Side mount diving
Sidemount is 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. It began as a configuration popular with advanced cave divers, as smaller sections of cave can be penetrated and tanks can be changed with greater ease. The same benefits for operating in confined spaces were also recognized by divers who conducted technical wreck diving penetrations.
Sidemount diving is now growing in popularity within the technical diving community for general decompression diving, and is becoming an increasingly popular specialty training for recreational diving, with several diver certification agencies offering recreational and technical level sidemount training programs.
- 1 Terminology
- 2 Benefits
- 3 History
- 4 Configurations
- 5 Equipment
- 6 Training
- 7 Procedures
- 8 References
- 9 Further reading
- 10 External links
- Sidemount diving
- Sidemount diving is the, now increasingly formalized, approach towards conducting dives with 2 or more primary cylinders secured at the side of the body and in line with the torso – with no cylinders on the diver’s back. A common feature that defines sidemount configuration is the use of bungee cords to provide an upper attachment on the cylinder valve, normally routed from behind the diver’s upper back, whilst the lower cylinder is secured to the diver’s lower harness (butt-plate or waist D-rings) via bolt-snaps.
- Sidemount staging
- Sidemount staging is the practice of using sidemount configuration (bungee loops and/or buttplate rails) as a means for stowing stage/deco cylinders in a streamlined manner against the sides of the torso, when otherwise diving in back-mounted doubles or CCR.
- Stage-only diving
- Stage-only diving is the use of standard deco/stage cylinder configuration, without back-mounted cylinders, on an otherwise standard, or partially modified tec/rec BCD. The lower cylinder is attached to waist D-Rings and the upper cylinder to shoulder D-Rings, both via direct bolt-snap. Some divers (untrained in specific sidemount diving techniques) often incorrectly assume this approach to be sidemount diving. No bungee cord is utilised in the upper attachment, causing the cylinders to hang low beneath the diver and not in line (parallel) with their torso beneath their armpits. Without modification of the BCD design, the lack of back-mounted cylinders also causes taco’ing of wing designs and considerable instability.
- Monkey diving
- Monkey diving is the use of sidemount configuration/procedures, whilst only carrying a single cylinder. It is presented as an option on some recreational level sidemount courses (dependant on agency) and may also be a considered strategy for certain overhead-environment (cave/wreck) penetrations. The use of a single cylinder may require a strategy of counter-weighting to prevent diver instability in the water, depending on the buoyancy of the chosen cylinder.
- No-mount diving
- No-mount diving is a specialized overhead-environment strategy for dealing with particularly tight restrictions. This may involve divers wearing a very basic harness under their existing configuration, or simply hand-carrying cylinders. Upon reaching a restriction through which they couldn’t otherwise pass, they will ‘strip down’ out of their primary gear, hand-hold or attach a cylinder/s to their ‘no-mount’ harness and move forwards. A ‘no-mount’ harness can consist of nothing more than a weight-belt with several D-rings attached. The evolution of sidemount techniques and configurations has largely made this approach unnecessary, as a minimalist sidemount harness/BCD can be worn beneath back-mounted doubles, or even a CCR.
The sidemount diving approach offers divers significant benefits to the flexibility of their approach. Unlike back-mounted doubles, acquiring and transporting sidemount suitable cylinders is often much more convenient and accessible. Sidemount diving configuration allows the travelling diver to conduct technical and/or overhead environment dives without having to source traditional back-mounted twin cylinders. When diving in remote locations, the transportation of diving cylinders, especially by hand, is considerably less physically taxing.
Sidemount diving equipment is also considerably lighter, and less bulky than back-mounted alternatives – allowing for easier and cheaper (considering the rate of many airlines’ excess baggage costs) travel.[dubious ]
Unlike back-mounted cylinders, the sidemount diver has immediate access to, and observation of, the regulators and tank valves of their cylinders. This enables immediate problem identification and allows swifter resolution, without recourse to ‘behind the head’ shut-down drills that require a higher level of mobility, flexibility and freedom to operate.
Sidemount diving configuration places the cylinders under the diver’s armpits, in line with their body. This decreases water resistance (improving air consumption and reducing fatigue) whilst also allowing the diver to pass through smaller restrictions than would otherwise be possible in back-mounted cylinders. The flexibility to remove tanks, and propel them in front, allows the diver to pass through very small passages and holes when penetration diving – being limited only by the size of their bodies and exposure protection.
