Buoyancy compensator (diving)

Surfaced divers with inflated horsecollar BCs

An adjustable buoyancy life jacket (ABLJ) is fitted around the neck and over the chest, secured by straps around the waist and usually between the legs. They are sometimes referred to as "horse collars" because of their resemblance, and are historically derived from the inflatable underwater demolition team (UDT) vest or Mae West life jacket issued to World War II flyers and divers.

They were developed in the 1960s and have been largely superseded by wing and vest type BCs, primarily because the buoyancy is concentrated in front of the diver when full, and behind the neck when partially filled, producing a tendency to shift the diver's center of buoyancy towards the head with inflation, which adversely affects the diver's trim underwater.[4] The ABLJ's location on the diver's chest and round the neck provides the best buoyancy distribution of the buoyancy compensater designs when it comes to floating a distressed, fatigued or unconscious diver face-up on the surface in the event of a problem.

The Dacor Seachute BC4 had unique upper and lower bladders. The upper bladder was around the neck and could be inflated by the CO₂ cartridge for use as a surface life jacket. The lower bladder was over the diver's stomach area, and was inflated by LP gas from the regulator, for buoyancy control underwater. This arrangement provided better buoyancy distribution for trim control while diving than most other front inflation systems.[5]

Diver wearing a stabiliser jacket

Vest BC, stab jacket, stabiliser jacket, stab, waistcoat or (disparagingly) "Poodle Vest" BCs are inflatable vests worn by the diver around the upper torso, which incorporate the cylinder harness. The air bladder extends from the back around the diver's sides.

Wraparound bladders are favored by some divers because they make it easier to maintain upright attitude on the surface. However, some designs have a tendency to squeeze the diver's torso when inflated, and they are often bulky at the sides or front when fully inflated. Back inflation BCs are less bulky at the sides but may have a tendency to float the diver tilted forward on the surface depending on weight and buoyancy distribution, which presents a possible hazard in an emergency if the diver is unconscious or otherwise unable to keep his or her head above the water.

Vest BCs typically provide up to about 25 kilograms of buoyancy (depending on size) and are fairly comfortable to wear, if of the correct size and adjusted to fit the diver. Vest BCs are the most common type among recreational divers because they can integrate buoyancy control, weights, attachment points for auxiliary gear, and cylinder retention in a single piece of gear. The diver need only attach a cylinder and regulator set in order to have a complete scuba set. Some "tech-rec" (technical and recreational) vest BC's have the ability to carry multiple cylinders - Twin sets on the back, and sling cylinders at the sides, suspended from D-rings. The lack of flexibility of positioning the D-rings due to structural constraints on some designs is partly compensated by fitting larger numbers of D-rings, some of which may be in the right place for a given diver.

Three main wraparound configurations can be distinguished:

• The original stabilizer jacket patent by Scubapro featured a buoyancy bladder which allowed air flow around the arms and around the backpack - the trademarked 360° flow-through design. This was a complex bladder to manufacture.[6]
• Over-the-shoulder bladder, separated under the arms, which has a centre of buoyancy fairly high on the body when fully inflated, which tends to hold the diver upright at the surface. These are bulky over the front of the torso, particularly in the shoulder and chest area, and relatively clear to the sides under the arms.
• With extensions of the bladder from the lower back forward under the arms, but separated at the shoulders, and no buoyancy on the upper chest, which has a lower centre of buoyancy when fully inflated, and tends to tilt the diver backwards when fully inflated at the surface. These can be very bulky under the arms when integrated weighting and/or pockets are added, and for smaller waisted divers, also to the front of the waist area, but are relatively clear in the chest and shoulder area. The "shell" or bladder casing is usually separate from the cummerbund, which can be snugly fitted while leaving the bladder a relatively loose fit around the torso, to avoid restricting breathing when fully inflated.

