Unlike earlier diving, which relied either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas, (usually compressed air), allowing them greater freedom of movement than with an air line. Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in free-diving.
The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed air carried in back mounted cylinders is inhaled through a demand regulator and then exhaled into the water adjacent to the tank. The single hose two stage scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first example of this type of regulator, known as the Porpoise, which was developed because patents protected the Aqualung's twin hose design. The single hose regulator separates the cylinder from the demand valve, giving the diver air at the pressure at his mouth, not that at the top of the cylinder.
The open circuit compressed air systems were developed after Cousteau had a number of incidents of oxygen toxicity using an oxygen rebreather, in which exhaled oxygen is passed through an absorbent chemical to remove carbon dioxide before being breathed again. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are available, and form the second main type of scuba unit, mostly used for technical and military diving.
The term "SCUBA" (an acronym for self-contained underwater breathing apparatus) originally referred to United States combat frogmen's oxygen rebreathers, developed during World War II by Christian J. Lambertsen for underwater warfare.
"SCUBA" was originally an acronym, but is now generally used as a common noun or adjective, "scuba". It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.
Diving activities associated with scuba
Scuba diving may be performed for a number of reasons, both personal and professional. Recreational diving is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.
Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba.
There are a fair number of divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised by national legislation.
Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform search and recovery or search and rescue operations and to assist with the detection of crime which may involve bodies of water. In some cases diver rescue teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving.
Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology.
The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.
Diving activities commonly associated with scuba may include:
|Type of diving activity||Classification|
|aquarium maintenance in large public aquariums||commercial, scientific|
|boat and ship inspection, cleaning and maintenance||commercial, naval|
|cave diving||technical, recreational, scientific|
|fish farm maintenance (aquaculture)||commercial|
|fishing, e.g. for abalones, crabs, lobsters, scallops, sea crayfish,||commercial|
|frogman, manned torpedo||military|
|media diving: making television programs, etc.||professional|
|mine clearance and bomb disposal, disposing of unexploded ordnance||military, naval|
|pleasure, leisure, sport||recreational|
|policing/security: diving to investigate or arrest unauthorized divers||police diving, military, naval|
|search and recovery diving||public safety, police diving|
|search and rescue diving||police, naval, public service|
|surveys and mapping||scientific, recreational|
|scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology and medicine)||scientific|
|underwater archaeology (shipwrecks; harbors, and buildings)||scientific, recreational|
|underwater inspections and surveys (occasionally)||commercial, military|
|underwater photography||professional, recreational|
|underwater tour guiding||professional, recreational|
Water normally contains the dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills. Humans lack gills and do not otherwise have the capacity to breathe underwater unaided by external devices. Although the feasibility of filling and artificially ventilating the lungs with a dedicated liquid (liquid breathing) has been established for some time, the size and complexity of the equipment allows only for medical applications with current technology.
Early diving experimenters quickly discovered it is not enough simply to supply air to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs—approximately 1 bar (14.7 pounds per square inch) for every 33 feet (10 m) of depth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs. It becomes virtually impossible to breathe unpressurised air through a tube below three feet under the water.
By always providing the appropriate breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and without excessive effort, regardless of depth.
Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.
The most commonly used scuba set today is the "single-hose" open circuit 2-stage diving regulator, connected to a single high pressure gas cylinder, with the first stage connected to the cylinder valve and the second stage at the mouthpiece. This arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung, in which the cylinder pressure was reduced to ambient pressure in one or two stages which were all in the housing mounted to the cylinder valve or manifols. The "single-hose" system has significant advantages over the original system for most applications.
In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of up to about 240 bar (3000 psi) to an intermediate level of about 10 bar (145 psi) above ambient pressure. The second stage demand valve regulator, supplied by a low pressure hose from the first stage, delivers the breathing gas at ambient pressure to the diver's mouth. The exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one outlet port delivering breathing gas at unreduced tank pressure. This is connected to the diver's submersible pressure gauge or dive computer, to show how much breathing gas remains in the cylinder.
Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers, which unlike open-circuit sets that vent off all exhaled gases, process each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.
Rebreathers release little or no gas bubbles into the water, and use much less stored gas volume for an equivalent depth and time because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout and used the first electronic control system. Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes.
In a closed-circuit rebreather the oxygen partial pressure in the rebreather is controlled, so it can be increased to a safe continuous maximum, which reduces the inert gas (nitrogen and/or helium) partial pressure in the breathing loop. Minimising the inert gas loading of the diver's tissues for a given dive profile reduces the decompression obligation. This requires continuous monitoring of actual partial pressures with time and for maximum effectiveness requires real-time computer processing by the diver's decompression computer. Decompression can be much reduced compared to fixed ratio gas mixes used in other scuba systems and, as a result, divers can stay down longer or decompress faster. A semi-closed circuit rebreather injects a constant flow of a fixed nitrox mixture into the breathing loop, or changes a fixed percentage of the respired volume, so the partial pressure of oxygen at any time during the dive depends on the diver's oxygen consumption or breathing rate. Planning decompression requirements requires a more conservative approach for a SCR than for a CCR, but decompression computers with a real time oxygen partial pressure input can optimise decompression for these systems.
For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used, so long as the diver is properly trained in their use. The most commonly used mixture is Nitrox, also referred to as Enriched Air Nitrox (EAN), which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the likelihood of decompression sickness or allowing longer exposure to the same pressure for equal risk. The reduced nitrogen may also allow for no stops or shorter decompression stop times and a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.
Several other common gas mixtures are in use, and all need specialized training for safe use. The increased oxygen levels in nitrox help reduce the risk of decompression sickness; however, below the maximum operating depth of the mixture, the increased partial pressure of oxygen can lead to an unacceptable risk of oxygen toxicity. To displace nitrogen without the increased oxygen concentration, other diluents can be used, usually helium, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox.
For technical dives, some of the cylinders may contain different gas mixtures for the various phases of the dive, typically designated as Travel, Bottom, and Decompression gases. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.
Controlling buoyancy underwater
To dive safely, divers must control their rate of descent and ascent in the water. Ignoring other forces such as water currents and swimming, the diver's overall buoyancy determines whether he ascends or descends. Equipment such as diving weighting systems, diving suits (wet, dry or semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy. When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimizes gas consumption caused by swimming to maintain depth.
The buoyancy force on the diver is the weight of the volume of the liquid that he and his equipment displace minus the weight of the diver and his equipment; if the result is positive, that force is upwards. The buoyancy of any object immersed in water is also affected by the density of the water. The density of fresh water is about 3% less than that of ocean water. Therefore, divers who are neutrally buoyant at one dive destination (e.g. a fresh water lake) will predictably be positively or negatively buoyant when using the same equipment at destinations with different water densities (e.g. a tropical coral reef).
The removal ("ditching" or "shedding") of diver weighting systems can be used to reduce the diver's weight and cause a buoyant ascent in an emergency.
Diving suits made of compressible materials decrease in volume as the diver descends, and expand again as the diver ascends, causing buoyancy changes. Diving in different environments also necessitates adjustments in the amount of weight carried to achieve neutral buoyancy. The diver can inject air into dry suits to counteract the compression effect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy. For open circuit divers, changes in the diver's average lung volume during a breathing cycle can be used to make fine adjustments of buoyancy.
Neutral buoyancy in a diver is a metastable state. It is changed by small differences in ambient pressure caused by a change in depth, and the change has a positive feedback effect. A small descent will increase the pressure, which will compress the gas filled spaces and reduce the total volume of diver and equipment. This will further reduce the buoyancy, and unless counteracted, will result in sinking more rapidly. The equivalent effect applies to a small ascent, which will trigger an increased buoyancy and will result in accelerated ascent unless counteracted. The diver must continuously adjust buoyancy or depth in order to remain neutral. This is a skill which improves with practice until it becomes second nature.
Water has a higher refractive index than air – similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people.