Increased accessibility to life-supporting regulators, first-stages and valves improves efficiency and speed of critical cylinder shut-down procedures, allows immediate gas-loss identification and provides the diver with quick access to alternative safety procedures; such as regulator swapping (between cylinders), valve-‘feathering’ to access gas within a cylinder whose regulator is malfunctioned/free-flowing... or even breathing directly from a tank valve.
In addition, stowage of the cylinders next to the diver’s torso, and beneath his armpits, serves to protect vulnerable valves and regulator first-stages from collision, impact and abrasion damage, or accidental shut-down through contact with a ceiling. It also significantly reduces the risk of entanglement behind the diver, where it is least easy to rectify.
Many divers will testify that sidemount diving configuration offers greater stability and easier-to-attain trim and control in the water. It is also less physically tiring to carry, and get into, sidemount equipment than with traditional back-mounted doubles – especially when operating from a small boat or a rough shore entry.
The ability to attach, remove and replace cylinders whilst in the water allows the diver to avoid ever having to carry heavy-weight back-mounted cylinders. This is combined with reduced physical exertion when conducting regulator shut-down procedures, which is a major benefit to technical divers who suffer from shoulder or back discomfort or reduced mobility from old injuries.
Redundancy of gas
Whilst technical divers have always utilized a redundant gas system, either isolated-manifold or independent back-mounted cylinders, recreational divers have traditionally resorted to using ‘pony cylinders’ or ‘ascent bottles’ as contingencies against out-of-air emergencies. Whether attached to the primary cylinder, or slung at the chest, these cylinders often presented problems with stability and streamlining, whilst simultaneously only providing a bare minimum supply of air for emergency ascent.
Sidemount diving with two cylinders helps resolve stability and streamlining issues, and ensures that a truly capable redundant supply of air is maintained.
Technical divers debate the pros and cons of independent cylinders versus isolated-manifold doubles. Back-mounted manifold cylinders provide easy access to complete gas supplies, in the event of a regulator failure and shut-down. However, the manifold itself creates additional o-ring failure points and a failure in that component will deprive the technical diver of, at least, one-half of his remaining gas supply. Independent cylinders, when sidemounted, provide true gas redundancy, whilst offering access (via switching regulators between cylinders or feather breathing) of all remaining gas.
Sidemount for the recreational diver
The benefits for cave diving and wreck diving with tight penetrations are largely accepted, but they are not so clear for the typical recreational diver.
Most recreational divers rely on their buddy for bailout gas, and do not carry a redundant gas supply. When there is only one cylinder, there is only no need to control several valves. The position of the cylinder valve behind the head has proven to be reasonably safe in millions of dives, though some divers do have physical difficulty reaching the valve while wearing the set, particularly if the cylinder is mounted relatively low on the harness.
In single cylinder diving there is seldom a reason to shut a cylinder valve while diving, and there is no need for changing cylinders or managing different gases. The recreational diver with a single cylinder is not supposed to enter low overhead spaces, so the single valve behind the divers head is unlikely to come into contact with objects which might roll it closed, as the diver tends to avoids situations where the head might impact with obstructions.
Since many recreational divers prefer to swim with their arms crossed in front of the chest, a side mounted cylinder might get in the way. Carrying one cylinder on one side does not increase stability or control, especially when it comes to a rough shore exit and other situations when freedom of movement of the arms is needed.
There are rarely any transportation benefits since the detached transportation of a back mounted cylinder is possible, and carrying the weight on the back is less stress on the spine than carrying it to one side when out of the water. Since backmount equipment designed for travelling is readily available, the weight advantages are unclear.
The 1960s - UK sump diving
The concept of sidemounting cylinders originated from cave diving in the UK, during the 1960s. During 'dry' explorations of Wookey Hole, the River Axe and other underground systems, divers occasionally encountered submerged passages that blocked further exploration. These cavers began incorporating scuba equipment specifically to progress beyond underwater areas. However, because they operated in very confined spaces, and most exploration remained primarily 'dry', they began experimenting and improvising with extremely minimalist configurations, minimising bulk, allowing cylinders to be easily removed and replaced, and retaining the capacity to squeeze through the tightest restrictions.
