration of a free dive.
2.  Rapid breathing as the body's response to hypercapnia.
3.  Rapid, often shallow breathing, associated with panic.
hyperventilation-induced blackout
See: underwater blackout syndrome

Main article: Hypocapnia

Abnormally low tissue and blood carbon dioxide concentration.

Main article: Hypothermia

A lowering of core body temperature, usually due to heat loss.

Main article: Hypoxia (medical)

Abnormally low tissue oxygen concentration: Insufficient oxygen in the body to support normal activities or consciousness.
hypoxic training
prolonged underwater distance swimming or extended breath-hold intervals.[68]



See: International Association of Oil and Gas Producers


See: isobaric counterdiffusion

International Diving Regulators and Certifiers Forum, previously International Diving Regulators Forum. A voluntary group of diving regulators and certifying agencies formed to work together toward mutual recognition and to identify and implement best practices in diver training with the object of harmonizing cross border diver training standards outside Europe. Members include Australia, Canada, France, Norway, South Africa and United Kingdom.[69]
International Diving Schools Association: Formed with the primary purpose of developing common internationional diving standards for all occupational divers, Offshore, Inshore and Inland, and specialist related non-diving qualifications e.g. Supervisor, DMT and LST.[70]

See: International Marine Contractors Association.

International trade association for the marine contracting industry.
inert gas

See: Decompression theory#Inert gas uptake (Ingassing)

A gas which is not metabolically active, used to dilute the breathing gas.

See: Decompression theory#Inert gas uptake (Ingassing)

Inert gas uptake in body tissues during a dive or other hyperbaric exposure.
inherent unsaturation

See: Decompression theory#Inherent unsaturation

Metabolic reduction of total gas pressure in the tissues.
inshore diver
Colloquial term for a diver who works on inland dive sites or coastal waters not associated with the oil and gas industry. Also referred to as "civils" as much of this work is connected with civil engineering works.[4]
integrated weight system
Any system for carrying dive weights on the buoyancy compensator or diving safety harness, avoiding the use of a separate weight harness or weightbelt.
internal condition of cylinder
The state of the internal surface of a cylinder regarding corrosion, contamination and cracking.
interstitial emphysema
Gas trapped in the spaces between organs after lung barotrauma.
International Association of Oil and Gas Producers

See: International Association of Oil and Gas Producers

The international forum of the oil and gas producing industry.
inverter line

See: tripping line

in-water recompression

See: In-water recompression, Hyperbaric treatment schedules#In-water recompression schedules

Recompression of a diver by returning to a specified depth in the water, followed by decompression on a specified gas, commonly oxygen, as treatment for decompression sickness or as prophylaxis for incomplete or missed decompression after a dive.
Intermediate Pressure, or Interstage Pressure. The reduced pressure between the first and second stages of a diving regulator. Also referred to as LP (Low Pressure) in this context.
isobaric blackout

See: constant depth blackout

isobaric counterdiffusion

See: Isobaric counterdiffusion

The diffusion of gases in opposite directions caused by a change in the composition of the external ambient gas or breathing gas without change in the ambient pressure.
isolation manifold
Connection between two scuba cylinders which when open allows free flow of gas in both directions between the cylinders, but has an isolation valve to block this flow.
isolation valve
1.  valve in an isolation manifold (q.v.) used to close the gas passage through the manifold and isolate the contents of the two cylinders. Used to prevent a leak on one cylinder from causing the other cylinder to also lose gas.



See: Jackstay#Diving jackstay

A line secured at both ends to serve as a support[71] or guide.

See:Underwater searches#Jackstay search

Underwater search techniques using one or more jackstays to guide the searcher.
jack-up rig

Main article: Jackup rig

A type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of sea. The buoyant hull enables transportation of the unit and all attached machinery to a desired location. Once on location the hull is raised to the required elevation above the sea surface on its legs supported by the sea bed.
Bulk gas storage cylinder with internal volume 50 litres.
Jersey upline

See: upline

JIC fitting

Main article: JIC fitting

A type of pipe and hose fitting with a 37-degree flare seating surface. Commonly used in umbilical hose connections.
JIM suit

Main article: JIM suit

An atmospheric diving suit manufactured by Underwater Marine Equipment Limited.
jocking strap
webbing strap system used with diving helmets to hold the helmet assembly down on the diver to prevent buoyancy lifting it when underwater.

Main article: Jonline

A short line used to connect to a shotline or anchor line, allowing the diver to move a short horizontal distance away to decompress. The line helps compensate for vertical movement in the anchor line or shot line due to waves.[72]
job safety analysis

Main article: Job safety analysis

A procedure to integrate health and safety principles and practices into a particular task or job. Each basic step of the job analysed to identify potential hazards and controls for each hazard.

See: Underwater searches#Variations on the jackstay search

A variation on the movable jackstay search suitable for a single diver.
A path from a main guideline to another which is not in contact
jump camera
A camera mounted on a frame, which when lowered to the bottom of a body of water, takes a photograph, usually of the bottom under the camera. The frame constrains the camera to a fixed camera to subject distance, resulting in photographs of uniformly sized areas of bottom, equivalent to quadrats.
Jump jacket
A harness with integral buoyancy jacket specifically designed for commercial diving work with helmets and bells.[73]
jump line

See: Cave diving

A short cave line used to connect between two permanent lines that are not in contact. May also be used to search for the other end of a break in a cave line and repair the break
jump reel
jump spool
A reel or spool with a relatively short line intended to be used as a jump line (q.v.).

See: Scuba cylinder valve#Reserve valves

Scuba cylinder valve with lever operated reserve mechanism.[74]


kayak diving

Main article: Kayak diving

Diving from a special purpose kayak used to get to the site where the distance from a suitable entry and exit point is inconvenient for shore diving.
Bulk high pressure gas storage cylinder size designation (approximately 50 litres internal volume)
Kelly tube

See: tube


See: Scuba cylinder valve#Plain valves

Scuba cylinder valve without reserve mechanism.



Main article: Lanyard

A piece of cordage used to secure or lower things; usually it is used where there is a risk of losing the object

See: Launch and Recovery System


Main article: Laryngospasm

An uncontrolled or involuntary muscular contraction (spasm) of the laryngeal cords which causes a partial blocking of breathing in, while breathing out remains easier.
latent hypoxia
While freediving, an arterial oxygen partial pressure which is sufficient to sustain consciousness at depth, but when ascending drops to hyoxic levels due to the reduction of ambient pressure, associated with ascent blackout.
launch and recovery system
Mechanised system for lowering a diving bell, diving stage, submersible or ROV from a vessel, offshore platform, dockside or other platform, and lifting it back on board.
lay barge

Also: pipelay barge

See: Pipe-laying ship and Submarine pipeline

Barge on which pipeline sections are assembled, welded and laid on the seabed as the barge is moved forward.[4]
lay line
laying line

See: Cave diving#Line management

To run line (unreel under light tension while advancing) and place it
lazy shot

See: Diving shot#Lazy shot

A shot line (q.v.) which does not reach the bottom
Local dive shop. A retailer of recreational diving equipment, which may also sell cylinder fills, service equipment, provide training or book or organise recreational dives for customers.

See: Light emitting diode

A semiconductor device for producing light from an electrical current commonly used in dive lights.
lens port
Transparent window on an underwater camera housing through which light reaches the lens.

See: low impact diving


See: Lifeline (diving)

A line connected securely to the diver at one end and anchored at the other end at the diving control point, which is handled by a line tender, and is used to communicate with the diver and provide a means of finding the diver for a surface standby diver, and for assisting the diver to the surface and back to the control point if necessary.
life support
life support system

See: Life-support system#Underwater and saturation diving habitats

Equipment vital to the short term survival of the diver. Most notably the breathing gas supply, and for saturation diving, equipment for providing a correctly pressurised environment. In some cases thermoregulatory equipment is also considered life support, and in saturation diving, all of the peripheral systems essential to maintaining a habitable saturation system.
life support package (LSP)

Also: "fly-away package"

A collection of equipment and supplies kept in a suitable location such that when a hyperbaric rescue chamber or self-propelled hyperbaric lifeboat arrives at the safe haven it is available to support or complete decompression by using the LSP components to maintain the decompression environment by way of power, gas mixtures, heating and cooling.[13]
life support supervisor
A senior life support technician (q.v.) appointed by the diving contractor to supervise the operation of saturation life-support systems.[20]
life support technician

Also: "LST"

A person who operates and maintains the life support systems of a saturation diving system.
lift bag
lifting bag

Main article: Lifting bag

A robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy when filled with air.
Alternative term for a viewport of a standard diving helmet.
light head
The part of a canister light (q.v.) which emits light, and is held in the hand or mounted on the helmet.
lightweight helmet

Main article: Diving helmet#Lightweight demand helmets

Low volume, close fitting diving helmet, usually with demand valve. Neutrally buoyant, and moves with the diver's head.
line marker
Line arrows, cookies and sometimes clothes pegs. Tags used to indicate the direction to an exit, midway point between exits, jumps and personal markers to identify divers on a guide line in an overhead environment.
line tender

See: diver's attendant

line trap
Narrow gap where the guide line passes but divers can not get through.[7]
lipid pneumonia

Main article: Lipid pneumonia

A lung inflammation (pneumonia) that develops when lipids enter the bronchial tree

Main article: Liveaboard

A large boat which provides transport, accommodation and services for vacationing recreational divers
1.  Diving from a boat which is under way (not moored). The major implication is that the engines will be running and propellers or thrusters may be engaged while divers are in the water, a significant hazard.
2.  (offshore diving) Diving from a vessel underway, making way, but not using dynamic positioning. In offshore diving, diving from a DPV is not considered live-boating.[13]

Also: "air-lock", "airlock" and "air lock"

Main article Airlock

Compartment of a hyperbaric habitat or chamber which can be entered through two or more openings which can be closed and sealed, and the pressure adjusted relative to the adjacent chambers. Used to transfer personnel or equipment between areas of different pressure.
Enter a pressurised environment through an airlock
Disconnect a mobile hyperbaric chamber from another hyperbaric chamber after sealing the doors between them and venting the connection space.
lock-off time
The time at which a diving bell under pressure is locked off (disconnected) from the compression chamber(s) on deck.[13]
Connect a mobile hyperbaric chamber to another hyperbaric chamber so that the pressures can be equalised to allow transfer under pressure between them.
Lock-on time
The time at which a diving bell under pressure is locked on (reconnected) ready for equalisation to the compression chamber(s) on deck.[13]
Exit from a pressurised environment through an air-lock

See: Lockout–tagout

Safety procedure when working on a hazardous energy source.[75]
lockout mode

See: Dive computer#Lockout mode

Factory set function of some dive computers which disables the computer from decompression calculation after a violation of a factory set limit for depth, decompression ceiling or other violation of approved operating range for a period sufficient for tissues to fully desaturate if the diver survives uninjured. Some models will lockout immediately, usually to gauge mode which provides depth and time data but no decompression information, leaving the diver without some safety-critical information, others will continue to provide the diver with best estimate decompression information until the end of the dive.
log book
Record of dives kept as proof of experience. Optional for recreational divers, but legally required for professional divers in many jurisdictions.
long hose
5 ft to 7 ft interstage hose used on one of the regulators used by cave and other technical divers, which allows gas sharing through narrow spaces where the divers must pass through in single file.
longshore current

Main article: Longshore current

Mass transport of water along a shoreline, often due to wave action at an angle to the shoreline.