BC attachment systems are generally intended to limit the shifting of the BC as a result of the lifting forces, including minimizing the tendency to slide towards the head when the diver is upright while the bladder is inflated. If the diver is wearing a weight belt, this will pull in the opposite direction to BC lift when the diver is upright, and can result in the diver sagging down in the jacket when floating at the surface. Solutions to this problem include the cummerbund (a broad adjustable waist band) and the crotch strap (a strap between the legs). The crotch strap, when adjusted correctly, is effective at preventing this shift, but may prevent the weight belt from falling clear of the diver if dropped in an emergency. Fitting the weight belt over the crotch strap after putting the BC on can be difficult. The cummerbund is an attempt to avoid this problem, as the weight belt can not be snagged on it in the same way, but the weight belt must then be worn either under the cummerbund, obstructing access to the buckle, or below the cummerbund. The effectiveness of a cummerbund depends on a waistline which is smaller than the circumference of the upper torso, and it may constrain free breathing if fitted too tightly.

This tendency of the inflated BC to shift towards the head is less of a problem when the weights are carried in integrated weight pockets on the BC, but it may then have a tendency to slide towards the head when deflated on an inverted diver underwater. This is less of a problem for the average recreational diver, who does not spend much time head down underwater, but can increase the difficulty of recovering from a dry-suit inversion where the air in the suit flows to the feet and the weights in the BC shift towards the head. A crotch strap will prevent this.

Back inflation buoyancy compensators are typified by the stainless steel backplate and wing arrangement popular with technical divers, but other arrangements are also available. Wings or Backplate and wing consist of an inflatable bladder worn between the diver's back and the cylinder(s). Invented by Greg Flanagan in 1979 for North Florida cave divers, and further developed by William Hogarth Main,[7] the back plate and wing configuration is not a recent development, but has gained popularity because of suitability for technical diving where it is often used, as the technical diver often carries multiple cylinders on his back and/or clipped to D-rings on the harness webbing. The bladder and cylinders or rebreather are fastened to a backplate which is strapped to the diver by the harness. The wing design frees the divers sides and front and allows for a large volume bladder with high lift capacity (60 lbs /30 liter wings are not uncommon). Some designs use elasticated webbing or bungee cords around the bladder to constrict the bladder when not inflated, although there is dispute regarding the safety and utility of this addition.[8] The distance between boltholes on the centreline of the backplate has standardised at 11 inches (280 mm) between centres.[7]

Other back inflation buoyancy compensators are more like the jacket style regarding the structure, attachment to the diver, and accessories, differing mainly in the bladder position, which is similar to a wing, being entirely behind the diver, without extensions to the sides or front.

A few short-lived rigid air compartment back inflation BCs were marketed in the 1970s.

A hybrid arrangement is also possible, which has most of the buoyancy in the back, but has a small amount to the sides below the arms.

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  Diver wearing a wing buoyancy compensator
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  Wings for twin-cylinder set: the cylinders are fastened together by 2 metal bands which are bolted through the wing to the backplate
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  Bladder of a wing buoyancy compensator, showing the side which is away from the diver. The two pairs of slits allow use of cambands to hold a single cylinder
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  Divers preparing for a decompression dive using backplate and wing with sling mounted decompression gas cylinders.

A variation on the back mounted buoyancy compensator is used without a backplate for side mount diving This arrangement is functionally similar to wearing the buoyancy compensator sandwiched between the cylinder(s) and backplate, but there is no backplate or back mounted cylinder. The buoyancy cell may be mounted between the sidemount harness and the diver, or on top of the harness.[9] The sides of the bladder may be restrained from floating upwards when inflated by bungee cords clipped to the waistband in front of the diver or clipped to each other, forming an elastic belt across the front of the hips, well below the diaphragm.

Some side mount harnesses are adaptable for use with a back mount cylinder as an option, without the rigid backplate.[9]

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  Minimalist sidemount harness showing webbing, sliders and D-rings, buoyancy compensator, integrated weight holders and cylinder
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  Top view of sidemount diver
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  Sidemount diver in cave showing 2 cylinders and BC back view
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  Combination sidemount/backmount harness. Front view
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  Combination sidemount/backmount harness. Back view
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  Apeks sidemount harness upper side
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  Apeks sidemount harness underside

The buoyancy compensator must fit the diver comfortably and must stay securely in place without constraining the diver's freedom of movement. There is some conflict between allowing easy adjustment to fit a range of diver builds, and setting up the harness to optimum fit for a specific diver in a specific diving suit. This is a particular problem with jacket style BCDs which are inherently less adjustable for fit than backplate harnesses, which are more adjustable, but take more time to adjust.