Diving masks and helmets solve this problem by providing an air space in front of the diver's eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in salt water than they actually are. Therefore total field-of-view is significantly reduced and eye–hand coordination must be adjusted.
(This also affects underwater photography: a camera seeing through a flat port in its housing is affected in the same way as its user's eye seeing through a flat mask viewport, and so its operator must focus for the apparent distance to target, not for the real distance.)
Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Generic and custom corrective lenses are available for some two-window masks. Custom lenses can be bonded onto masks that have a single front window or two windows.
As a diver descends, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalization. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze.
Water preferentially absorbs red light, and to a lesser extent, yellow and green light, so the color that is least absorbed by water is blue light.
|Color||Average wavelength||Approximate depth of total absorption|
|Ultraviolet||300 nm||25 m|
|Violet||400 nm||100 m|
|Blue||475 nm||275 m|
|Green||525 nm||110 m|
|Yellow||575 nm||50 m|
|Orange||600 nm||20 m|
|Red||685 nm||5 m|
|Infra-red||800 nm||3 m|
A diver cannot talk underwater unless he is wearing a full-face mask, but divers can communicate, using hand signals.
Table of Hand Signals
|1.||Hand raised, fingers pointed up, palm to receiver.||STOP||Transmitted in the same way as a traffic police officer’s STOP|
|2.||Thumb extended downward from clenched fist.||GO DOWN or GOING DOWN|
|3.||Thumb extended upward from clenched fist.||GO UP or GOING UP|
|4.||Thumb and forefinger making a circle with three remaining fingers extended (if possible).||OK! or OK?||Divers wearing mittens may not be able to extend 3 remaining fingers distinctly.|
|5.||Two arms extended overhead with finger tips touching above head to make a large O shape.||OK! or OK?||A diver with only one free arm may make this signal by extending that arm overhead with finger tips touching top of head to make the O shape. Signal is for long-range use.|
|6.||Hand flat, fingers together, palm down, thumb sticking out, then hand rocking back and forth on axis of forearm.||SOMETHING IS WRONG||This is the opposite of OK! The signal does not indicate emergency.|
|7.||Hand waving over head (may also thrash hand on water).||DISTRESS||Indicates immediate aid required.|
|8.||Fist pounding on chest.||LOW ON AIR||Indicates signaler's air supply is reduced.|
|9.||Hand slashing or chopping throat.||OUT OF AIR||Indicates that the signaler cannot breathe.|
|10.||Clenched fist on arm extended in direction of danger.||DANGER|
All signals are to be answered by the receivers repeating the signal as sent. When answering signals 7 & 9, the receiver should approach to offer aid to signaler.
Hazards of scuba diving
According to a 1970 North American study, diving was (on a man-hours based criteria) 96 times more dangerous than driving an automobile. According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving. A big difference between the risks of driving and diving is that the diver is less at risk from fellow divers than the driver is from other drivers.
Injuries due to changes in pressure
Divers must avoid injuries caused by changes in pressure. The weight of the water column above the diver causes an increase in pressure in proportion to depth, in the same way that the weight of the column of atmospheric air above the surface causes a pressure of 101.3 kPa (14.7 pounds-force per square inch) at sea level. This variation of pressure with depth will cause compressible materials and gas filled spaces to tend to change volume, which can cause the surrounding material or tissues to be stressed, with the risk of injury if the stress gets too high. Pressure injuries are called barotrauma and can be quite painful, even potentially fatal – in severe cases causing a ruptured lung, eardrum or damage to the sinuses. To avoid barotrauma, the diver equalizes the pressure in all air spaces with the surrounding water pressure when changing depth. The middle ear and sinus are equalized using one or more of several techniques, which is referred to as clearing the ears.
The scuba mask (half-mask) is equalized during descent by periodically exhaling through the nose. During ascent it will automatically equalise by leaking excess air round the edges. A helmet or full face mask will automatically equalise as any pressure differential will either vent through the exhaust valve or open the demand valve and release air into the low pressure space.