Also: "breathing loop"

Main article Rebreather#Concentric bellows counterlungs

The circuit in a rebreather through which the breathing gas passes during a breathing cycle.
loop mix
loop mixture
1.  The breathing gas in the loop of a rebreather.
2.  Composition of the breathing gas in the loop of a rebreather, usually after passing through the scrubber, where it is analysed for oxygen partial pressure.
loop volume
Volume of the breathing loop of a rebreather.
loop vent valve
An over-pressure relief valve in a rebreather loop.[28]
lost buddy drill
standardised procedure followed when a diver realises that their buddy is not where they should be. Procedures may vary depending on the circumstances and training organisations.
lost line drill
Standardised procedure to be followed when the guideline to the surface is lost in a penetration dive, often in conditions of low visibility and darkness.
low impact diving

Main article: Low impact diving

Diving with low environmental impact. Diving in a way that avoids contact with or disturbance of sensitive organisms and adversely affecting the environment. Usually applied to recreational diving.
low pressure
1.  Also: "intermediate pressure"
In diving, low pressure usually refers to the gas pressure provided from the first stage regulator to the demand valve for reduction to ambient pressure, the pressure provided by a low-pressure breathing air compressor to the gas panel for supply to the diver's umbilical or airline, or the pressure supplied to buoyancy compensator, dry suit or supply for lift bag inflation gas or pneumatic tools. These pressures are generally less than 30 bar, and in the case of scuba regulators are usually about 9 to 11 bar above ambient pressure.
2.  In the context of diving cylinders, in the US, a low pressure cylinder has a working pressure below 2,500 pounds per square inch (170 bar)
low visibility
low viz

See: Underwater vision#Low visibility

Water where, regardless of illumination, the distance over which objects can be seen is small. It is a term with highly variable and often relative meaning, but it would be almost universally accepted that less than 0.5 metres (1.6 ft) visibility would be considered low visibility. For operational purposes NOAA define it as "When visual contact with the dive buddy can no longer be maintained", and DAN Southern Africa suggest less than 3 metres (9.8 ft).[76][77]

See: low pressure

LP compressor
Low-pressure compressor. Used for breathing air supply for surface supplied air diving.
LP cylinder
Low-pressure cylinder (US) with working pressure less than 2,500 pounds per square inch (170 bar).
LP port
Opening on the first stage of a regulator through which regulated gas is supplied.

See: life support technician

lung packing

See: glossopharangeal insufflation


mammalian diving reflex

Also: "diving response"

A reflex response to breathhold and chilling of the face diving response expressed by the cardiovascular system, which exhibits hypertension, bradycardia, oxygen conservation, arrhythmias, and contraction of the spleen.[51]

See: twin cylinder manifold

manifold (breathing gas supply)

Also: "gas panel"

Panel for the distribution of diver breathing gas.[13]
manifold operator

Also: "gas man"

A person such as a life support technician (LST), diving supervisor, or mixed-gas diver, who is designated to perform the duties of gas distribution on a surface-supplied mixed gas (HeO2) diving operation, who is competent in the operation of the manifold and whose primary responsibility is to operate the manifold.[13]
manual bypass valve
A valve that allows the diver to manually inject gas into the breathing loop of a rebreather, bypassing the valve operated by the control system.[28]
manual closed circuit rebreather

Also: "mCCR", "MCCR". See also: diver controlled closed circuit rebreather

See: Rebreather#Closed circuit mixed gas rebreathers

A closed cirduit rebreather which relies on the diver to monitor and control the gas mixture in the loop.
manufacturing standard

Also: "manufacturing code"

Set of design and manufacturing rules intended to produce uniform and safe products by several manufacturers in an industry.
Marsh Marine connector
One of the popular underwater plug connector systems for diver communications cables.[78]
Martini's law
Rough rule of thumb for estimating nitrogen narcosis effects based on equivalence to consumption of dry martinis: Variously quoted as one martini per 10 m or one martini per 50 ft depth.
(Rigging): The large heavy duty link to which the legs of a chain sling are attached, and which is the attachment point on the sling for the lifting hook or shackle
maze cave
Cave structure characterised by multiple branches and changes in direction.

See: manual closed circuit rebreather

metres sea water

Main article: metres sea water

Unit of pressure equal to 1/10 bar. Not a linear measure of depth.[79]
Microscopic bubbles which are not detectable by ultrasound or Doppler ultrasound, yet can affect the likelihood of DCS by slowing off-gassing.[6]
Microscopic cavities that function as bubble seeds by absorbing dissolved gas.[6]
Significantly distant from both the bottom and the surface.
medical lock
Small lock on a decompression chamber used for transfer of medical equipment and other supplies into and out of the chamber while the chamber remains under pressure.
Short for trimix breathing gas
mixed gas
Breathing gas for diving other than air, but usually implies a helium based mixture.
mixed mode diving
A mixed mode dive team is a buddy team where the divers use different modes of diving on the same dive, such as one diver on open circuit and the other on rebreather[80]
mixed platform diving
Mixed platform rebreather diving refers to the use of different makes or models of rebreather on the same dive.[80]
mobile diving system

Also: "portable diving system"

A diving system (q.v.) which is installed on a vessel or installation on a temporary basis and that is not fixed, i.e. can be demobilised, transported and re-sited. Includes surface supplied air, nitrox, heliox and saturation diving systems.[13]
mobile offshore drilling unit
A generic term for several classes of self-contained floatable or floating drilling rigs such as drilling vessels, semisubmersibles, submersibles, jack-ups, and similar facilities that can be moved without substantial effort. These facilities may have self-propulsion equipment on board and may require dynamic positioning equipment or mooring systems to maintain their position.[81]
Maximum operating depth: Limiting depth for safety based on partial pressure of oxygen of a breathing gas mixture.
modified flutter kick
Version of the flutter kick finning style which reduces risk of silting by directing thrust more directly backwards. Two techniques exist: One version has the legs bent at the knees so that the fins are placed relatively high and on average are aligned more horizontally. The other version has one fin stationary below the moving fin to deflect downwash.[7] Leg movement is restrained, and ankle movement used for precision manoeuvring.
modified frog kick
Version of the frog kick finning style which reduces risk of silting by directing thrust more directly backwards. Performed with bent knees and fins raised above the line of the torso.

See: mobile offshore drilling unit


Main article: Monofin

A type of swimfin typically used in finswimming and free-diving. It consists of a single surface attached to footpockets for both of the diver's feet.
molecular sieve

Main article: Molecular sieve

Material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids. Molecules small enough to pass through the pores are absorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level.
monkey diving

See: Sidemount diving#Terminology

The use of sidemount configuration and procedures with a single cylinder.

Main article: Moonpool

An opening in the floor or base of the hull, platform, or chamber giving access to the water below.

See: metres sea water

multi-level dive

Also: "multilevel dive", "multi level dive"

See: Multi-level dive

A dive profile in which the diver remains in more than one distinct depth ranges (excluding decompression stops) for a significant period before beginning final ascent to the surface.
A brown organic deposit usually found on the ceilings of caves which is easily dislodged by diver's exhaust bubbles and then drifts down through the water.
multiple stage compressor

See: Compressor#Reciprocating compressors

Compressor in which gases are compressed more than once, usually with cooling between stages. Used to improve efficiency and reduce temperatures.
mushroom valve
A rubber non-return valve flap which is circular or oval, with a stem in the middle to attach it to the holder in the centre of the grating over the orifice.

See: Decompression theory#M-values

At a given ambient pressure, the M-value is the maximum theoretical value of absolute inert gas pressure that a tissue compartment can take without presenting symptoms of decompression sickness.


National Association for Cave Diving

See: nitrogen narcosis

Main article: Navy Experimental Diving Unit

The United States Navy Experimental Diving Unit, the primary source of diving and hyperbaric operational guidance for the US Navy, located in Florida

Main article: U.S. Navy Seals

A US Navy trained combat diver.

See: No decompression limit

neck dam
The lower part of a lightweight diving helmet which includes a neoprene or latex neck seal similar to the neck seal on a dry suit, to prevent ingress of water. The alternative is to clamp the helmet to the diver's drysuit.
Bungee loop attached to the secondary regulator second stage, worn around the neck to store the secondary close under the chin, where it is protected and the diver is immediately aware of a free-flow. With a little adjustment and practice it is possible to pick it up by head and mouth movement alone, not needing use of a hand. The bungee is attached to the second stage by a breakaway connection, often a close-fitting loop over the mouthpiece, so that it can be moved away from the diver's head in an emergency without disturbing the primary second stage or the mask.
neck thread

See: Diving cylinder#Cylinder neck

Internal screw thread in the bore of the cylinder neck to fit a cylinder valve, which controls flow of gas into and out of the cylinder.

See: Navy Experimental Diving Unit

negative buoyancy

Also: just "negative" in context

See also diving heavy

Buoyancy less than weight. Insufficient upward force due to buoyancy to keep afloat or remain at constant depth
negative buoyancy entry
negatively buoyant entry
negative entry

See: Scuba skills#Entries and exits

Entry into the water in a buoyancy condition that will sink by default. When intentional, generally after reducing buoyancy of BC and, if applicable, dry suit by venting to ensure that the diver will not float back to the surface, but will continue to descend.

Also: "neo-Haldanean"

Main article Decompression theory#Critical difference hypothesis

Decompression models based on later modifications of the principles described by John Scott Haldane.

Main article: Neoprene

Synthetic elastomer used in the form of foamed sheets as the material for most wetsuits and some drysuits.
neutral buoyancy

See: Buoyancy

Having a fully immersed buoyancy exactly equal to weight, so that the forces are balanced and the person or object statically remains at a constant depth. Effectively average density is equal to that of the surrounding fluid medium. The state of neutral buoyancy is typically metastable for a compressible system.

Main article: Newtsuit

An atmospheric diving suit designed and originally built by Phil Nuytten.
net cutter
A handle with a hooked blade used to cut netting or cordage to free the diver from entanglement
Non-freezing cold injuries: Permanent tissue damage due to low temperature exposure without any freezing damage.

See: Decompression sickness#Signs and symptoms

Minor symptoms characteristic of mild decompression sickness.
night diving

Main article: Night diving

Diving during the hours of darkness.
Nickel-metal hydride. A technology for rechargeable battery cells.

Main article: Nitrile rubber

A synthetic elastomer used for most standard O-ring seals.