It is critically important that the fully inflated buoyancy compensator can support the diver with the maximum equipment load on the surface at the start of a dive, and with maximum suit compression at the maximum depth before much gas is used up. There have been fatalities due to overloading the BCD. On the other hand, buoyancy control is easiest with the lowest practicable volume of gas in the BCD and dry suit, as these volumes change with depth changes, and must be adjusted to remain neutral.

It must be possible to remain neutral at the end of the dive, at the shallowest decompression stop, when almost all the diver's breathing gas has been used up. It is not sufficient to only be able to remain neutral with reserve gas, as if the reserve gas is nearly used up ue to a problem, the diver will not want to be struggling or unable to stay down to decompress.

Weighting must be sufficient to allow the diver to stay at the shallowest stop with empty cylinders, and available buoyancy volume must allow the BCD to support the full cylinders. The absolute minimum volume for the BCD is enough to support the total mass of breathing gas in all the cylinders the diver will carry, plus lost volume due to suit compression at depth. This will be enough only if the diver carries no excess weight. It is easier to allow for a slight weight excess and use a slightly larger volume BCD, but if taken to excess this will make buoyancy control more difficult and labour-intensive, and will use more gas, particularly during ascent when it is most critical. A BCD designed for recreational diving or for a small person may not have sufficient volume for technical diving.

An unnecessarily large volume BCD constitutes a greater risk of loss of control of ascent rate, particularly when combined with carrying more weight than is necessary to allow neutral buoyancy at the end of the dive with empty cylinders. On the other hand, a large volume gives greater comfort and security when floating at the surface before and after a dive.

The diver needs to be able to establish three states of buoyancy at different stages of a dive:[10]

1. negative buoyancy: when the diver wants to descend or stay on the seabed. Recreational divers seldom need much buoyancy deficit, but commercial divers may need to be heavy to facilitate some kinds of work.
2. neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort. This is the desired state for most of a recreational dive.
3. positive buoyancy: when the diver wants to float at the surface or ascend under some emergency circumstances.

To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted to counteract the buoyancy of both the diver and the equipment.

When underwater, a diver often needs to be neutrally buoyant and neither sink nor rise. A state of neutral buoyancy exists when the weight of water that the diver and equipment displaces equals the total weight of the diver and equipment. The diver uses a BC to maintain this state of neutral buoyancy by adjusting the volume of gas in the BCD and therefore its buoyancy, in response to various effects, which alter the diver's overall volume or weight, primarily:

• If the diver's exposure suit is made of a compressible gas-filled material such as foamed Neoprene, the volume of the material will change (Boyle's Law) as the pressure changes when the diver descends and ascends.[11] The volume of air in the BC is adjusted to compensate for this.
• Gas contained in the flexible air spaces within the diver's body and equipment (including gas in the BC) is compressed on descent and expands on ascent. The diver normally counteracts this by adding gas to the space or drysuit, in order to avoid "squeeze".[11] Gas content in the BC is adjusted to correct buoyancy if these other corrections are not enough.[10]
• As the dive proceeds, gas is consumed from the diving cylinders of the breathing equipment. This represents a progressive loss of mass which makes the diver more buoyant; the diver's overall buoyancy must be reduced by venting air from the BC. For this reason the diver needs to configure his equipment to be a little overweight at the beginning of the dive so that neutral buoyancy can be achieved after the loss of the weight of the breathing gas.[12] Air or nitrox weighs about 1.3 grams for every litre at standard pressure. Thus, the magnitude of weight change from loss of air during a dive varies from roughly 4.3 kg (9.5 lbs) representing the total air content of a steel 15 litre cylinder at 230 bar/3500 psi (in practice, reserve requirements dictate that only about 8 lbs of this will be breathed if the dive goes to plan), to about 5 lbs difference for the smaller 80 ft³ aluminum-80 (AL80) tank (11.1 litres internal capacity) pressurised to 200 bar/3000 psi, and again assuming that only 5/6ths of the air in the tank is used, leaving a typical safety reserve.