The nature of these 'dives' in cramped sumps did not prioritize the need for buoyancy control or underwater propulsion – so the bare minimum needed was a mask, a cylinder, a regulator, a method of attachment to the body and, only on rare occasions, a set of fins.
Many of these early sump explorers adopted an approach based upon a sturdy belt, with attached cam-band, that allowed a cylinder to be dropped in and carried alongside the outer thigh. This allowed them to crawl, or wriggle, through the dry cave sections, whilst presenting a secure method of attachment for passing through submerged areas. Swimming efficiency, reduced water resistance, trim and buoyancy control were not generally required due to the nature of those caves. At the time, this approach to 'wet' cave exploration was generally called the 'English System'.
The 1970s - Florida
During the 1970s the 'English system' began to be incorporated by American cave divers, operating in Florida. Those cave systems were predominantly 'wet' and involved prolonged swimming with SCUBA; thus more emphasis was paid towards developing the diving performance of the system, in particular buoyancy and trim. Divers required buoyancy control devices for extended fining and began shifting the location of the cylinders from against the thigh, up to the armpit and against the torso.
These exploratory level cave divers began by making their own systems, using and adapting 'off-the-shelf' SCUBA equipment for their needs or creating configurations ‘from scratch’, based upon webbing harnesses and improvised bladders for buoyancy. One popular approach to improvised bladders was the MSR Dromedary hydration system (like a 'Camelbak'), which would be sewn or bolted to the rear of the harness and operated by orally inflating and deflating through the adapted 'drinking tube'.
The 1990s - release of first commercial rig
In the mid-1990s, Dive Rite produced the first commercial sidemount diving system, focused on the newly released 'Transpac' harness. Other cave divers continued to manufacture their own DIY configurations.
At this time, the use of sidemounted configuration was primarily restricted to a small number of exploration-grade cave pioneers.
The 2000s - cave diving popularity and sidemount evolution
The widespread popularity of sidemount diving systems did not emerge until the mid-2010s, when the growing popularity of technical and cave diving became exposed to sidemount proponents on the internet who were offering an alternative approach that matched the minimalism and functionality of the popular 'DIR/Hogarthian' back-mounted systems, whilst offering advantages in flexibility, comfort, accessibility and – highly debated online – safety.
The increasing interest in sidemount diving configurations prompted several manufacturers and individuals to design and sell their own designs of a sidemount system. Hollis, OMS, UTD developed equipment, while Steve Bogaerts (a UK-born cave pioneer, who lives and cave-dives in Mexico) released the very popular 'Razor' system and began teaching a specific model training program for his rig.
At this time, several technical scuba agencies developed formal sidemount training programs and incorporated sidemount diving configuration as an equipment option within existing technical diving programs.
When PADI instructor, Jeff Loflin, devised a distinctive sidemount diving speciality course, it proved extremely popular, being replicated by many PADI technical-level instructors. This soon led to PADI devising standardised sidemount diving programs at both recreational and technical levels, making sidemount a viable and mainstream option for both recreational and technical divers. Other agencies, such as ANDI, IANTD, SSI, TDI and UTD also incorporate sidemount training at varied levels.
Various harness/BCD configurations have been used to sidemount cylinders. The choice between different configuration approaches is typically determined by the nature of the diving undertaken (open water, technical, wreck or cave) and by the divers' existing equipment, financial budget and whether they have a preferred approach to diving philosophy (minimalist, DIR, Hogarthian, etc.). The size/ material/ volume of diving cylinders to be used also has a large impact on sidemount BCD requirements.
Backplate and wing harness adaptation
Rigid Hogarthian style backplate and wing BCD systems may be modified by adding butt-plates, bungee cords and optional special sidemount buoyancy compensators. Cylinders are supported at the valve end by bungee loops that run from the backplate to the front chest D-rings. The lower cylinder clip attaches to D-rings mounted on the waist belt or 'rails' on a butt plate.