Main article: Nitrogen

The major component gas of air and many breathing gas mixtures used in diving. Important in diving as an active agent in nitrogen narcosis and decompression sickness.
nitrogen narcosis

Main article: Nitrogen narcosis

Also known as narcs, inert gas narcosis, raptures of the deep, Martini effect: A reversible alteration in consciousness that occurs while breathing gases containing nitrogen under elevated partial pressure similar to alcohol intoxication or nitrous oxide inhalation, and can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters.

Also: "Enriched Air Nitrox (EAN)"

Main article Nitrox

Mixture of nitrogen and oxygen for use as breathing gas. Usually with oxygen percentage higher than air.
nitrox stick

See: Gas blending for scuba diving#Mixing the gases

A mixing tube used to blend oxygen with air before compressing to make nitrox breathing gas.
National Oceanographic and Atmospheric Administration (US)
no decompression limit

See: Decompression practice#No decompression limit

No decompression limit. The maximum time which a diver can remain at a specified depth without incurring a stage decompression obligation in terms of the specified decompression tables or algorithm.
nominal capacity
Volume of free gas contained by a scuba cylinder when filled to charging pressure at constant temperature.
No-mount diving

See: Side mount diving#Terminology

A specialized overhead-environment strategy for dealing with particularly tight restrictions which may involve divers wearing a very basic harness or simply hand-carrying cylinders.
1.  A breathing gas mixture with oxygen content approximating atmospheric air.
2.  A breathing gas for diving which contains sufficient oxygen to minimise risk of hypoxia at atmospheric pressure.
National Speleological Society

Main article: Nystagmus

An oscillation of the eyes alternating a slow eye movement in one direction, and a fast eye movement in the other direction.


octopus breathing
Sharing air using an octopus regulator
octopus regulator

Also: "octo reg", "safe second"

See: Diving regulator#Octopus

A secondary demand valve fitted to a diving first stage for use as an alternative air source for another diver in case of an emergency. Also occasionally useful as a backup in case of some kinds of malfunction of the primary
Not an integral part of the scuba unit. Usually applied to gas cylinders carried additional to the onboard gas of a rebreather.
offboard gas
Gas carried in cylinders not integrally mounted on a rebreather, but plumbed into the unit.

See: outgassing

1.  A significant distance away to seaward from the shoreline.
2.  Outside the national maritime border, in international waters, though generally still inside the exclusive economic zone.[82]
offshore diver

Main article Commercial offshore diving

Colloquial term for a diver who works in the offshore oil and gas industry.[4] A professional diver who works in regions outside the jurisdiction of national occupational health and safety and labour laws.
Method for installation of sub-sea pipeline for the oil and gas industry. The pipe is constructed in an onshore construction yard, moved into the water and bent into a spiral without causing plastic deformation of the material. When the pipeline is sufficiently long it is transported with the help of tugs to the installation area. At the installation area the pipeline is unwound and pulled over a simple lay barge with stinger and installed on the bottom.[83]
An integral part of the unit. Usually applied to gas cylinders mounted on a rebreather frame or housing, or emergency gas supply cylinders mounted on a diving bell or stage.
onboard gas
Gas carried in cylinders mounted as part of a unit, usually a rebreather or diving bell or stage.

See: ingassing


See: inshore

open circuit
Breathing apparatus which discharges exhaled gas into the environment, without any further use.

Main article: Optode

Optical sensor device to measure a specific substance, usually with the aid of a chemical transducer.
Out of air. An emergency situation where the supply of breathing gas to the diver has stopped.
open water
1.  Water where there is no physical obstruction to a direct ascent to the surface.[84]
2.  Water which is open to influences by weather and climatic conditions.[37]
Over-pressure valve. A pressure relief valve which automatically opens at a set pressure to allow excess gas to escape.

Main article: O-ring

A mechanical gasket in the shape of a torus; a loop of elastomer with a circular cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the contact surfaces.
oro-nasal mask
ori-nasal mask
A breathing mask that covers the mouth and nose only. It may be an independent item, such as an oxygen mask or BIBS mask, or it may be a component inside a full face diving mask or helmet to reduce the amount of dead space

See: oxygen toxicity unit


See: Decompression theory#Outgassing of tissues

Diffusion of gas out of the tissue into the blood, and transport to the lungs where it diffuses into the lung gas and is eliminated by exhalation.
A physical or procedural obstruction to a direct ascent to the surface. Physical overheads include cave, cavern or culvert ceilings, fishing nets, ship hulls, and wreckage. Procedural overheads are generally a decompression obligation.
Excessive rotation of a reel or spool by inertia, causing the line to unwind beyond the amount paid out, and lie in loose coils on the spool which can jam or tangle.

See also: diving heavy

Carrying more ballast weight than is necessary to achieve neutral buoyancy at all times during a dive. In scuba diving usually a dangerous error, but used in surface-supplied diving to stabilise the diver when working on the bottom.

Main article: Oxygen

Important component gas of atmospheric air and essential component of any breathing gas. Required to sustain life.
oxygen analyser
oxygen analyzer

Main article: Oxygen analyzer

Instrument for measuring the partial pressure of oxygen in a gas mixture
oxygen cell
Electro-galvanic oxygen sensor for measuring partial pressure of oxygen in the breathing gas using an oxygen analyser
oxygen clean

See: Oxygen compatibility#Oxygen cleaning

Cleaned for oxygen service by appropriate methods and materials and tested for contaminants. Verified that particulates, fibres, oils, greases and other contaminants are absent.[65]
oxygen clock

See: Oxygen toxicity#Underwater

A notional alarm clock, which accumulates hyperbaric oxygen exposure at a rate which increases with higher oxygen partial pressure toward the maximum single exposure limit recommended. This function is implemented as cumulative oxygen exposure for acute (CNS) oxygen toxicity in some dive computers.
oxygen compatible

Main article: Oxygen compatibility

Made from materials which are suitable for oxygen service. Capable of coexisting with elevated oxygen concentrations and a potential source of ignition without flashing, based on a system’s maximum operating pressure and temperature.[65]
oxygen compatible air
Air which has been filtered to reduce contaminants to a level suitable for blending with high pressure oxygen. Air with a reduced level of condensable hydrocarbon mist or vapour.[65]
oxygen design
oxygen service design

See: Oxygen compatibility#Oxygen service design

Design that minimizes any tendency for heat generation, ignition of particulates, or the accumulation of contaminants for an intended partial pressure of oxygen and temperature.[65]
oxygen fraction

See: Breathing gas#Classification by oxygen fraction

Fraction by volume or pressure of the gas mixture made up by oxygen
Oxygen Pete
Imaginary monster associated with CNS oxygen toxicity, also a CNS oxygen toxicity seizure.
oxygen rebreather

See: Rebreather#Oxygen rebreathers

A closed circuit rebreather that uses only oxygen as a gas supply.[28]
oxygen sensor
A component that produces a signal in response to the presence or concentration of oxygen. In diving this is usually a galvanic cell that generates a current and voltage proportional to the partial pressure of oxygen in a breathing gas.[28]
oxygen service

See:Oxygen compatibility#Oxygen service

Suitable for operating with significantly higher levels of oxygen than normal atmospheric air. Often implies special cleaning procedures, use of oxygen compatible materials, and design to reduce ignition risk. System or component that has been designed and tested for oxygen use, has been tested as oxygen clean and is oxygen compatible.[65]
oxygen toxicity

Main article: Oxygen toxicity

A condition resulting from the harmful effects of breathing molecular oxygen (O2) at elevated partial pressures.
oxygen toxicity unit
Unit of exposure to toxic concentrations of oxygen in breathing gas, primarily in connection with pulmonary oxygen toxicity and oxidative damage to cell membranes. The accumulation of OTUs depends on time and partial pressure of exposure, and dissipation is a function of recovery time at non-toxic partial pressures.
oxygen window

Main article: Oxygen window in diving decompression

Inherent unsaturation due to metabolic reduction of total gas pressure in the tissues.



Main article: Panic

A sudden sensation of fear which is so strong as to dominate or prevent reason and logical thinking, replacing it with overwhelming feelings of anxiety and frantic agitation consistent with an animalistic fight-or-flight reaction
parallel compartments

See: Decompression theory#Perfusion limited tissues and parallel tissue models

A decompression model comprising a group of tissues with varied rates of perfusion, but supplied by blood of approximately equivalent gas concentration. It is assumed that there is no gas transfer between tissue compartments by diffusion. This results in a parallel set of independent tissues, each with its own rate of ingassing and outgassing dependent on the rate of blood flowing through the tissue
partial pressure

Main article: Partial pressure

(Abbreviation PP or pp) The pressure that a component gas of a gas mixture would exert if it alone was present in the volume occupied by the gas mixture.[13]
passive addition

See: Rebreather#Passive addition semi-closed circuit

Feed gas addition system for semi-closed circuit rebreathers which discharges a part of the gas in the breathing circuit. fresh gas is added when the volume of the circuit decreases during inhalation and triggers the addition valve. Compare with active addition.
patent foramen ovale

See: patent foramen ovale

A common form of congenital heart defect that enables blood flow between the left and right atria through a gap in the interatrial septum. In some circumstances this may increase risk of decompression sickness if venous blood carrying gas bubbles is shunted into the arterial system, bypassing the pulmonary capillary network filter.
pendulum rebreather

Also: "push-pull rebreather"

Rebreather with a single breathing hose from the mouthpiece to the scrubber and counterlung. Gas passes through it in both directions, unlike the one-way breathing loop configuration. The volume of the hose between the mouthpiece and scrubber is dead space

See: Penetration diving

Entering a region with no direct vertical access to the surface, such as a cave or the interior of a wreck.
penetration line

See: distance line

Cave diving: Bubbles making their way to the walls and the ceiling of the cave and dislodging silt.[85]

Main article: Perfusion

The process of delivery of blood to a capillary bed in the biological tissue.[6]
perfusion limited

See: Decompression theory#Deterministic models

The assumption in a decompression model that perfusion has the dominant influence on gas uptake and release. Compare with diffusion limited.
permanent stamp markings
permanent markings
stamp markings

See Diving cylinder#Permanent stamp markings

Text stamped into the metal of the shoulder of a diving cylinder providing obligatory and optional information about the cylinder.
personal locator beacon

Also: "PLB"

Main article: Emergency locator beacon

Radio beacons for personal use which are intended to indicate a person in distress who is away from normal emergency services.
personal safety equipment

Main article: Personal protective equipment

Equipment worn by personnel to reduce risk of injury at sites where it is not practicable to eliminate the hazard, including ear protectors, safety glasses, hard hats, gloves, overalls, respirators etc. Diving suits and underwater breathing apparatus are also personal protective equipment.

See: patent foramen ovale

photo quadrat

See: jump camera

A photograph of a quadrat taken for later analysis. Common in marine ecological research where in situ counting would be impractical. Quadrats may be identified by rigid frames or by a fixed camera to subject distance.