If a drysuit is worn, it must be equalized by inflation and deflation, much like a buoyancy compensator. Most dry suits are fitted with an auto-dump valve, which, if set correctly, and kept at the high point of the diver by good trim skills, will automatically release gas as it expands and retain a virtually constant volume during ascent. During descent the dry suit must be inflated manually.
Although there are many dangers involved in scuba diving, divers can decrease the risks through proper procedures and appropriate equipment. The requisite skills are acquired by training and education, and honed by practice. Open-water certification programs highlight diving physiology, safe diving practices, and diving hazards, but do not provide the diver with sufficient practice to become truly adept.
Effects of breathing high pressure gas
The prolonged exposure to breathing gases at high partial pressure will result in increased amounts of non-metabolic gases, usually nitrogen and/or helium, (referred to in this context as inert gases) dissolving in the bloodstream as it passes through the alveolar capillaries, and thence carried to the other tissues of the body, where they will accumulate until saturated. This saturation process has very little immediate effect on the diver. However when the pressure is reduced during ascent, the amount of dissolved inert gas that can be held in stable solution in the tissues is reduced. This effect is described by Henry's Law.
As a consequence of the reducing partial pressure of inert gases in the lungs during ascent, the dissolved gas will be diffused back from the bloodstream to the gas in the lungs and exhaled. The reduced gas concentration in the blood has a similar effect when it passes through tissues carrying a higher concentration, and that gas will diffuse back into the bloodsteam, reducing the loading of the tissues.
As long as this process is gradual, all will go well and the diver will reduce the gas loading by diffusion and perfusion until it eventually re-stabilises at the current saturation pressure. The problem arises when the pressure is reduced more quickly than the gas can be removed by this mechanism, and the level of supersaturation rises sufficiently to become unstable. At this point, bubbles may form and grow in the tissues, and may cause damage either by distending the tissue locally, or blocking small blood vessels, shutting off blood supply to the downstream side, and resulting in hypoxia of those tissues.
This effect is called decompression sickness or 'the bends', and must be avoided by reducing the pressure on the body slowly while ascending and allowing the inert gases dissolved in the tissues to be eliminated while still in solution. This process is known as "off-gassing", and is done by restricting the ascent (decompression) rate to one where the level of supersaturation is not sufficient for bubbles to form. This is done by controlling the speed of ascent and making periodic stops to allow gases to be eliminated. The procedure of making stops is called staged decompression, and the stops are called decompression stops. Decompression stops that are not computed as strictly necessary are called safety stops, and reduce the risk of bubble formation further. Dive computers or decompression tables are used to determine a relatively safe ascent profile, but are not completely reliable. There remains a statistical possibility of decompression bubbles forming even when the guidance from tables or computer has been followed exactly.
Decompression sickness must be treated as soon as practicable. Definitive treatment is usually recompression in a recompression chamber with hyperbaric oxygen treatment. Exact details will depend on severity and type of symptoms, response to treatment, and the dive history of the casualty. Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although death or permanent disability may still occur.
Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anesthesia. Being "narced" can impair judgment and make diving very dangerous. Narcosis starts to affect some divers at 66 feet (20 m). At this depth, narcosis manifests itself as a slight giddiness. The effects increase drastically with the increase in depth. Almost all divers are able to notice the effects by 132 feet (40 meters). At these depths divers may feel euphoria, anxiety, loss of coordination and lack of concentration. At extreme depths, hallucinogenic reaction and tunnel vision can occur. Jacques Cousteau famously described it as the "rapture of the deep". Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, so that divers often fail to realize they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions. However, diving with trimix or heliox dramatically reduces the effects of inert gas narcosis.
Oxygen toxicity occurs when oxygen in the body exceeds a safe partial pressure (PPO2). In extreme cases it affects the central nervous system and causes a seizure, which can result in the diver spitting out his regulator and drowning. While the exact limit is idiomatic, it is generally recognized that Oxygen toxicity is preventable if one never exceeds an oxygen partial pressure of 1.4 bar. For deep dives—generally past 180 feet (55 m), divers use "hypoxic blends" containing a lower percentage of oxygen than atmospheric air. For more information, see oxygen toxicity.
