A buoyancy compensator (BC), also called a buoyancy control device (BCD), stabilizer, stabilisor, stab jacket, wing or adjustable buoyancy life jacket (ABLJ), depending on design, is a type of diving equipment which is worn by divers to establish neutral buoyancy underwater and positive buoyancy at the surface, when needed.
The buoyancy is usually controlled by adjusting the volume of gas in an inflatable bladder, which is filled with ambient pressure gas from the diver's primary breathing gas cylinder via a low-pressure hose from the regulator first stage, directly from a small cylinder dedicated to this purpose, or from the diver's mouth through the oral inflation valve. Ambient pressure bladder buoyancy compensators can be broadly classified as having the buoyancy primarily in front, surrounding the torso, or behind the diver. This affects the ergonomics, and to a lesser degree, the safety of the unit. They can also be broadly classified as having the buoyancy bladder as an integral part of the construction, or as a replaceable component supported inside the structural body.
The buoyancy compensator requires a significant amount of skill and attention to operate, because control is entirely manual, adjustment is required throughout the dive as weight reduces due to gas consumption, and buoyancy of the diving suit and BC generally varies with depth. Fine buoyancy adjustment can be done by breath control on open circuit, reducing the amount of actual BC volume adjustment needed, and a skilled diver will develop the ability to adjust volume to maintain neutral buoyancy while remaining aware of the surroundings and performing other tasks. The buoyancy compensator is both an important safety device when used correctly and a significant hazard when misused or malfunctioning.
The ability to control trim effectively is dependent on both appropriate buoyancy distribution and ballast weight distribution. This too is a skill acquired by practice, and is facilitated by minimising the required BC gas volume by correct weighting.
Function The buoyancy compensator is used by ambient pressure divers using underwater breathing apparatus to adjust buoyancy underwater or at the surface within the range of slightly negative to slightly positive, to allow neutral buoyancy to be maintained throughout the depth range of the planned dive, and to compensate for changes in weight due to breathing gas consumption during the dive. Where staged cylinders are used, it may also be used to compensate for weight changes when dropping and retrieving these cylinders. Variations in the buoyancy of wetsuits depend on the volume and density of the suit and the ambient pressure, but for thick suits at depth it can be in the order of 10 kg. Variations in the buoyancy of dry suits should be compensated by maintaining a constant volume of gas inside the suit, by manual addition and a combination of automatic and manual dumping, independently of the adjustments to the buoyancy compensator made to compensate for gas usage.
Scope of application The buoyancy compensator is a standard item of scuba diving equipment, though not always necessary, and an optional item for surface-supplied diving, where neutral or positive buoyancy may not be necessary or desirable. Breathhold divers do not have a gas supply to operate a buoyancy compensator, so they cannot use them, though they may wear an inflatable vest lifejacket for positive buoyancy at the surface. Atmospheric pressure diving suits may use a trim tank similar to that on a submarine for small adjustments, but can be ballasted to be almost precisely neutral, and are virtually incompressible within their designed operating range.
Accurate and reliable depth control is necessary for safe decompression. The surface-supplied diver has the option to use the umbilical for depth control with the assistance of the line tender, and a tethered scuba diver can use the lifeline in the same way. Similarly, any diver using a shotline or jackstay to navigate between the surface and the work site can use it for depth control, making a buoyancy compensator non-essential provided the diver can find the shotline when needed.
In most recreational and professional scuba, neutral buoyancy during most of the dive is necessary or desirable, as it gives the diver enhanced mobility and maneuverability, and allows the diver to avoid contact with delicate benthic organisms, and to fin without disturbing sediment which can rapidly reduce visibility. For this function a buoyancy compensator is necessary.
Positive buoyancy at the surface is a safety requirement for any diver who must swim to or from the point of descent or surfacing, but this does not need to be precisely controllable buoyancy.
The buoyancy compensator is intended to control buoyancy of a diver and their personal diving equipment, including stage and bailout cylinders, and for minor additional equipment such as reels, cameras and instruments that are lightweight or near neutral buoyancy. It is not a buoyant lifting device for heavy tools and equipment. If a diving task requires the diver to work heavy, it is almost always better, and always safer, to use surface supplied equipment.
If used by saturation divers to allow mid-water work, precautions must be taken to limit possible uncontrolled upward excursion. This may be possible by limiting excursion umbilical length or using an underwater tending point.