See: Pigging#Pipeline Inspection Gauge

Pipeline inspection gauge: A tool that is sent through a pipeline and propelled by the pressure of the product in the pipeline itself, for purposes such as cleaning, dewatering, inspecting, measuring, separation of contents, etc.[13]
Coiled section of pipe to provide greater flexibility.
pillar valve

See cylinder valve

pin index connection

Main article: Pin Index Safety System

Standard connection type for portable medical oxygen cylinder valves and regulators.
pipeline end manifold

Also: "PLEM"

See: Pipeline end manifold

The place where a flexible underwater hose string is attached to connect a seabed pipeline with a single point mooring.[4]
piston bolt

See: bolt snap

Securing the guideline as it is being run and the choice of primary and secondary tie offs.[7]

See personal locator beacon


See: pipeline end manifold

pneumo breathing

See: Surface-supplied diving skills#Breathing from a pneumofathometer hose

Use of the pneumofathometer hose to supply breathing gas to a surface supplied diver in an emergency. Supply can be from the diver's own pneumo hose or from a standby diver's pneumo hose.

Also: "pneumo", "Kluge-Pneumo"

See: Pneumofathometer

Instrument to measure the depth of a diver, which measures the ambient pressure at the diver by measuring the pressure in a hose filled with air with an open end at the diver and with the surface end connected to a gas supply, control valve and pressure gauge calibrated in msw, fsw, or often both.[13]

Main article: Pneumothorax

Air or other breathing gas in the chest cavity, outside of the lung, particularly between the pulmonary pleurae, sometimes resulting in a collapsed lung.
positive displacement
(of compressor) A configuration that compresses gas by reducing the volume of the compression chamber (eg. cylinder) by mechanical means (eg. piston) to produce higher pressure of the contents which flow out via the exhaust valve.
pony cylinder

Main article: pony cylinder

Relatively small scuba set carried as an independent alternative breathing gas source by a scuba diver.
A large bore quick connector fitting designed and used by Draeger on diving and firefighting breathing apparatus, which has been used in rebreathers, particularly modifications.
pre-fill external inspection
Examination of the external condition of a pressure vessel and fittings to ensure that it complies with requirements before accepting for filling.[31]
Nitrox blend with high oxygen content used to mix nitrox by topping up the decanted premix with air.
pressure gradient
The rate of change of partial pressure of dissolved gas through a solvent, which is the driving mechanism for diffusion through the solvent. Also loosely used to refer to the difference between the dissolved gas pressure in a tissue and ambient pressure.[6]
primary light
The main light to be used on a dive. Usually the most powerful.
primary regulator
the regulator which the diver intends to breathe from for most of the dive. Particularly when diving with back-mounted manifolded twin cylinders.
primary tie-off
First tie off of the guideline in a penetration dive. This is usually made in a place with free vertical access to the surface.
professional diving

Main article: Professional diving

Diving which is done as part of the diver's employment or professional occupation.
progressive penetration
An incremental approach to cave and wreck exploration. Each dive goes a bit further so that the divers develop a familiarity with the environment.[7]
Pressure relief valve
Pounds per square inch. Unit of pressure in the Imperial system.
Push to talk: Voice communication systems which require the user to press a button to transmit. Used with through water systems to conserve battery power.
public safety diving

Main article: public safety diving

The underwater work conducted by law enforcement, fire department rescue, and search & rescue/recovery dive teams.
1.  A relatively long pull on a lifeline when used for rope signals. (see "bell" for comparison).
2.  Part of an ascent between the bottom and a decompression stop, between decompression stops, or to the surface. Possibly deriving from the practice of pulling a surface supplied diver up by the umbilical or lifeline.
pulmonary over-inflation syndrome

See Barotrauma

Pulmonary barotrauma of ascent. Lung over-pressure injury.
To press the purge button on a demand valve to induce a gas flow which is intended to clear the demand valve interior of water or other substances.
purge button
Button or flexible area on the front or side of a demand valve which allows the user to manually open the second stage valve to provide gas flow without inhalation.
purge valve
Valve in snorkel or mask which allows water to drain either under gravity or as a result of exhalation into the air space
push gradient
Tech diving jargon: Decompress at a high gradient factor, particularly when exceeding the baseline M-value. This will expose the diver to a higher risk of developing decompression sickness while reducing time decompressing in the water.[86]
push-pull rebreather

See: pendulum rebreather


Also: "Pee-valve"

See: Dry suit#The P-valve

A valved catheter fitted to a dry suit, which enables a diver to urinate at any time without having to get out of the water.
Pyle stops

See: Decompression theory#Pyle stops

Named after Richard Pyle, an early advocate of deep stops. An additional brief deep decompression stop, typically 2 minutes long and half way between the maximum depth and the first conventional decompression stop.


Oval connector shaped like a chain link with a screw gate on one side.
A group of high pressure gas storage cylinders mounted upright on a frame and manifolded together. Usually in 4, 6, 9, 12, or 16 cylinder arrangements.

Main article Quadrat

See also: photo quadrat

A small, typically rectangular plot used in ecology and geography to isolate a standard unit of area for study of the distribution of an item over a large area. The quadrat is suitable for sampling plants and slow-moving or sessile animals.


rams head

Also: "cobra guard", valve cage

Frame attached to the top of back-mounted scuba cylinders to protect valves, manifold, and regulator first stages from impact with the surroundings.
rapture of the deep

See: nitrogen narcosis

rash guard
rash vest

Main article: rash guard

A shirt made of spandex and nylon or polyester, worn to protect against rashes caused by abrasion, and jellyfish stings. These shirts can be worn by themselves, in tropical water, or under a wetsuit.
Rat hat
Ratcliffe diving helmet, Designed by Bob Ratcliffe, later produced by Oceaneering International. No longer in production.[87]
ratio decompression

Also: "ratio deco"

Main article: Ratio decompression

A technique for calculating decompression schedules for scuba divers engaged in deep diving without using dive tables, decompression software or a dive computer.[88][89]

See: Recreational Dive Planner


Main article: Diving rebreather

See also: gas extender and reclaim system

Underwater breathing apparatus which recycles most of the exhaled gas, removing carbon dioxide and topping up oxygen before the gas is breathed again.
reciprocating compressor

Main article: Reciprocating compressor

Compressor in which the volume of the compression chamber/s is cyclically changed by reversing linear motion. E.g.: a piston moving back and forth in a cylinder.
reclaim helmet

See: Saturation diving

Diving helmet with a reclaim gas regulator allowing exhaled gas to be safely returned to the surface through an additional hose on the umbilical.
reclaim system

See: Saturation diving

System for recovering helium based breathing gas used by divers and recycling it.
Subjecting a diver to pressure after an ascent from a dive as treatment for decompression illness or to prevent decompression sickness.[20] Preferably done in a recompression chamber, but occasionally done as in-water recompression.
recompression chamber

Main article: Recompression chamber

A hyperbaric chamber used to treat divers suffering from certain diving disorders such as decompression sickness.
recovery breathing

Also: "hook breathing"

A technique used by freedivers on surfacing to reduce the risk of surface blackout. A partial exhalation is made, followed by a quick inhalation, then the diver closes the airway and pressurises for a few seconds as if about to cough. This is repeated a few times over the first 30 seconds or so on the surface. The aim is to keep thoracic pressure slightly raised to artificially raise arterial oxygen partial pressure or prevent it from dropping in the critical seconds until newly oxygenated blood can reach the brain, and thereby prevent surface blackout. This is the same technique used by pilots during high-g maneuvers, and by mountaineers at high altitude.
recreational diving

Main article: Recreational diving

Recreational diving or sport diving is a type of diving that uses scuba equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels of training, experience and equipment. In other circles, technical diving is considered a subset of recreational diving as opposed to "professional diving", which is done as part of the diver's work.
Recreational Dive Planner

Also: "RDP"

Main article Recreational Dive Planner

A no-stop decompression table developed by DSAT.
red tide

See: algal bloom

reduced gradient bubble model

Main article: Reduced gradient bubble model

A computationally intensive bubble model decompression algorithm developed by Bruce Wienke.

See: Engineering redundancy

Technical diving philosophy of ensuring that a spare or backup is available for any item of life-support equipment that would immediately endanger the diver if it were to fail.[90]
redundant breathing gas supply

Also: redundant gas supply, fully redundant gas supply, redundant air supply etc.

See Redundant breathing gas supply

A breathing gas supply, carried by the diver, which is both suitable for the depths at which it may be breathed, and sufficient to allow the diver to make a safe and controlled return to the surface or other place where more breathing gas is available, which is not used during the dive, and is stored in one or more cylinders which are mot the one the diver is breathing from at any given time.
redundant equipment
duplicated or spare equipment carried by the diver or team to substitute for vital primary equipment in case of a malfunction. In some cases, such as cave lights, multiple redundancy may be desirable. A redundant breathing gas supply is the most common example.
Mechanism used to store, deploy and recover long lengths of line at low tension.
reference temperature
Temperature at which a cylinder may be safely and legally filled to the nominal charging pressure without corrections.[31]
1.  A mechanism for controlling the output pressure of a high pressure gas supply.
2.  As 1, with a demand valve (q.v.) incorporated which provides the diver with breathing gas at ambient pressure.
regulator changeout
regulator swap-out
Scuba emergency skill of swapping a correctly functioning regulator with one that is malfunctioning to gain access to breathing gas in a cylinder while underwater. Usually applied to side- or sling-mounted cylinders.
regulator freeze

See Ice diving#Regulator freezing

Locking of the regulator mechanism caused by freezing of the water due to expansion cooling of the regulated air. Often causes the mechanism to lock open, causing free flow and further cooling.
reject criteria
Features, states or conditions which are not acceptable.[31]
repetitive dives

See Decompression practice#Repetitive dives

Any dive which is done while the tissues retain residual inert gas from a previous dive.
Method of estimating tolerable repetitive exposure to high partial pressure oxygen.[91]
rescue tether
A short lanyard or strap carried by a surface supplied stand-by diver to be used to tether an unresponsive diver to the standby diver during a rescue. It is attached at one end to a D-ring on the stand-by diver's harness, and has a clip at the other end which may be secured to a D-ring on the casualty's harness to allow the rescuer the use of both hands during the return to the bell or surface.
reserve gas

See Scuba gas planning#Reserve pressure

Gas which is not intended to be used during the dive, and is reserved for contingencies.
reserve valve
Cylinder valve with a lever operated bypass valve to release the gas below reserve pressure
residual gas
Gas remaining in a cylinder from the previous fill.
residual nitrogen

See: Decompression practice#Residual nitrogen time

Nitrogen in excess of normal atmospheric saturation remaining in the diver's tissues after a dive.
residual nitrogen time

Also: "RNT"

See: Decompression practice#Residual nitrogen time

Time penalty in a repetitive dive equivalent to time at depth which would produce the residual nitrogen in the diver at the start of the dive.
residual pressure
Pressure of the gas remaining in a partly used cylinder from the previous fill.
respiratory minute volume

Also: "minute ventilation"

Main article Respiratory minute volume

The volume of gas inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute.
respiratory quotient

Main article Respiratory quotient

The ratio of carbon dioxide produced as a metabolic product to the oxygen consumed.
Section of a cave which is difficult to pass through due to lack of space. A minor restriction is too small for two divers to swim through together, a major restriction requires the diver to remove equipment to fit through.
Certification that an item of equipment continues to be fit for service. Usually after passing the required periodic inspection and testing by an approved or authorised practitioner.
reverse jump
Procedure to cross a gap during an exit which allows the line to be retrieved without returning to the start point. In the event of a line break or removed primary reel, one diver holds the end of the search line at the break point, and the other searches for the other part of the broken line or the exit while laying line from the reel. When the original line or exit is found, the reel handler signals to the static diver with line pulls and the static diver swims in the direction of the line while the reel handler reels it in, recovering the line.[92]
reverse profile
Repetitive dive which is deeper than the previous dive
Multilevel dive in which a later level is deeper than an earlier level.