Hazards due to failure of diving equipment
Hazards of the diving environment
Loss of body heat
Water conducts heat from the diver 25 times better than air, which can lead to hypothermia even in mild water temperatures. Symptoms of hypothermia include impaired judgment and dexterity, which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need the thermal insulation provided by wetsuits or drysuits.
In the case of a wetsuit, the suit is designed to minimize heat loss. Wetsuits are usually made of neoprene that has small closed gas cells, generally nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene, and to a larger extent the nitrogen gas, in this case acts as an insulator. The effectiveness of the insulation is reduced when the suit is compressed due to depth, as the nitrogen filled bubbles are then smaller and conduct heat better.
The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver's skin and the insulating suit itself. Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at all openings (neck, wrists, ankles zippers and overlaps with other suit components), this reduces flow of cold water over the surface of the skin, and thereby reduces loss of body heat by convection, which helps keep the diver warm (this is the principle employed in the use of a "Semi-Dry" wetsuit)
In the case of a drysuit, it does exactly what the name implies: keeps a diver dry. The suit is waterproof and sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are usually worn under a drysuit to keep a layer of air inside the suit for better thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit. Usually this bottle contains argon gas, because of its better insulation as compared with air. Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.
Drysuits fall into two main categories: neoprene and membrane; both systems have their good and bad points but generally their thermal properties can be reduced to:
- Membrane or Shell drysuits: usually a trilaminate construction; owing to the thinness of the material (around 1 mm), these require an undersuit, usually of high insulation value if diving in cooler water.
- Neoprene drysuits: a similar construction to wetsuits; these are often considerably thicker (7–8 mm) and have sufficient insulation to allow a lighter-weight undersuit (or none at all); however on deeper dives the neoprene can compress to as little as 2 mm thus losing a proportion of its insulation. Compressed or crushed neoprene may also be used (where the neoprene is pre-compressed to 2–3 mm) which avoids the variation of insulating properties with depth. These drysuits function more like a membrane suit.
Injuries due to contact with the solid surroundings
Hazards of marine animals
Hazards inherent in the diver
Pre-existing physiological and psychological conditions in the diver
Diver behaviour and competence
Inadequate learning or practice of critical safety skills may result in the inability to deal with minor incidents, which consequently may develop into major incidents.
Overconfidence can result in diving in conditions beyond the diver's competence, with high risk of accident due to inability to deal with known environmental hazards.
Inadequate strength or fitness for the conditions can result in inability to compensate for difficult conditions even though the diver may be well versed at the required skills, and could lead to over-exertion, overtiredness, stress injuries or exhaustion.
Peer pressure can cause a diver to dive in conditions where he may be unable to deal with reasonably predictable incidents.
Diving with an incompetent buddy can result in injury or death while attempting to deal with a problem caused by the buddy.
Overweighting can cause difficulty in neutralising and controlling buoyancy, and this can lead to uncontrolled descent, inability to establish neutral buoyancy, inefficient swimming, high gas consumption, poor trim, kicking up silt, difficulty in ascent and inability to control depth accurately for decompression.
Underweighting can cause difficulty in neutralising and controlling buoyancy, and consequent inability to achieve neutral buoyancy, particularly at decompression stops.
Diving under the influence of drugs or alcohol, or with a hangover may result in inappropriate or delayed response to contingencies, reduced ability to deal timeously with problems, leading to greater risk of developing into an accident, increased risk of hypothermia and increased risk of decompression sickness.
Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on the details.
Hazards of the dive task and special equipment
Diving longer and deeper safely
There are a number of techniques to increase the diver's ability to dive deeper and longer:
- Technical diving – diving deeper than 40 metres (130 ft), using mixed gases, and/or entering overhead environments (caves or wrecks)
- Surface supplied diving – use of umbilical gas supply and diving helmets.
- Saturation diving – long-term use of underwater habitats under pressure and a gradual release of pressure over several days in a decompression chamber at the end of a dive.