In practice, the diver doesn't think about all this theory during the dive. To remain neutrally buoyant, gas is added to the BC when the diver is negative (too heavy), or vented from the BC when the diver is too buoyant (too light). There is no stable equilibrium position for a diver with any compressible gas space. Any change in depth from a position of neutral buoyancy and even small changes in volume, including the act of breathing, result in a force toward an even less neutral depth. Thus, maintenance of neutral buoyancy in scuba is a continuous and active procedure—the diving equivalent of balance, in a positive feedback environment. Fortunately, the diver's mass provides a source of inertia, as does the liquid medium, so small perturbations (such as from breathing) can be compensated for easily by an experienced diver.[10]

A feature of diving which is often non-intuitive for beginners, is that gas generally needs to be added to the BC when a diver descends in a controlled manner, and vented (removed or dumped) from the BC when the diver ascends in a controlled manner. This gas (added or vented) maintains the volume of the gas in the BC during depth changes; this bubble needs to remain at approximately constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas in the BC decreases in volume due to the increasing pressure, resulting in a decrease in buoyancy and faster descent with greater depth, until the diver hits the bottom. The same runaway phenomenon, an example of positive feedback, can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety (decompression) stop. This effect is greatest near the surface where volume change is greatest in proportion to depth change.

With practice, divers learn to minimize this problem, starting by minimizing the volume of gas required in their BC's. This is done by using the minimum weighting needed for their equipment, which keeps the volume of the gas in the BC as small as possible at the beginning of a dive. Just enough gas will be added tp the BC to compensate for the slow loss of weight as the dive progresses, as a result of gas use, which will vary according to the dive, but is limited by the cylinder contents. (in practice, for a recreational diver, this will be about 2 to 4.5 kilograms (4.4 to 9.9 lb) per cylinder).[10]

Somewhat complex trained reflex behaviors may be developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutrally buoyant from minute to minute during a dive, without having to think much about it. Skilled scuba divers may be identified by their ability to maintain constant depth in horizontal trim, without fin use.

Divers with neutral buoyancy and horizontal trim are less likely to touch or disturb the bottom

The vertical-horizontal orientation, or trim, of the submerged diver is influenced by the BC and by other buoyancy and weight components and contributed to by the diver's body, clothing and equipment. The scuba diver typically wishes to be trimmed nearly horizontally (prone) while under water, to be able to see and swim efficiently, but more nearly vertical and perhaps partly supine, to be able to breathe without a regulator when on the surface. Buoyancy and trim can significantly affect hydrodynamic drag on a diver and the effort required to swim. The effect of swimming with a head up angle, of about 15° as is quite common in poorly trimmed divers, can be an increase in drag in the order of 50%.[13]

The static and stable orientation of an object floating in water, such as a diver, is determined by its center of buoyancy and its center of mass. At stable equilibrium, they will be lined up by gravity and buoyancy with the center of buoyancy vertically above the center of mass. The diver's overall buoyancy and center of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit. The diver's mass on a typical dive does not generally change by what seems like much (see above—a typical dive-resort "aluminum 80" tank at 207 bars (3,000 psi) contains about 2.8 kilograms (6.2 lb) of air or nitrox, of which about 2.3 kilograms (5.1 lb) is typically used in a dive, although any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure. Larger changes in buoyancy are possible if the diving weights are jettisoned, or a heavy object is picked up.