See: reduced gradient bubble model

Rimbach system
System of touch contact signalling used in cave diving. Push forward = go, pull back = back up, squeeze = stop.[7]
ring bungee

See also: bungee (sidemount)

Length of shock cord with metal rings and a clip used to control the position of the top end of a sidemounted cylinder.[21] Differs from a regular bungee in having the rings.[21] A popular configuration is a bolt snap connected to a ring by a quick link, with a length of bungee from the ring to another quick link which is used to connect the assembly to a D-ring on the back of the harness. The bolt snap is clipped to the shoulder D-ring and the cylinder neck bolt snap is clipped to the ring.
rip current

Main article Rip current

A strong localised flow of water to seaward from near the shore, typically through the surf line

Main article Drilling riser

A conduit that provides a temporary extension of a subsea oil well to a surface drilling facility

See: respiratory minute volume

Royal Navy Physiological Laboratory

See: residual nitrogen time

rock bottom gas management

See: Scuba gas planning

Retention of a breathing gas reserve based on calculated values for the amount of gas required for a safe ascent from any point in the planned dive profile. Factors such as emergency supply of gas to a buddy, air consumption rates under stress and decompression gas requirements are considered in the calculations.
Closing of a cylinder valve as a result of scraping contact between the valve knob and the overhead or other surroundings. The left hand knob is more likely to roll-off, closing the backup regulator in the standard twins configuration. Roll-off is an emergency if the diver does not have an immediately accessible alternative breathable gas supply.[92]
Remotely operated underwater vehicle – A tethered underwater mobile device operated by a remote crew.
Member of the drilling crew who works under the direction of the driller to make or break connections as drillpipe is tripped in or out of the hole.[4]
Any unskilled manual laborer on the rigsite.[4]
rule of thirds

See: Rule of thirds (diving)

Cave and wreck penetration breathing gas management convention where no more than one third of the gas in a cylinder may be used on the inward part of the dive, and the other two thirds is kept for exit: One third for the planned exit, and one third in case of an emergency.}}

run line
running line
Unreeling line under light tension while advancing. Usually in the context of laying a distance line.
run time
Time elapsed since the start of a dive.
running stop
A pause made during ascent to adjust the average ascent rate to the nominal value. For example a nominal ascent rate of 3 m per minute might use an actual ascent rate of about 5 m per minute and make a stop every 3m until the end of the current minute, resulting in a slightly stepped profile with an overall ascent rate corresponding to the nominal rate.[88]
run time schedule
Decompression schedule and dive plan based on elapsed time from the start of the dive, All waypoints and events are specified in terms of elapsed time with start of descent at zero.

Also: "positive reserve valve", "automatic valve", or "calibrated orifice"

A cylinder valve which limits the outflow by a calibrated orifice when in the "on" position.[74]


SAC rate

See: surface air consumption rate

safe air
Term used for nitrox by American Nitrox Divers International (ANDI)
safe second
Obsolete term for octopus regulator.[92]
safety-critical element
safety-critical equipment
Item of equipment or process with the purpose to prevent or limit the consequences of a high risk hazard, that if realised, could result in the fatality or severe injury of one or more divers or support crew.[13]
safety reel
Reel with relatively short line for use in an emergency, usually for searches to find a lost buddy or lost guideline or to jump a line break.[92]
safety spool
Spool with relatively short line for use in an emergency, usually for searches to find lost buddy or lost guideline or to jump a line break.[92]
safety stop

See: Decompression practice#Safety stop

A voluntary (not required by the decompression schedule) additional decompression stop intended to further reduce risk of decompression sickness.
salt water aspiration syndrome

Main article: Salt water aspiration syndrome

A reaction of the lungs to inhalation of a mist of salt water
Involuntary muscular contractions experienced by breathhold divers when approaching hypoxic blackout.[93]
A shallow water recreational airline diving system using a scuba cylinder on a float towed by the diver.
Condition where the inert components of the breathing gas dissolved in a diver's tissues are in equilibrium with the gas in the lungs.[20]
saturation diving

Main article: Saturation diving

Diving mode where the divers remain pressurised for periods of several days or weeks and decompress only at the end of the period.
saturation diving system
saturation spread

Main article: Saturation diving system

The combination of equipment and services to operate a saturation diving project. It would include the closed diving bell, the accommodation modules, decompression chamber, life-support systems, gas storage and supply systems, pressurisation equipment, underwater breathing apparatus, and launch and recovery systems. In may also include a hyperbaric evacuation system.
Self-contained breathing apparatus (not for underwater use).
schrader valve

Main article: Schrader valve

Valve using a standard automotive tyre valve insert, common in low pressure inflation hose female connectors and BC inflation valves.[92]
scientific diving

Main article: Scientific diving

Diving for purposes of scientific research. The rules and constraints of scientific diving vary in different jurisdictions, but generally allow different options to mainstream commercial diving.
Diver propulsion vehicle used by scuba divers to increase range underwater.[92]
scooter ring
D-ring on scuba harness used to attach to scooter tow line. Usually on front of the harness crotch strap.[92]
1.  Semi-closed circuit rebreather: A rebreather (q.v.) which either dumps part of each breath to the environment or continuously adds gas and dumps the excess. Compare with closed circuit rebreather (CCR) (q.v.).
2.  Surface Consumption Rate - An alternative term with the same meaning as Surface Air Consumption (SAC) (q.v.). Not to be confused with Semi-closed Circuit Rebreather.
screw gate carabiner

Main article: Carabiner

A clip mechanism which can be locked in the closed position by turning a threaded barrel.

See: Rebreather#Carbon dioxide scrubber

Canister containing material (sorb) which chemically combines with carbon dioxide to remove it from the gas passed through the canister.

Main article: Scuba set

Self-contained underwater breathing apparatus. May be open or closed circuit.
scuba orienteering

Main article: Scuba orienteering

Competitive underwater sport in which scuba divers attempt to swim a circuit marked by buoys, without surfacing, using compass navigation and mechanical distance measurement. Points are awarded for time and accuracy according to the specific course definition and length.
scuba replacement
Mobile surface-supplied diving equipment using portable gas storage cylinders for primary and reserve breathing gas supply. Preferred to scuba for commercial diving applications due to lower perceived risk compared to scuba, and because equipment and procedures are otherwise identical to surface supplied diving using compressors for air supply. Used when logistical constraints or air quality issues preclude use of a compressor.[13]
Safety drill. An air sharing exercise based on deploying the long hose primary regulator.
search line

See: Underwater searches

Cord used to indicate the transect for a search. The diver moves along the line, searching by sight or feel on one or both sides of the line. After the transect has been searched, the line is moved to the next transect. usually a short distance offset from the previous position, at a distance which depends on the visibility and the size of the target.
search pattern

See: Underwater searches

Systematic procedure for covering the search area sufficiently to be reasonably sure of finding a given target. Several patterns are in general use for underwater searches, depending on the target, the terrain, and available facilities.
secondary drowning

Main article: Secondary drowning

A complication of aspiration of water or other fluids into the lungs.
secondary regulator

See: backup regulator

second stage

See: demand valve

The part of a diving regulator which provides pressure reduction from intermediate pressure to ambient pressure on demand. Demand valve.
self-propelled hyperbaric lifeboat (SPHL)

Also: "hyperbaric lifeboat" (HLB), "hyperbaric rescue vessel" (HRV)

A pressure vessel adapted for use as a means of hyperbaric evacuation, and fitted to a conventional lifeboat hull.[13]
semi-dry suit

See: Diving suit#Semi-dry suit

A wet suit with wrist and ankle seals, and usually a more watertight zipper than usual, to reduce flushing of water through the suit.

Main article: Vapor–liquid separator

Device which facilitates the separation of liquid particles from the compressed gas, usually with a drain to periodically remove accumulated liquid from the system.
serial compartments

See: Decompression theory#Diffusion limited tissues and the "tissue slab", and series models

A decompression model based on the assumption that diffusion is the limiting mechanism of dissolved gas transport in the tissues, in which there is perfusion transport for one compartment, and diffusion between a series of compartments,
serial number

Main: Serial number

1.  Number stamped on the cylinder by the manufacturer in the shoulder area which identifies the cylinder. In combination with the manufacturer’s identification this will be unique to the cylinder.[31]
2.  Character string, including number, marked on an item of equipment by the manufacturer to uniquely identify it.
set point

See: Rebreather diving#Set-points

Reference value for oxygen partial pressure in an electronically controlled closed circuit rebreather. The control system monitors the real time value of oxygen partial pressure in the breathing loop and automatically adjusts the composition by adding gas to keep the concentration between the upper and lower set-points.