Sidemount divers who conduct penetration diving in confined overhead environments (wreck diving or cave diving) will generally prefer a soft fabric backplate, or webbing harness only, owing to the risk of a solid backplate becoming stuck in a small restriction.
An example of a commercial backplate and wing harness adapter, including integrated butt plate and attachment points for bungee loops, is the OMS Profile.
Specialised and hybrid harnesses
Specialised sidemount harnesses are available 'off-the-shelf' commercially. Some of these are designed specifically for sidemounting only, but others are 'hybrid' designs, enabling the diver to swap between sidemount and back-mounted cylinders, as needed.
Examples of dedicated sidemount rigs:
- Hollis SMS50 
- Razor 2 
- DiveRite Nomad LT 
- XDeep Stealth 2.0 
- Golem Gear A2 and S 
- UTD 'Z-system' 
Examples of hybrid sidemount rigs:
- Hollis SMS100 or SMS75 
- OMS Tesseract 
- DiveRite Nomad XT or EXP
- Custom Divers Rhino 
- White Arrow S-Wing 
Some manufacturers now provide sidemount rigs targeted for recreational diving use. These are typically variants of existing dedicated sidemount rigs, with the low pressure inflator (LPI) mounted at the top of the BCD (rather than at the bottom corner) for an 'over the shoulder' configuration more familiar to diver's transitioning to sidemount from a traditional BCD.
Examples of dedicated recreational sidemount rigs:
Minimalist webbing harness
A webbing harness with shoulder straps, waist belt and crotch strap, supporting a variety of sliders and D-rings for attachment of cylinders and accessories, with or without integrated weighting or separate weight belts, and with or without a back mounted buoyancy compensator, which may be attached to the harness, or directly to the diver. Cylinders are usually attached to a shoulder or chest D-ring and waist belt D-ring on each side. Additional accessories may include canister lights and clip-on pockets for small equipment. This style of harness may be off the shelf from an original equipment manufacturer or retailer, or home made, as most of the parts are freely available or relatively simple to make.
Belt style 'Sump' harness
In the UK, cave diving was an additional skill learned by cavers to explore flooded parts of a cave system, rather than divers choosing to explore caves. The early equipment was little more than cylinders fitted with belt loops and slid onto the standard caver's belay or battery belt along with any extra weights needed to achieve neutral buoyancy, and a caver's belt mounted battery pack. This simple sidemount configuration was particularly low profile and suited to small cylinders, and worked well for low visibility, usually fairly shallow dives, which were often more of a crawl or wriggle though tight confines than a swim.
As penetration distances into caves increased, the basic belt was replaced by a more sturdy harness with shoulder straps and padding on the waist band to distribute the load more comfortably.
A disadvantage of this arrangement is that the cylinders and lead weights must be fitted to the harness before it is put on. This usually required the diver to lie down to fit the harness, and if the cylinders were large, to be assisted to a standing position.
- Cylinders of varying sizes and materials can be used for sidemount diving. The optimum choice of cylinder will be determined by the water conditions and/or choice of exposure protection used.
- Regulator set
- Each primary sidemount cylinder requires a regulator 1st stage, a regulator second stage and a submersible pressure gauge (SPG). The left hand cylinder will also include a low-pressure inflator (LPI) for BCD inflation. If a drysuit is used, the drysuit inflator will be attached to the right hand cylinder. The most common  regulator configuration mimicks hogarthian regulator set-up, with a long (5–7 foot) hose on the right cylinder and a short hose on the left cylinder, where the regulator is contained in a bungee necklace. However, some sidemount divers prefer an opposite configuration, two long hoses or two short hoses.
- Cylinder straps
- Sidemount cylinders are secured to the diver's butt-plate or rear harness D-rings via a cam-band or worm clamp to which a bolt-snap is attached via cord. The placement of the cam-band/worm clamp along the cylinder length is determined by characteristics needed to ensure cylinder trim in line with the diver's torso. The upper cylinder may be secured directly via the bungee strap, or by using a choker into a ring-bungee configuration.
- Choker (#1)
- A bolt-snap secured around the cylinder neck via a short cord loop that provides additional cylinder security and stability when conducting water entries with the sidemount cylinders in place.