Scuba diver training and certification agencies
Recreational scuba diving does not have a centralized certifying or regulatory agency, and is mostly self regulated. There are, however, several large diving organizations that train and certify divers and dive instructors, and many diving related sales and rental outlets require proof of diver certification from one of these organizations prior to selling or renting certain diving products or services.
The largest international certification agencies that are currently recognized by most diving outlets for diver certification include:
- American Canadian Underwater Certifications (ACUC) (formerly Association of Canadian Underwater Councils) – originated in Canada in 1969 and expanded internationally in 1984
- British Sub Aqua Club (BSAC) – based in the United Kingdom, founded in 1953 and is the largest dive club in the world
- European Committee of Professional Diving Instructors (CEDIP) based in Europe since 1992
- Confédération Mondiale des Activités Subaquatiques (CMAS), the World Underwater Federation
- National Association of Underwater Instructors (NAUI) – based in the United States
- Professional Diving Instructors Corporation (PDIC) – based in the United States
- Professional Association of Diving Instructors (PADI) – based in the United States, largest recreational dive training and certification organization in the world
- Scottish Sub Aqua Club (SSAC or ScotSAC) the National Governing Body for the sport of diving in Scotland.
- Technical Diving International (TDI), Scuba Diving International (SDI), ERDi– based in the United States, TDI is the world's largest technical diving agency, SDI is the recreational division focusing on new methods and online courses, and ERDi is the public safety component.
- Scuba Schools International (SSI) – based in the United States with 35 Regional Centers and Area Offices around the globe.
- YMCA Scuba – based in the United States, provided by Young Men's Christian Association (YMCA) of the USA; discontinued on 31 December 2008.
The current record for the longest continuous submergence using SCUBA gear was set by Mike Stevens of Birmingham, UK at the National Exhibition Centre, Birmingham, UK during the annual National Boat, Caravan and Leisure Show between February 14 and February 23, 1986. Mike Stevens was continuously submerged for 212.5 hours beating his own previous record of 121.5 hours. The record was ratified by the Guinness Book of Records. Mike used a standard regulator and mask and wore only a t-shirt and swim shorts and an 8 pound weight belt, he had no surface breaks during the 212.5 hours. A team of divers attended Mike throughout the dive. The team was led by Diving Officer Trevor Parkes. The dive raised £10,000 for the Birmingham Children's Hospital from donations by the public.
- Altitude diving
- Aqualung, a type of breathing set
- Artificial gills (human)
- British Sub-Aqua Club
- Decompression (diving)
- Decompression sickness
- Diver training
- Divers Alert Network (DAN)
- Diving equipment
- Diving hazards and precautions
- Diving physics
- Diving signal
- Diving suit
- Drift diving
- Engineer Diver
- See Frogman#Mistakes in fiction for common mistakes in depicting scuba gear.
- Green Fins
- Sea Hunt, a television fiction series about scuba diving.
- Sea Trek
- Sport diving, a competitive underwater sport based on recreational scuba diving practice.
- Technical diving
- Timeline of underwater technology
- Underwater diving
- Underwater orienteering, a competitive underwater sport focused on underwater navigation.
- Underwater photography
- Underwater videography
- Wreck diving
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- Brubakk, Alf O; Neuman, Tom S (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN 0-7020-2571-2.
- Cousteau J.Y. (1953) Le Monde du Silence, translated as The Silent World, Hamish Hamilton Ltd., London; ASIN B000QRK890
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- "Compact Oxford English Dictionary – scuba". Oxford University Press.
- HSE press release E061:05 – 5 May 2005 HSE issues warning over recreational dive training http://www.hse.gov.uk/press/2005/e05061.htm
- Statutory Instruments 1997 No. 2776 HEALTH AND SAFETY, The Diving at Work Regulations 1997, http://www.legislation.gov.uk/uksi/1997/2776/contents/made
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- Richardson, D; Menduno, M; Shreeves, K. (eds). (1996). "Proceedings of Rebreather Forum 2.0.". Diving Science and Technology Workshop.: 286. Retrieved 20 August 2008.
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