See also: SAC

Surface Gas Consumption
shallow water blackout
Loss of consciousness during a dive associated with occurrence at a shallow depth. Used for several different mechanisms, depending on context, therefore often leading to confusion.
1.  See: Shallow-water blackout
Loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres (16 feet), when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it.[68]
2.  See: Freediving blackout#Terminology and latent hypoxia
Loss of consciousness caused by cerebral hypoxia at the end of a deep breath-hold dive during the latter part of the ascent or immediately after surfacing due to lowered pO2 caused by reduction in ambient pressure, see also latent hypoxia.
3.  Loss of consciousness while ascending on a rebreather due to sudden drop of pO2 in the breathing loop, usually associated with manual CCR and SCR.
shark pod
shark shield
Electrical device carried by a diver intended to repel sharks by electrical field pulses.[94]
shock cord

See: bungee cord

shore diving
Diving from a shore entry point.
shot line

Main article: Diving shot

A rope between a float at the surface, and a sufficiently heavy weight holding the rope approximately vertical.[27]

Main article: Sidemount diving#Terminology

A scuba diving equipment configuration which has diving cylinders mounted alongside the diver, below the shoulders and along the hips, instead of on the back of the diver.
sidemount staging

See: Sidemount diving#Terminology

The practice of using sidemount configuration (bungee loops and/or butt-plate rails) as a means for stowing stage/deco cylinders when otherwise diving in back-mounted scuba
silent entry

See: Scuba skills#Entries

An entry technique which minimises noise and splash, suitable for entry from a low platform. The diver sits with feet dangling over or into the water, turns their torso sideways, takes their weight on their hands, then swings off the surface and drops feet first into the water, slowed by their arms, and lets go with the hands when in the water.[95]: 251 
silica gel

Main article: Silica gel

A desiccant filter medium used to adsorb water.
silt out

Main article: Silt out

A situation when underwater visibility is rapidly reduced to zero, usually when a diver disturbs silt deposits.
silt screw

See: Cave diving#Equipment

Device which is inserted into silt or sand to provide an anchor point, such as for a tie-off on a cave line.[21] Silt screws are generally stakes made from small bore plastic (PVC) pipe with a sharpened end.
simultaneous operations
Two or more potentially clashing operations occurring, for example, at the same time and same location.[13]
single point mooring

Main article: Single buoy mooring

A loading buoy anchored offshore, that serves as a mooring point and interconnect for tankers loading or offloading gas or liquid products. SPMs are the link between geostatic subsea manifold connections and weathervaning tankers. They are capable of handling any size ship, even very large crude carriers (VLCC) where no alternative facility is available[4]

Main article: Sinkhole

A natural depression or hole in the Earth's surface caused by karst processes — the chemical dissolution of carbonate rocks - or suffosion processes for example in sandstone

Main article: Sintering

Particulate or granular material bonded together by the application of pressure and sufficient heat to partially melt the surface of the particles and weld them together. The product is usually porous.
Place where water from a stream flows into the ground. also "swallow hole"[7]

Main article: Skandalopetra diving

A flat stone, usually of marble or granite, weighing between 8 and 14 kg, with rounded corners and edges, and tied to a rope, historically used by Greek sponge free-divers to assist descent. When the diver wishes to ascend the rope is used to signal the tender on the boat who then pulls in the rope. Currently skandalopetra diving is a competitive sport.

See: dive skins

skip breathing

See: Hypercapnia#Skip breathing

Breathing pattern where the diver holds each breath a while to conserve breathing gas, which can cause hypercapnia which can lead to headaches, aggravate nitrogen narcosis, increase risk of oxygen toxicity, and reduce physiological reserves in an emergency.

See: Submarine pipeline#The S-lay system

Method of laying undersea pipelines by welding the sections together on the lay barge and deploying them from the stern horizontally guided by a "stinger" - a structure that supports the pipe string to control its bend radius.[96] The term refers to the shape of the bend in the pipe which transitions from roughly horizontal onboard where the sections are welded together, to angled downward over the stinger, to roughly horizontal again as it settles on the seabed.
Rigid plastic tablet used for writing messages or notes.
sling cylinder

See: Diving cylinder#Open-circuit

Independent cylinders with their own regulators carried clipped to the harness at the side of the diver.
slingshot valve

See: Y-valve

See Scuba cylinder valve#Other distinguishing features

A cylinder valve with two valved outlets angled to left and right of the centreline
slob knob
Flexible extension used for operating valve knobs on a manifold

See: surface marker buoy

snag line
Line used for underwater search intended to snag on the target. May be dragged by boats or by divers. May be weighted if dragged over a smooth bottom.
snap shackle

See: Shackle#Snap shackle

A clip connector mechanism which locks when closed, can be operated without tools, and can usually be released under load.
snoopy loop

See: Rubber band#Snoopy loop

A heavy duty elastic band made from a slice of inner tube.

See: Underwater photography#Snoot

Accessory for a photographic strobe which limits illumination of the subject to a very small area, leaving the background datk, and virtually eliminating backscatter.

Main article: Snorkel (swimming)

Tube with a bend and mouthpiece used for breathing air from above the water surface when the wearer's mouth and nose are submerged.

Main article: Snorkeling

Swimming at the surface of the water while breathing through a snorkel. the snorkeller is almost always equipped with a diving mask or swim goggles, and usually swimfins.
snorkel keeper

See: Snorkel (swimming)#Construction

Device to hold a snorkel in place at the side of a diving mask by fixing it to the mask strap.

See also: Hookah

Main article: Snuba

Snuba is a portmanteau of "snorkel" and "scuba", referring to a proprietary recreational surface supplied underwater breathing system supplied from a cylinder mounted on a small raft towed by the diver.
solo diving

Main article: Solo diving

The practice of scuba diving alone, without a dive buddy or in-water standby diver .
sonic orifice

See: Choked flow

Metering device to provide constant mass flow of a gas.

Also: "sodasorb", "sodalime" and "sofnolime"

Carbon dioxide absorbent material used in rebreather or life support system scrubber to remove carbon dioxide from the breathing gas so it may be recycled.

Main article: Speargun

Hunting weapon for shooting fish underwater which propels a barbed steel spear a short distance forward using stored energy from stretched rubber strips or compressed air behind a captive piston.
Slang term for breathhold spearfisher.
speech unscrambler

Also: "helium unscrambler" and "unscrambler"

An electronic device to render words spoken in a hyperbaric helium environment intelligible.[13]

Main article: Speleogen

Dissolution features in bedrock.

Main article: Speleothem

Also known as a cave formation: A secondary mineral deposit formed in a cave.

See: submersible pressure gauge


See: Self-propelled hyperbaric lifeboat


See: Full-face diving mask#Straps

Mask strap system for full-face masks with three or more straps. (commonly 5)

See: Standard diving dress#Helmet

A valve in the side of a copper diving helmet which could be used by the diver to suck in a mouthful of seawater to spit onto the inside face of a viewport to wash off condensation droplets to improve their view.

See: single point mooring

1.  See: Distance line#Reels and spools
Circular device for storing line, comprising a short tubular section with a relatively large flange at each end. No moving parts. A small, compact, economical and reliable alternative to a reel for relatively short lines.
2.  Short, straight section of pipe or tube with a flange at each end.
The topside base for (usually) surface supplied commercial diving operations. Also "air spread" and "saturation spread" (q.v.) depending on the diving mode.[97]

See: Spring (hydrology)

Place where a concentrated flow of water emerges from the ground.
spring strap


Fin strap using a stainless steel spring to secure the fin to the foot.
spring suit
A wetsuit that covers the torso and has short sleeves and long or short legs
spud can
The foot on a leg of a Jack-up rig oil platform designed to spread the load so that the rig does not sink too deeply into the sea-bed.[4]
South Pacific Underwater Medicine Society
square profile

See: Dive profile#Square profile

Dive profile where the diver descends continuously to the maximum depth and stays there for the duration of the dive before ascending directly at a stedy rate to the surface or first decompression stop. This profile provides worst case exposure for gas absorption by the body tissues for a given depth and bottom time, and is assumed for most decompression planning using decompression tables.

See: barotrauma

Injury or discomfort caused by increase or decrease in volume of gas space in the diver's body or equipment due to a change in ambient pressure.
Surface-supply diving equipment.
stabilisor jacket
stab jacket

See: buoyancy compensator

1.  See: diving stage
2.  See: stage cylinder
stage cylinder
1.  A cylinder used for a stage of a long penetration dive, also known as drop cylinder, which is placed on the distance line to be collected on the return.
2.  Also generically used to refer to decompression gas cylinders carried as sling cylinders (q.v.)
staged decompression
The practice of making decompression stops.
Placing a stage cylinder at the distance line for planned later use.
stage-only diving

See: Sidemount diving#Terminology

The use of standard deco/stage cylinder configuration, without back-mounted cylinders, on an otherwise standard, or partially modified tec/rec BCD. The cylinders are attached to waist and shoulder D-Rings by direct bolt-snap, and no bungee cord is used in the upper attachment. Similar to, and may be confused with sidemount
standard diving dress
standard dress

Also: "heavy gear"

Main article: Standard diving dress

Early free flow surface supplied diving equipment using a heavy rubberised canvas suit, copper helmet and corselet, and weighted boots. Still in use in some parts of the world.[20]
standard operating procedure

Main article: Standard operating procedure

Procedure compiled by an organisation prescribing the processes to be followed when performing specified tasks.
standby diver
stand-by diver

See: Standby diver

(Professional) diver functioning as a safety backup to the working diver. Often on the surface at the dive control point, but ready to enter the water at very short notice on the instruction of the supervisor. In bell diving the stand-by diver would be the bellman.[13]
static apnea
static apnoea

Main article: Static apnea

Underwater breath-holding without changing location
static lung load

Also: "hydrostatic imbalance", "SLL"

The pressure difference between gas inside the lungs and the gas inside the mouthpiece of the breathing apparatus or inside the helmet. In most rebreathers this is the hydrostatic pressure at depth of the counterlung. In ballasted bellows counterlungs the pressure is modified by the force exerted by the ballast weight.[28]
A full one piece Wetsuit that covers the torso and the full length of the arms and legs.
An overboard extension at the stern of an offshore pipe lay barge used to provide additional support at the over bend of an S-lay during offshore construction.[96]

See: decompression stop

living depth

See: storage depth

storage depth

Also: "living depth"

See: Saturation diving#Accommodation

Depth pressure in a saturation system at which divers live between compression and decompression when not locked out on a dive.[79]
stride entry

Also: "giant stride entry"

See: Scuba skills#Entries

Entry technique from a platform a small to moderate distance above the water surface, which is suitable to stand on before entry. The diver faces the water and steps out with the leading foot, pushing away from the platform with the back foot, and drops into the water while maintaining a vertical posture until fully submerged. It is usually advisable to hold loose equipment against the body, particularly the mask and DV, and if there is no crotch strap, the buoyancy compensator is held to prevent it riding up the torso on impact. The feet can be brought together after initial impact to limit depth of immersion in moderately shallow water, by applying fin thrust downwards.[19]
Derogatory term used by DIR zealots to describe divers who do not follow DIR procedures and use DIR approved equipment and configurations.
subcutaneous emphysema

See: Subcutaneous emphysema

Gas under the skin resulting from lung overpressure injury.[11]
submersible compression chamber
submersible decompression chamber

See: closed bell

A closed diving bell, used for transferring divers under pressure to and from the worksite. Particularly if used fpr decompressio.[20][13]
submersible pressure gauge

Also: "SPG"

See: Submersible pressure gauge

Gauge attached to the first stage regulator and used to monitor pressure remaining in the diving cylinder.
suicide clip
Derogatory term for clip mechanisms which are capable of clipping onto a line or other object without the active and intentional intervention of a user.
suit blowup

See: Scuba skills#Dry suit blowup

Excessive inflation of a dry suit leading to uncontrolled ascent.

Main article: Sump (cave)

A passage in a cave that is submerged under water.
sump pack
Tough waterproof bag with watertight seal used to carry dry equipment in caves, including through water filled passages.[21]
superoxide scrubber
Rebreather scrubber which not only removes carbon dioxide from the exhaled air, but also replenishes the oxygen by chemical reaction.
A temporary and thermodynamically unstable condition of a solvent containing more dissolved gas than it can hold in solution over the long term for the prevailing conditions.
supersaturation limit
The theoretical pressure ratio between tissue gas concentration and ambient pressure above which the probability of bubble formation is unacceptably high.[6]
Supervised Diver

Main article: Supervised Diver

EN 14153-1 / ISO 24801-1 standard competence for recreational scuba diver. The level 1 "Supervised Diver" has sufficient knowledge, skill and experience to dive, in open water, to a recommended maximum depth of 12 m, which do not require in-water decompression stops, under the direct supervision of a dive leader, in groups of up to four level 1 scuba divers per dive leader provided the dive leader is capable of establishing physical contact with all level 1 scuba divers at any point during the dive, only when appropriate support is available at the surface, and under conditions that are equal or better than the conditions where they were trained.[98]

See: diving supervisor

supply lock
Small lock on a saturation life support habitat for transfer of relatively small items.