- Choker (#2)
- A small strap around the neck of a sidemount cylinder used to pull the bolt-snap closer to the cylinder neck. Primarily used in conjunction with the DiveRite 'Ring Bungee' mounting method; this has the effect of constraining the top of the cylinder to lie closer to the shoulder of the diver for a lower profile. The choker is passed through the jaw of the snap and tightened, then the clip is clipped to the bungee which is clipped to the shoulder D-ring, allowing the clip to slide along the bungee.
- The use of bungees for cylinder attachment and trim is a quintessential identifier of sidemount configuration. The bungee is attached to the upper harness/BCD and routes under the diver's armpits to the shoulder D-ring area. Appropriately sized bungees ensure that the sidemount cylinders remain in trim with the diver's side torso with the valves under the armpits. The cylinder/s should neither rise above, or drop below, the diver when they are in flat, horizontal trim position. Bungees are typically wrapped around the cylinder valve handle and/or cylinder neck, to secure the cylinder whilst retaining flexibility to maneuver the cylinders into a forward position.
- Double bungee
- Some configurations use two lengths of bungee, typically attached to the rear of the harness/BCD via stainless steel quick-links. They may attach via dedicated D-rings on the diver's BCD or a 'daisy chain' length of looped nylon webbing that permits easy adjustment of bungee length/fit.
- Continuous Bungee
- Other configurations make use of a single length of bungee cord, routed from one shoulder D-ring to the other, via the rear of the diver.
- Ring Bungee
- The 'Ring Bungee' introduces a metal ring along the bungee length (in the armpit area). This permits the use of a choker to connect tanks to the ring, making bungee wrapping of the cylinder valve/neck unnecessary. A popular configuration has the 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. The choker is used to minimise the distance the cylinder top can dangle away from the ring bungee.
- BAT wing configuration
- The buoyancy and trim wing system uses a buoyancy compensator air cell behind the harness, attached at top and bottom ends to the harness, and with an elastic waist strap, usually of shock cord, which keeps the wing close to the waist and prevents the sides from floating above the diver. Buoyancy of the cell is usually concentrated in the lower part of the wing to correspond to the centre of mass of the diver, so that trim changes are minimised with changes of volume
- Captive wing arrangements
- These buoyancy cells are sandwiched between the harness and the diver.
- Cutting devices
- Sidemount divers, particularly those operating in overhead environments will carry two or more cutting devices. These should be stowed where they can be easily accessed, even when in a tight passage. Popular stowage locations include; a sheath mounted on the harness or lower arm. Redundant/back-up devices may be carried in a pocket. Short bladed (often titanium) knives, trauma shears or emergency line cutters (for instance, the Eezycut Trilobite ) are the most popular selections for primary and back-up cutting tools.
- Sidemount divers who enter overhead environments will typically carry a primary torch, often of umbilical/canister design, along with one or two back-up torches. Specific sidemount umbilical/canister torches are now sold, with a 90 degree junction for the umbilical which permits them to be mounted on the butt-plate or upper-rear crotch strap. Back-up torches are typically a robust LED design, with a battery/burn life indicated to be 2-3x that of the planned dive duration.
- Reels and spools
- Sidemount divers will typically carry one or more reels/spools. In open-water these may be used for deployment of delayed surface marker buoys (DSMB). In overhead environments, these will include a primary guideline and safety/jump spools - in like with cave or technical wreck diving protocols. These are typically attached to the diver on rear D-rings, normally on the butt-plate or rear waist strap, to prevent them dangling below the diver.
- Line arrows and markers
- Sidemount divers in overhead environments will carry a selection of directional (arrows) and non-directional (cookies) line markers. The use of Referencing Exit Markers (REM) is also becoming popular with sidemount divers.
- Helmets are particularly popular with sidemount divers, as they provide a convenient mounting point for primary and/or back-up lights. The primary function of the helmet remains to provide head protection in overhead environments. Sidemount configuration can mean that the more typical method of mounting back-up lights below the diver's shoulder D-rings becomes less favorable. This area is more heavily loaded with bungee and deco stage attachments, reducing access to back-up lights stowed from the D-ring. Mounting primary lights on the helmet can be advantageous as it frees up the hands in very confined spaces and/or when heavily task-loaded with other equipment. The draw-backs to helmet mounted lights are that it can increase the risk of snagging the helmet on obstructions and an increased potential to inadvertently dazzling dive buddies. Sidemount divers may choose to make their own helmet by adapting an existing climbing, kayaking or skate-boarding helmet. Off-the-shelf cave/sidemount diving helmets are also sold by companies like Light Monkey
- A pocket is useful for carrying small accessories and spares. A small nylon pouch which clips,via double-ended bolt-snap/s onto the rear/butt D-rings has the advantages of streamlining; while permitting easy access to the contents by removing/replacing the pouch as necessary.
