The inflatable buoyancy compensator is operated by adjusting the volume of gas contained in the bladder, using an inflation valve to inject gas and one or more deflation valves, or dump valves to release gas. The gas is usually supplied from a low pressure port of the diving regulator on a breathing gas cylinder, or orally, as exhaled gas, though dedicated gas cylinders can be used. At the surface, the bladder is inflated to provide positive buoyancy, allowing the diver to float in a preferred orientation, or deflated to let the diver start to sink to initiate a dive. During the dive, gas is added or released using the same valves, as required to provide the desired buoyancy.
Buoyancy control The diver needs to be able to establish three states of buoyancy at different stages of a dive: • 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. A feet first descent may make ear equalisation easier for some divers, and this is difficult unless buoyancy is slightly negative. • neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort, and no other support. This is the desired state for most of a recreational dive, and allows trim which minimises environmental impact. This state is also optimal for a number of professional diving activities. • 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 BC 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 gas bubbles in the material will change (following
Boyle's law) as the pressure changes when the diver descends and ascends. The volume of gas 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", or releasing the excess. Gas content in the BC is adjusted to correct buoyancy if these other corrections are not enough. • As the dive proceeds, gas is consumed from the
scuba cylinders carried by the diver. This represents a progressive loss of mass which makes the diver more buoyant, and the diver's overall buoyancy must be reduced by venting air from the BC. For this reason the diver needs to configure their 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 all the breathing gas carried. Air or
nitrox weighs about 1.3 grams per litre at standard pressure. Thus, the magnitude of weight change from use of breathing gas during a recreational dive usually varies from roughly for a 10-litre 200 bar cylinder breathed down to 50 bar, to roughly for a steel 15 litre cylinder breathed down to 50 bar, or about 5 lbs difference for the common 80 ft3 aluminium (AL80) cylinder (11.1 litres internal capacity) pressurised to 3000 psi, breathed down to 500psi, though in technical diving using multiple cylinders the mass loss can be considerably more, and in an emergency the reserve gas may also be used. 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. There is a depth range in open circuit diving in which effectively stable neutral buoyancy can be maintained by adjusting the lung volume during the breathing cycle. This depth range depends on the volume of ambient pressure gas spaces in and connected to the diver, and the ambient pressure, representing the depth, of the neutrally buoyant diver, with a lung at half tidal volume at the reference depth. The volume changes of external ambient pressure gas spaces are the perturbing influence, and the variation of lung volume achievable by the diver is the restoring influence. This pseudo-stable range of depths is greater at greater depths since a larger depth change is needed to change pressure, and thereby volume, by the same proportion. Similarly, the range is greater for a smaller total volume of non-respiratory ambient pressure gas space, as the variation in buoyancy is also proportional to this quantity, while the lung capacity of the diver is almost constant. 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 minimise this problem, starting by minimizing the volume of gas required in their BCs. 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 vented from 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 per cylinder). The need to compensate for excessive ballast weight by a larger volume of gas in the BC bladder considerably reduces the depth range in which breath volume adjustment can compensate for changes in BC gas volume. 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. Ease and accuracy of buoyancy control is affected by awareness of changes of depth. Precision control is relatively easy while there is a clear visual reference, but more difficult when the only reference is instrumentation. The most difficult circumstances for most scuba divers are during ascent in low visibility in mid-water without an ascent line, a time when depth control is most important for decompression safety.
Orientation in the water 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%, which will adversely affect gas consumption. The static and stable orientation of an object floating in water, such as a diver, is determined by its centre of buoyancy and its centre of mass. At stable equilibrium, they will be lined up by gravity and buoyancy with the centre of buoyancy vertically above the centre of mass. The diver's overall buoyancy and centre 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 contains about of air or nitrox, of which about 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 centre 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 movement of gas, 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
centre of gravity. 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 centre of mass. Systems that integrate the weights into the BC, can provide improved comfort so long as the BC 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 BC 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 at 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 at the lower 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 centre of buoyancy is behind the centre of gravity. This floating orientation is generally considered undesirable and can be minimised by relocation of some of the weights further to the rear, and using higher density cylinders (typically steel), which also move the centre 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 centre of buoyancy further forward when filled, as this has the same net effect. Any or all of these options can be utilised to trim the system out to its desired characteristics and many factors can contribute, such as the number and position of
diving cylinders, the type of
diving suit, the position, size, and buoyancy distribution 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 supine) to some degree.
Inflation gas supply and consumption 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 the slight additional 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 pressurised inflation system. Emergency inflation by expendable CO2 cartridge is provided on some older 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 neutralise 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, and the use of small dedicated cylinders for inflation was considered adequate, but not necessary. For deep technical diving it is considered prudent to supply the BC from a different regulator or cylinder to dry suit inflation gas, as this reduces the risk of simultaneous failure of both buoyancy control options by an order of magnitude. When used with a full-face mask or helmet, or with a rebreather, 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 BC are compatible and the gas supplies independent. The dry suit can also usually be used for additional buoyancy in an emergency. The use of compatible quick connectors for both the dry suit and buoyancy compensator is also a way of reducing the risk of both items becoming unavailable during a dive, providing the diver has the dexterity and strength to disconnect and reconnect the fittings underwater. == Hazards and malfunctions ==