See: surface decompression


See: Breaking wave

The mass or line of broken water formed by waves breaking on a shore or reef
surface air consumption rate
A measure of air consumption in units of pressure over time, usually psi/minute, adjusted to surface pressure, used to estimate air endurance of a cylinder of specific size. Useful for those who work in imperial units.[57] SAC has a constant value for a given diver and represents gas used on the surface at rest.[47] Surface gas consumption (SGC) is an alternative term referring to alternative breathing gas mixtures. Occasionally also termed surface consumption rate (SCR)[99][100]
surface compression chamber
A hyperbaric chamber for surface use for routine decompression or therapeutic recompression.[20]
surface decompression

See: Decompression practice#Surface decompression

A procedure in which some or all of the staged decompression obligation is done in a decompression chamber immediately after surfacing instead of in the water.
surface detection aids
Equipment, such as flags, surface marker buoys, flares, EPIRBs, radio beacons mirrors, and whistles, carried by divers to maintain contact with dive boats or attract rescue when lost at sea.
surface equivalent volume

See also: free gas volume

Gas volume calculated as expanded to surface pressure.
surface interval

See: Decompression practice#Surface interval

The time spent by a diver at surface pressure after a dive during which inert gas which was still present at the end of the dive is further eliminated from the tissues.
surface marker buoy

Also: "SMB"

Main article: Surface marker buoy

A buoy towed by a diver to indicate the diver's position to people at the surface.
surface orientated diving
Any diving operation in which the diver is decompressed to surface pressure after the dive. Compare to saturation diving .[20]
Southern Underwater Research Group. An organisation based in Cape Town, which publishes marine ecology field guides, dive travel guides and underwater maps.[101]

See: Waves and shallow water

Reciprocating water movement parallel to the bottom surface caused by the passing of a wave overhead, by analogy with the transient linear motion of a ship in the direction of travel also called surge.
sustained load cracking

Main article: Sustained load cracking

The development of cracks in a material subjected over long term to static stress significantly less than the yield stress. There is a low but significant risk of this mode of failure in pressure vessels of AA6351 aluminium alloy.[102]

Main article: Swell (ocean)

A series of surface gravity waves that is not generated by the local wind.
swim line
Line used to space divers across the search area for a swim line search. Each diver holds the line at a distance from the previous diver of somewhat less than twice the visibility distance. The line is used to keep the divers spaced evenly across the search area while swimming perpendicular to the line.
A submerged arch or short tunnel that a diver can swim through without needing to remove equipment. It is usually possible to see natural light at the far end in good visibility and illumination. Usually refers to a natural formation. Technically an overhead environment, but usually with no risk of getting lost, though entrapment may be possible.


tank factor

See: baseline


Main article: Taravana

A form of decompression sickness originally observed among Polynesian island natives who habitually did multiple repetitive deep breath-hold dives.[103][104]
task loading

Main article: Task loading

A multiplicity of responsibilities leading to an increased risk of failure on the part of the diver to undertake some key basic function which would normally be routine for safety
taut wire system
A constant tension wire from a vessel to a weight on the seabed used as a reference to detect movement of a dynamically positioned vessel from the reference point.[4]
technical diving

Main article: Technical diving

An extension of the scope of recreational scuba diving to applications with greater technical complexity and higher inherent risk. Definitions vary, but diving with multiple breathing gases, helium based gases, closed circuit rebreathers, or under extensive overheads are generally considered as technical diving. There is no sharp distinction from other forms of recreational diving.
tech ring
D-ring welded to a belt slide so that it can not fold down against the webbing. Intended to make it easier to fit and remove snaps.[7]
temperature stick

Also: "thermal profile monitor", "TPM"

An array of temperature sensors mounted in a rebreather scrubber canister along the path of gas flow to monitor the temperature as an indication of the advance of the exothermic reaction front of the scrubber, providing an indication of scrubber depletion.[28]
tension leg rig

Main article: Tension-leg platform

A vertically moored floating structure normally used for the offshore production of oil or gas, particularly suited for water depths between 300 and 1500 meters. The platform is permanently moored by means of tethers at each of the structure's corners and virtually all vertical motion of the platform is eliminated.
test of pressure
Diagnostic procedure for decompression sickness. The diver is recompressed, and if the symptoms reduce, it may be assumed that the diver has decompression sickness and hyperbaric treatment will be effective. The test is not entirely reliable[105]
test pressure
Pressure at which a gas storage cylinder will be hydrostatically tested for revalidation. Usually 1.5 or 1.67 x working pressure.[31]
tethered ascent

See also: Scuba skills#Emergency ascents

Ascent controlled by a line from the diver to a fixed point at the bottom. This may be used to control depth and rate of ascent when the diver has inadvertently lost complete control of buoyancy due to loss of ballast weight, so cannot attain neutral buoyancy at some point during the ascent, and needs to do decompression. CMAS require this skill for their Self-Rescue Diver certification.[106]
tethered diving
Diving with a lifeline between the diver and a surface tender.
Thalmann algorithm

See also: VVAL18

Main article: Thalmann algorithm

The Exponential/linear decompression algorithm used in the 2008 US Navy decompression tables
therapeutic recompression

See: Hyperbaric medicine, Decompression practice#Therapeutic decompression, and Decompression sickness#Treatment

A procedure for treating decompression sickness by recompressing the diver, thus reducing bubble size, and allowing the gas bubbles to re-dissolve, then decompressing slowly enough to avoid further formation or growth of bubbles, or eliminating the inert gases by breathing oxygen under pressure
therapeutic schedule
Procedure for hyperbaric treatment involving recompression to relieve symptoms, followed by decompression at a rate unlikely to cause a relapse. Use of special breathing gas, particularly oxygen, to increase the rate of elimination of inert gases is common.[20]
thermal profile monitor

See: temperature stick


Main article Thermocline

A thin but distinct layer in a large body of fluid, in which temperature changes more rapidly with depth than it does in the layers above or below
thermodynamic decompression model

See: Decompression theory#The no-supersaturation approach

Hypothesis that bubble formation during decompression will not occur provided absolute ambient pressure exceeds the total of the partial gas tensions in the tissue for each gas.

See: rule of thirds

three part shackle

Also: "bolt and nut shackle", "bolt shackle"

A shackle which uses a bolt as the pin, secured with a nut. The nut may be locked with a split pin for greater security. The bolt may rotate in the shackle under load without great risk of unscrewing the pin
through-water communications
Wireless voice communications transmitted through the water
thumb the dive
Terminate the dive by signalling exit to surface at a time or place other than the planned turning point.[7]
Friction initiated noisy but relatively harmless pyrotechnic device designed for military exercises, with civilian use for diver recall.[107][108]
time to fly
The surface interval necessary after diving to reduce tissue gas concentrations to a level where the risk of decompression sickness due to the pressure reduction experienced in normal commercial airliners is acceptable.[11]

Main article Tinnitus

The perception of sound within the ear in the absence of corresponding external sound. Usually a constant tone.
tissue compartments

See: Decompression theory#Tissue compartments

Hypothetical body tissues which are designated as fast and slow to describe the rate of saturation.
tissue half times

See: Decompression theory#Tissue half times

The time it takes for the tissue to take up or release 50% of the difference in dissolved gas capacity at a changed partial pressure.
1.  Total Nitrogen Time: Equivalent time of hyperbaric exposure for a repetitive dive used with some decompression tables.
2.  Trinitrotoluene, a high explosive
toolbox talk

Also: "TBT"

A meeting held at the start of each shift or prior to any project critical operation, where the diving supervisor or the diving supervisor’s delegate and shift personnel discuss the forthcoming tasks or jobs and the potential risks and necessary precautions to be taken.[13]
top up

See also: air top

To reconnect a partially filled cylinder and add gas until the pressure is within tolerance of the required charging pressure when corrected for temperature.
touch contact signals

See: Rimbach system

Toynbee manoeuvre

Main article Ear clearing

Method of equalising the middle ears by pinching the nose and swallowing.
trait anxiety

See: Anxiety#Trait

A tendency to respond with anxiety in the anticipation of threatening situations.

Main article Transect

A path along which one counts and records occurrences of the phenomena of study.

Also: "decant"

Fill cylinder with gas by transfer from a cylinder with higher pressure.
transfill whip
High pressure hose and end fittings used to decant between cylinders. Usually includes purge valve and may include pressure gauge.
trauma shears

Also: "bandage scissors" or "paramedic shears"

Main article Trauma shears

Blunt tipped slightly serrated shears with angled blades sometimes used by divers as a safety cutting implement in place of a knife or line cutter.
travel gas
Gas mixture used for descent and ascent when the bottom gas is not suitable for breathing at shallower depths.
Pass through a cave by entering at one point and exiting at another.[7]
treatment table
A depth, time and breathing gas profile designed to treat a diver for decompression illness.[13]

Main article Tremie

A pipe, through which concrete is placed below water level. The top of the tremie is above water and open, and the bottom end is kept below the surface of the poured concrete.
triangular profile
A triangular dive profile is one in which, after a descent at constant rate, and a short bottom time at maximum depth, the diver maintains a constant, slow ascent to the surface or first decompression stop. A plot of depth against elapsed time takes a triangular shape.

See: belt slider

Material used for dry suit shells made of a layer of waterproof rubber laminated between two layers of woven textile.

Main article Trimix (breathing gas)

Mixture of three gases for breathing. Oxygen, nitrogen and helium are the gases used.[65] The gas fractions will usually be specified.
trim weight

See Diving weighting system#Trim

Ballast weight placed to improve a diver's trim.
tripping line
A line attached to the top of an open parachute lifting bag and at the other end to an anchor point. Its purpose is to invert and thus empty the bag if it becomes detached from the load.[13]
Guided dive using unfamiliar equipment as a marketing technique, in the hope of convincing the diver that the product is worth buying. Some instruction or guidance on the use and characteristics of the product is usual. Similar in concept to test driving a motor vehicle, and similarly, generally offered to qualified operators, except for rebreathers, where one qualifies on a specific type.