Generally, the diver has a little control over the position of the center of buoyancy in the BC during a dive, the air in an incompletely inflated buoyancy compensator will rise to the shallowest part of the bladder unless prevented by a restriction to the flow. The position of this shallow point will depend on the diver trim and the geometry of the bladder. If the diver changes orientation in the water the gas will flow to the new high part if it does not have to flow down first to get there. as a result of this tendency, some buoyancy compensators will tend to hold the diver in the new position until actively changed. This is more likely in back mounted wing type bladders, where the gas can flow laterally to the high side and stay there. The diver can change the centre of gravity by adjustment of the equipment setup, which includes its configuration and position of weights, which ultimately influence where the effective BC lift is positioned relative to the Center of Gravity.[14]

Traditionally, weight belts or weight systems are worn with the weights on, or close to, the waist and are arranged with a quick release mechanism to allow them to be quickly jettisoned to provide extra buoyancy in an emergency. Weight carried on a belt can be distributed to shift the weight forward or backward to change the position of the diver's center of mass. Systems that integrate the weights into the BCD, can provide improved comfort so long as the BCD does not have to be removed from the body of the diver, for example in an underwater emergency such as an entanglement. When a weight integrated BCD is removed a diver wearing no weight-belt and any type of wetsuit or dry suit will be very buoyant.

By inflating the BC at the surface, a conscious diver may be able to easily float face-up, depending on their equipment configuration choices. A fatigued or unconscious diver can be made to float face up on the surface by adjustment of their buoyancy and weights so the buoyancy raises the top and front of the diver's body and the weights act on the lower and back of the body. An inflated horse-collar BC always provides this orientation, but an inflated vest or wing may float the diver face-down if the center of buoyancy is behind the centre of gravity. This floating orientation is generally considered undesirable and can be minimized by relocation of some of the weights further to the rear, and using higher density cylinders (typically steel), which also move the center of mass towards the back of the diver. The BC type can also be selected with this factor in mind, selecting a style with a center of buoyancy further forward when filled, as this has the same net effect. Any or all of these options can be utilized to trim the system out to its desired characteristics[15] and many factors can contribute, such as the number and position of diving cylinders, the type of diving suit, the position and size of stage cylinders, the size and shape of the diver's body and the wearing of ankle weights, or additional dive equipment. Each of these influence a diver's preferred orientation under the water (horizontal) and at the surface (vertical) to some degree.

The usual inflation system is through a low-pressure hose from the primary breathing gas supply, but a dedicated direct feed pony bottle was common on early buoyancy compensators, and remains an option for some models. Most BCs allow oral inflation both underwater and on the surface. This could theoretically reduce gas consumption, but is generally not considered worth the effort and slightly increased hazard of taking the DV out of the mouth underwater, and possibly having to purge it before breathing again. Oral inflation is, however, an effective alternative inflation method in case of a failure of the pressurized inflation system. Emergency inflation by expendable CO₂ cartridge is provided on some BCs.

Gas consumption varies depending on the dive profile and diver skill. The minimum consumption is by a diver who uses the correct amount to neutralize buoyancy and does not waste gas by overfilling, or by excessive weighting. The actual volume of the bladder should not affect gas consumption by a skilled user, as only enough gas to achieve neutral buoyancy is needed. Deep dives will require more gas, and dives in which the diver ascends and descends by large amounts and/or frequently, will require venting for each ascent and inflation for each descent. The amount of gas used during the dive during US Navy trials was generally below 6% of the total gas consumption,[5] and the use of small dedicated cylinders for inflation was considered adequate, but not necessary.

When used with a full-face mask or helmet, oral inflation becomes impracticable or impossible, and the reliability of the inflation system becomes safety-critical. Divers wearing dry suits have an alternative gas source available if the quick-connector systems for suit and BCD are compatible.

There are other types of equipment worn by divers that affect buoyancy:

• Backplate – Type of back-mount scuba harness
• Diving cylinder – High pressure compressed gas cylinder used to store and supply breathing gas for diving
• Diving weighting system – Ballast carried by underwater divers to counteract buoyancy
• Dry suit – Watertight clothing that seals the wearer from cold and hazardous liquids
• Wetsuit – Garment for water activities, providing thermal insulation but not designed to prevent water entering