Operating principle A buoyancy compensator works by adjusting the average density of the diver and their attached equipment to be greater than, equal to, or less than the density of the diving medium. This can be done in either of two ways: Variable volume, or inflatable: The volume of a flexible device can be varied by adding or removing ambient pressure gas, which has a relatively low density, or Variable density or compressible: The density of a rigid device can be varied by compressing or expanding the internal gas by adding or removing incompressible diving medium, which has a relatively high density. As of 2025, the overwhelming majority of BCs are variable volume types, inflated by gas at ambient pressure, and subject to unstable buoyancy variation with depth changes, but the variable density type is in use and available from some retailers who also offer the appropriate training.
Variable volume type
The common type of buoyancy compensator increases buoyancy by adding gas at ambient pressure to a flexible airtight bladder, thereby increasing the volume, and decreases buoyancy by releasing the gas into the water. This volume of gas will compress or expand as the ambient pressure varies with depth, following Boyle's law, and therefore the buoyancy of the system will increase and decrease in proportion to the absolute pressure variation and the volume of gas in the bladder. The variation of buoyancy for a given change of depth will be greater near the surface than at greater depth and greater for a large volume of gas than for a small volume. The range of depths for which the diver can compensate for these changes by voluntary adjustment of lung volume while breathing effectively is therefore dependent on the volume of gas in the bladder and the nominally neutral depth, where breathing at normal tidal volume of about 500 ml results in approximate dynamic equilibrium, and the diver remains at that depth without additional effort. This type of buoyancy compensator functions by increasing buoyancy from the most stable state, which is empty, so weighting is done for neutral buoyancy at the condition of least mass, which is at the end of the dive with the cylinders empty, at which point the diver should be able to stay at the last decompression stop without physical effort.
A few illustrative examples are presented here. They are simplified but numerically realistic:
Example 1a: A diver should be neutrally buoyant at the last decompression stop until breathing gas runs out, so that full use can be made in an emergency of all gas carried. At this point the BC should be empty if weighting is correct, and a dry suit should be at nominal inflation for the undersuit - just enough gas so that the undersuit is at optimum loft for insulation. A wetsuit will be very slightly compressed, so nearly at maximum buoyancy. An empty BC is incompressible and is not affected by depth changes.
Example 1b: If the same diver is decompressing at 3 m with reserve gas in all cylinders, the gas in the BC must support the weight of that reserve gas. A recreational diver with 50 bar of air or nitrox carried in a 12 litre cylinder will have about 0.780 kg of gas, and therefore about 0.78 litres of gas in the BC. A depth change of 1 m upwards will change ambient absolute pressure from 1.3 bar to 1.2 bar so the gas in the BC will expand to 1.3/1.2 × 0.78 = 0.845 litres, a difference of 0.065 litres, which can easily be compensated by reducing lung volume by that amount to stabilise, and a bit more to start sinking back to stop depth. Suit gas will also expand in the same proportion, and is likely to have a larger volume to start, so in practice more gas must be exhaled to get back to depth.
Example 1c: The same diver, but 2 kg overweighted, decompressing at 3 m with 50 bar reserve in a 12 litre cylinder will need about an additional 2 litres of gas in the BC for neutral buoyancy. The same depth change of 1 m upwards will increase the gas volume in the BC to 1.3/1.2 × 2.78 = 3.012 litres, a difference of 0.232 litres.
Example 2: The same diver as in example 1 at 30 m depth at the beginning of the dive with 200 bar in the 12 litre cylinder has about 3.1 kg gas in the cylinder, requiring about 3.1 litres of gas in the BC for neutral buoyancy. At this depth a depth reduction of 1 m will increase the gas volume in the BC to 4/3.9 × 3.1 = 3.18 litres, a difference of 0.08 litres.
Example 3a: A technical diver in a 7 mm wetsuit at the beginning of a dive to 60 m with 200 bar of normoxic trimix 20/30 bottom gas in a manifolded twin 12 litre set with 2 × 11 litre sling cylinders filled to 200 bar with nitrox decompression gases will be carrying about 10 kg of gas and have lost about 6 kg of buoyancy through suit compression, so will need to inflate the BCD with about 16 litres of gas to compensate. At this depth a 1 m decrease of depth will cause the gas in the BC to expand to 7/6.9 × 16 = 16.23 litres, a difference of 0.23 litres.
Example 3b: The same technical diver aborts the dive nearly immediately on reaching 60 m and surfaces with a short decompression obligation of 3 minutes at 3 m. Their back gas is down to 150 bar and the deco gas is unused when they reach the 3 m stop. The suit will have regained almost all of its buoyancy and they are carrying about 7.5 kg of gas so will need about 7.5 litres of gas in the BC to compensate. At this depth a 1 m decrease of depth will cause the gas in the BC to expand to 1.3/1.2 × 7.5 = 8.125 litres, a difference of 0.625 litres.
Variable density type