Level 1: "Recreational (Rec or Basic Sidemount)"
Basic sidemount training is offered by most agencies, and aims to develop initial equipment familiarity and operation. At this level, students are taught to adjust, fit and operate the sidemount rig, whilst diving with one or two cylinders maximum.
These training courses then develop core diving skills that are specifically relevant to sidemount – namely: trim, buoyancy, alternative propulsion techniques and cylinder handling in the water. Many basic courses also teach supplementary diving skills, such as DSMB deployment and variations on open water skills, such as inverted (upside-down) fining and inverted mask-clearing. Knowledge development covers topics such as: gas management, dive planning and equipment considerations/options.
Level 2: "Technical (Tec or Advanced Sidemount)"
Technical, or advanced, sidemount courses develop basic sidemount skills with a focus on technical diving activities. A higher level of equipment proficiency and diving skill is required, as additional stage/deco cylinders will be added to the rig, enabling mixed-gas and accelerated decompression procedures to be conducted. Technical sidemount courses often develop utilisation of the sidemount rig for increasingly demanding penetration skills – teaching students how to manipulate cylinders in order to pass through restrictions.
Some agencies/instructors also provide environment specific sidemount courses at this level, focusing on sidemount diving within the cave or wreck overhead environments. These courses might incorporate the respective knowledge, skills and procedures from the cave or wreck syllabus, but adapted and focused only on performance in sidemount configuration.
One of the advantages of the sidemount system is the easy access to cylinder valves, and the protected position they occupy in comparison with most back mount systems, where the valves are relatively vulnerable to bumping against an overhead and being "rolled off" (closed by rubbing against the overhead).
The standard arrangement for sidemount is that all cylinders are independent, and each is provided with a single demand valve, an SPG, and on one or two, a low pressure inflator hose for buoyancy compensator and, if used, the drysuit. This implies that if gas is shared in an emergency, the recipient will be breathing from a different cylinder to the donor, unlike the more usual arrangement with backmount, where both divers breathe off the same set.
In the case of recreational sidemount, with only one cylinder, the regulator would have an octopus demand valve for air sharing and the procedures would revert to much the same as for a single back mount cylinder.
The sidemount diver typically operates two independent tanks for 'bottom gas' during a dive. Compared with back-mounted double cylinders and an isolation manifold, the use of independent cylinders requires additional gas management skills. The sidemount diver has to swap regulators at planned intervals to ensure that the use of gas is balanced from both cylinders - thus ensuring good balance and trim in the water, whilst preserving a sufficient supply of air for emergency sharing.
As of 2013, the only sidemount system offering a manifold system is the UTD ‘Z-Manifold'. This equipment provides a link between cylinders and dispenses with the need to swap regulators and balance tanks. In all other instances, the sidemount diver will follow a gas management strategy, which enable roughly equal consumption of gas from both sidemount cylinders and retains a necessary minimum reserve in both cylinders to enable an air-sharing exit/ascent from the dive.
The basic principle of breathing gas management is never to run out of suitable breathing gas while underwater. The implications depend on the team size and the dive profile, and range from very simple for a situation where the diver can make a safe free ascent at any point of the dive, to complex, when a team of two or three divers is constrained from surfacing immediately by decompression obligations or a hard overhead, and rely on each other for emergency gas supply at any point of the planned dive, particularly when a variety of gases are carried which are each only suitable for a specific depth range.
It is standard practice to allow for the possibility of total failure of gas supply from any one cylinder at any point of the dive, and to turn the dive if the gas supply is compromised beyond the planned limits.