Also: "Kelly tubes" or "Kellys"

A seamless transportable compressed gas container, with a water capacity exceeding 150 litres (5.3 cu ft) but not more than 3,000 litres (110 cu ft);[31] Often mounted horizontally in manifolded groups on a trailer or intermodal container frame.[109]
Transfer Under Pressure: Transfer of personnel between hyperbaric environments, usually between a closed bell and a saturation system, or between a portable recompression chamber and a multi-occupant chamber

Main article Turbidity

The cloudiness or haziness of a fluid caused by individual particles (suspended solids) that may be invisible to the naked eye, similar to smoke in air
turn the dive
Start the return on a dive which has reached the planned turning point in terms of depth, time, gas supply or distance.[7]
twilight zone
Deeper than 60 m in the sea, or the part of a cave or cavern that has dim but discernible ambient light.
type 1 wet bell

See: Diving bell#Type 1 wet bell

Wet diving bell (q.v.) with no direct supply of gas and services to the bell. Diver umbilicals lead directly to the surface control point.
type 2 wet bell

See: Diving bell#Type 2 wet bell

Wet diving bell (q.v.) with umbilical supply of gas and other services to the bell, from which they are distributed to the divers umbilicals from a control panel in the bell.


Underwater Breathing Apparatus: Equipment used to supply breathing gas to an underwater diver. Usually refers to the part of the system carried underwater by the diver.
UDT vest
Underwater Demolition Team vest, An inflatable surface life-jacket worn by underwater demolition teams. Similar in style and a precursor to the horse-collar style buoyancy compensator.
Undersea and Hyperbaric Medical Society
umbilical cable

See: Umbilical cable#Diver

See also: bell umbilical, diver umbilical, excursion umbilical, and ROV umbilical

Life support hose and cable bundle connection to a surface supplied diver or diving bell. Comprises gas supply hoses, pneumofathometer, a strength member and communications cable, and may also include gas reclaim hose, hot water hose and hoses for hydraulic or pneumatic power, and electrical and optical cables for ancillary equipment. Underwater television cameras and cabling can also be carried as a component part of the umbilical or can be taped or banded to it on a temporary basis. Also refers to the power, control and instrument cable for a ROV.[13]
umbilical changeout
Emergency procedure to disconnect the umbilical and connect a replacement in situ during the dive. Usually only used when a delay in recovering the diver or bell is likely.
umbilical cutter
mechanism fitted to a closed bell which allows the occupants to sever the bell umbilical from inside the sealed and pressurised bell in the event of an umbilical snag that prevents bell recovery. The device is typically hydraulically operated using a hand pump inside the bell, and can shear the umbilical at or just above the point where it is fastened to the top of the bell.[110]

Main article Undertow (water waves)

A subsurface flow of water returning seaward from shore as result of wave action
underwater blackout syndrome

Also: "hyperventilation-induced blackout"

Loss of consciousness due to hypoxia during a breath-hold submersion preceded by hyperventilation where alternative causes of blackout have been excluded.[111][112]

See: speech unscrambler


Also: "Jersey upline"

A fairly substantial natural fibre rope which is deployed from the bottom using a small lift bag to provide the equivalent of a shotline. The lower end is tied off to the bottom, usually on a wreck, and the diver ascends on the line to avoid being swept away from the site by currents. After reaching the surface, the last diver cuts the line and it sinks back down, Natural fibre is used so the line rots away over a few years.[72]
Against the flow.
upstream valve

See: Diving regulator#Upstream vs downstream

Valve, (usually regulator first stage or demand valve), where the valve mechanism moves against the flow when opening, and the pressure difference over the valve tends to close it.

Main article Upwelling

An oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water


Valsalva maneuver

Main article: Valsalva maneuver

Technique for equalising the middle ear by moderately forceful attempted exhalation against a closed mouth and blocked nose
valve cage

See: Scuba cylinder valve#Valve cage

Structure or frame fitted to scuba cylinder to protect the cylinder valve or manifold and regulator first stage from impact damage and roll-off .
valve drill
Safety exercise in which the diver shuts down, tests regulators and re-opens the manifold valves on a twin set in a specific order.
valve guard
protective structure or frame fitted to the top of a bulk storage cylinder to protect the cylinder valve from mechanical damage.[31]
Enclosed portable compartment with diving spread support equipment. Often built into an intermodal container. Exanples:
  • DDC van, containing control panels for deck decompression chamber operation,
  • Machinery van, containing hydraulic power pack, compressor, air banks or similar equipment
Van der Waals equation

Main article: Van der Waals equation

Thermodynamic equation of state for a real (non-ideal) gas.

Main article: Vasoconstriction

The narrowing of blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles.

Main article: Vasodilation

The widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large veins, large arteries, and smaller arterioles.
venous gas embolism

See: Decompression (diving)#Bubble formation, growth and elimination

Inert gas bubbles formed in the venous circulation.
venting breath
Breathing pattern intended to vent gas from a rebreather loop, usually by exhaling through the nose.[28]
vertical entry
vertical drop entry

See: Scuba skills#Entries

An entry technique for relatively high drops, up to and sometimes exceeding 3 m. The feet are overlapped and the legs kept straight. The body and head are kept vertical and the mask and DV held against the face with one or both hands, elbows tucked in. The intention is to hit the water vertically, with the least likelihood of knocking off or damaging vital equipment.[95]: 249 

Main article: Vertigo

A type of dizziness, where there is a feeling of motion when one is stationary.

See: venous gas embolism

1.  Glass or plastic window on a diving helmet or mask.
2.  Window on a hyperbaric chamber or manned submersible .
Visual Inspection Programme (US). Annual visual internal inspection of a scuba cylinder.

See: Underwater vision#Visibility, turbidity and nephelometry

The distance through the water at which an object can just be seen against the background. Often defined as the distance at which a Secchi disc perpendicular to the sight line can first be seen when moving towards it. It can vary depending on direction illumination, and depth.
visual gap
Gap between guidelines which is small enough that each line can be seen from the other.[7]
visual jump
The procedure of crossing a visual gap (q.v.) without the use of a jump line.[7]
visual inspection
Internal and external inspection of a pressure vessel as part of revalidation procedure[31]
Visual Plus

See: Eddy-current testing

Crack detection test of parallel neck threads of aluminium cylinders

Main article: Viton

Synthetic elastomer suitable for oxygen service O-rings.
volume tank
A pressure vessel connected to the outlet of a gas supply and used as a gas reservoir.[13]
voting algorithm
voting logic

See: Redundancy (engineering)#Voting logic, Rebreather diving#Managing cell failure in an electronic rebreather control system

The logical procedure in which rebreather electronics compare output from multiple sensors when sensors produce significantly different values, suggesting that one or more are faulty, and choose which signals should be ignored, assuming statistical independence of the sensors, which may not be valid.[28]

Main article: Varying Permeability Model

(also Variable permeability model). A decompression model and associated algorithms based on bubble dynamics.

Also: "E-L algorithm", "Exponential-Linear algorithm", "Thalmann algorithm"

Main article: Thalmann algorithm

Exponential-Linear algorithm used for the 2008 US Navy tables, which assumes exponential ingassing and a combination of linear and exponential outgassing rates.


wall diving

See: Underwater diving#Diving environment

Recreational scuba diving along the face of a near vertical cliff wall, particularly if the bottom is below the range of the diver's equipment and certification. This requires good buoyancy control.
water capacity
Of a cylinder: The internal volume. The amount of water it would hold at ambient pressure at 20 °C (68 °F)[31]
water trap
Mechanism to trap liquid water carried by compressed gas.
weight belt

See: Diving weighting system#Weight belt

Ballasted waist belt worn by divers to compensate for excess buoyancy. For scuba and freediving, usually easily removed to establish positive buoyancy in an emergency.
weight harness

See: Diving weighting system#Weight harness

Webbing strap system to support diving weights, usually suspended from the shoulders and fastened around the waist. The harness may carry the weights directly, or they may be carried in pockets on the harness. For scuba diving the weights may be arranged for easy shedding.
weighting system

Main article: Diving weighting system

Weights, generally made of lead, to counteract the buoyancy of other diving equipment, and the belts, pockets or harnesses used to support them.
weight slider
weight stop

See: belt slider

welding shield
Cover for the viewport area of a helmet or mask to filter excessive light and UV when welding or oxy-arc cutting.

Main article: Wellhead

The assembly at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment.
wet bell

See: Diving bell#Structure of a typical wet bell

A mobile platform used to deploy and recover divers to and from working depth fitted with an air dome and on board emergency gas supply for use as safe haven in emergencies. There may be a main supply umbilical from the surface providing breathing gas to a manifold inside the wet bell and diver excursion umbilicals terminated at the wet bell, or the divers' umbilicals may be direct from the surface.[13]
wet filling

See: Diving cylinder#Temperature change during filling

Filling scuba cylinders using a water bath for cooling the cylinders.[55]
wet notes
A small notebook of waterproof paper carried by some divers[113]
wet pot
Water filled hyperbaric chamber, generally for experimental work or training.

Main article: Wetsuit

A close fitting, thermally-insulating, foam neoprene diving suit that allows a limited volume and movement of water inside the suit.
Flexible high pressure gas hose with connector at the free end, used for temporary connections. The other end may be permanently connected to an installation or other equipment, or may also have a temporary connector. Whips are commonly named for their intended use, e.g. filling whip, for filling cylinders, decanting, transfill or transfer whip for decanting between cylinders, oxygen whip for oxygen transfer, blending whip, for decanting gases when blending gas, etc. Accessories may include a flow control valve, bleed valve, pressure gauge, and/or whip check.[9]
whip check
A cable or webbing strap connecting a hose end to the attachment point in addition to the hose end fitting, which restrains the movement of the hose if the connection is broken under pressure. Whip checks connecting two hose ends may also be attached to an anchor point to limit motion further if this is practicable.[114]
whip sock

Also: containment grip[115]

Whip check device which contains a short section of the whip within a braided tube which reduces wear and point loading on the hose, and constrains motion of the hose end more than a standard whip check in case of disconnection under pressure.[116]

Main article: Backplate and wing

Back inflation buoyancy compensator cell.
Woodville Karst Plain Project, a project to survey subterranean aquifers in Florida.
woolly bear
A wool or synthetic pile thermal under-suit worn under a diving dry suit, particularly with standard diving dress, often one-piece.[117]
working pressure

Also: "charging pressure"

Maximum filling pressure rating for the cylinder at standard temperature.[31]
work of breathing

Also: "WOB"

Main article: Work of breathing

See also: Breathing performance of regulators

The effort expended in inhaling and exhaling the breathing gas.
wreck diving

Main article: Wreck diving

Recreational or technical diving on and inside of shipwrecks.
wrist slate
A small plastic writing surface attached to the diver's wrist



yoke adaptor
A fitting used to connect a regulator or filling whip with a DIN thread connection to a CGA 850 "international" connection cylinder valve.[1]
yoke fitting
yoke valve
A fitting or valve used to connect a regulator or filling whip to a diving cylinder using the CGA 850 "international" connection.[1]

Also: "slingshot valve"

See: Scuba cylinder valve#Dual outlet valves

Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to H-valve but in Y configuration.


Bühlmann decompression algorithms. Also ZHL-16a, b and c
zip tie

Also: "cable tie", "tie wrap"

Main article: Cable tie

Self-locking plastic strip used to connect objects together.
Line cutting tool with a replaceable blade in a slot.


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External links[edit]