Rule of thirds and reserves
Most technical and overhead-environment divers will maintain the rule of thirds as the least conservative option when diving sidemount. Because air-sharing will remain a possibility while a second diver is present, it is considered good practice that both cylinders retain independent reserves of sufficient gas for both divers to reach the surface safely from any point in the planned dive. one third of the capacity of the cylinder is considered a simple rule of thumb reserve sufficient for most circumstances, but there are times when it is not enough and times when it will be more than enough.
The principle of the rule of thirds is that one third of the gas supply is available for the penetration, one third for the return and one third is a reserve which allows for gas sharing on the return. Therefore the dive is turned when either cylinder drops below two thirds capacity.
It is not safe practice to drain one cylinder, retaining the entire reserve in the other, in case of a catastrophic failure of the second cylinder supply.
- Balancing cylinders - regulator switching
- To maintain balance (approximately matched buoyancy characteristics) between cylinders, there is a requirement for switching regulators during the dive.
- Swapping after 1/6
- One method of maintaining cylinder balance is to swap regulators for every 1/6 cylinder gas consumption. This would mean each regulator was breathed from twice, before reaching the gas turn-point (2/3) and a further two switches on each regulator until the reserve (1/3) was reached.
- Swapping after 1/3
- An alternative method (simpler, but less refined cylinder balancing) would be to switch regulators with every 1/3 gas consumed (on each cylinder). This would mean each regulator was used once, before reaching the gas turn-point (2/3) and once more, per regulator, before reaching reserve level (1/3).
- Draining a tank to 1/3, then switching
- It is not generally acceptable practice to breath from a single cylinder until the turn-point, and then switch as this might leave insufficient gas remaining in that cylinder for an air-sharing exit or ascent – as if the second cylinder supply fails catastrophically, the diver would only have 1/3 of a cylinder, not 1/3 total supply for that exit.
Regular regulator switching also helps to monitor that both regulators remain functional – an especially prudent measure when sidemount diving through restrictions in high silt and mud conditions.
Level 1: "Recreational (Rec or Basic Sidemount)"
- Equipment configuration and assembly
- Pre-dive safety checks
- Attaching tanks on land and at the surface (shallow and deep water)
- Descent procedures
- Trim and buoyancy
- Propulsion techniques – frog kick, modified flutter kick, back kick, helicopter turn
- Gas planning and management (independent cylinders)
- Regulator switching
- Air sharing - donor and receiver
- Tank positioning and adjustment for continual good trim
- Removal of single tank while swimming
- Removal of two tanks while swimming
- Inverted and/or vertical mask clearing
- DSMB deployment ascent procedures
Level 2: "Technical (Tec or Advanced Sidemount)"
- Equipment configuration and assembly
- Stage bottle configuration and assembly
- Water entry procedures
- Descent procedures
- Trim and buoyancy
- Propulsion techniques - frog kick, modified frog and flutter kick, back kick, helicopter turn, finger walking, pull and glide
- Sidemount diving mobility – inverted propulsion, rolls, loops etc.
- Gas planning and management, including decompression gasses.
- Team drills and development
- Guideline/penetration contingency drills
- Sidemount regulator shut-downs
- Feather breathing from a shut-down free-flowing regulator  (Manipulating the valve to temporarily supply air from a shut-down cylinder as you breath).
- Swapping regulators between cylinders underwater
- In-water stage bottle attachment, removal and replacement
- Sidemount decompression procedures
- Practice of all sidemount skills (Level 1 & Level 2) within specific over-head environment (cave/wreck)
Compatibility in mixed configuration team diving
The basic sidemount configuration requires different techniques for sharing gas, compared to the isolation manifolded twins back mount configuration. This means that a single set of standardised safety procedures is not possible while diving with team members using back mounted tanks.
The UTD Z-system provides an approach to emulate back mounted Hogarthian configuration by quick-connecting two primary gas supply sidemount cylinders to a manifold block mounted behind the shoulders on the Z-system harness, from which long hose primary and necklace secondary demand valves are supplied, reducing the demand valve arrangement to a functional approximation of the standard Hogarthian configuration at the expense of additional complexity of components.
The added failure points may not be necessary when using more than one bottle at the time.[clarification needed] Additional stage cylinders are side mounted with independent regulators.
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- Sidemount Diving Techniques