Buoyancy compensator
A buoyancy compensator (or buoyancy control device, BC or BCD) is a piece of diving equipment worn by divers to provide:
- life saving emergency buoyancy both underwater and on the surface.
- the ability to adjust and control the overall buoyancy of the diver and the diver's heavy equipment allowing the diver to achieve neutral buoyancy, remaining at constant depth, or to descend or ascend in a controlled way.
Some types of buoyancy compensator also built around the diver's scuba set or are built into its harness.

# Features
BCs can have the following features:
- A low pressure direct feed that transports gas from diving cylinder and diving regulator to the BC.
- An inflation valve that allows gas from the direct feed into the bladders of the BC.
- A vent valve that allows gas to escape from the bladders of the BC. Most BCs have at least two vents: one at the extreme top and the other at the bottom of the BC. As air migrates to whichever part of the BC is uppermost, the vent situated at the shoulder is used when upright and the vent situated at the diver's waist is used when inverted.
- An over pressurization valve that automatically vents the bladders if the diver over inflates the BC by ascending or by injecting too much gas.
- A harness that the diver wears with straps around the torso and over the shoulders
- A plastic or metal backplate to support diving cylinders
- Pockets for carrying diving reel, buoys and decompression tables
- An integrated diving weighting system - pockets for lead weights with a quick release mechanism. Integrated weights can eliminate the need for a separate weight belt.
- D rings or other anchor points, for clipping on other equipment such as torches, strobes, reels, cameras and stage cylinders
- Emergency inflation cylinders. This can either be a 0.5 litre air cylinder, filled from the diver main cylinder, or a small carbon dioxide cylinder. There is a risk that an emergency cylinder is accidentally opened during a dive causing a rapid ascent and barotrauma to the diver. Carbon dioxide, being poisonous at high partial pressures, is a dangerous gas to have in a BC because the diver may inhale it from the bag underwater.

# Types
There are three main types of BC:

## Wings
Wings consist of a pair of inflatable bladders worn behind and on either side of the diver. Wings are a recent development and often used in technical diving where the diver often carries two or four large cylinders. The bladders and cylinders are fastened to a backplate which is strapped to the diver. The bladders are capable of expanding to a large volume and can therefore be very buoyant: 30 litre wings are not uncommon. Elasticated webbing around each bladder is often used to allow them to expand but to draw them back to the backplate when not inflated. Heavy equipment such as diving cylinders can be fixed to or slung from the back plate.

## Stab jacket
Stab jacket, stabiliser jacket, stab, waistcoat, or vest BCs are inflatable vests worn by the diver around the upper torso, and also act as the cylinder harness. The air bladder may extend from the back around the diver's sides ("wraparound") or may only cover the back ("back inflation.") Wraparound bladders are favored by some divers because they may make it easier to maintain balance, both submerged and on the surface. However, they have the tendency to squeeze the diver's torso when inflated. Back inflation BC's do not have this problem but have a greater tendency to float the diver face-down on the surface, which presents an obvious hazard in an emergency. Jacket-style BC's typically provide up to 25 litres of buoyancy (depending on size) and are fairly comfortable to wear. Jacket-style BC's 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" jacket-style BC's have the ability to carry multiple cylinders.

## Diver's lifejacket
Adjustable Buoyancy Life Jackets, ABLJs or horsecollar BCs are worn around the neck and chest with straps around the waist and usually between the legs. They are cheap, light and small, providing up to 15 litres of buoyancy. They float an unconscious casualty face-up. But they are old-fashioned, uncomfortable with a strap between the legs and provide less buoyancy than the other types. The diver must use a separate cylinder harness as a platform for the aqualung.

# Neutral buoyancy
When underwater, a diver often needs to be neutrally buoyant so that the diver neither sinks nor rises but remains at constant depth. A state of neutral buoyancy exists when the weight of water that the diver displaces equals the total weight of the diver. The Buoyancy Compensator is used to maintain this state by adjusting its volume and therefore its buoyancy, in response to various effects, which alter the divers overall volume or weight, primarily:
- If the diver's exposure suit is made of a compressible material such as Neoprene, the volume of the material will reduce as the diver descends and increase when the diver ascends.
- Gas contained within the air spaces within the diver's body and equipment is compressed on descent and expands on ascent, resulting in volume changes similar to those caused by exposure suit compression, but of a greater magnitude. Gas contained inside a drysuit is also compressed as the diver descends; however, the diver normally counteracts this by adding gas to the drysuit, not the BC, in order to avoid "squeeze", mild barotrauma to the skin.
- As the dive proceeds, gas is consumed from the diving cylinders of the breathing equipment. For open circuit SCUBA, this represents a significant loss of mass from the diver, which makes him or her more buoyant; the divers overall buoyancy must be reduced by venting air from the BC. For this reason the open circuit diver needs to be a little overweight at the beginning of the dive so that there is air to be dumped. The magnitude of this phenomenon is 1.225 grams for every litre of air used, or about 4.3 kg for the air in a 15l cylinder.
In practice, the diver doesn't think about all this theory during the dive. To remain neutrally buoyant, gas is injected into the BC when the diver is too heavy, or is sinking, and vented from the BC when the diver is too buoyant, or is rising. A feature of diving is that there isn't any automatically stable equilibrium position for a diver wearing a BC, or even simply for a diver with lungs full of air. Any change in depth from a position of neutrality-- even a small change-- will result in a force toward an even less neutral depth. Thus, maintenance of neutral buoyancy in scuba must be a continuous and active procedure-- the diving equivalent of balance.
A feature of scuba which is often non-intuitive to beginning students of diving, is that gas generally needs to be added to the BC when a diver descends in a controlled manner, and valved-off (removed) from the BC when the diver ascends in a controlled manner. This gas maintains the volume of the gas bubble in the BC during depth changes; this bubble needs to remain at constant volume for the diver to remain even approximately neutrally buoyant. When gas is not added to the BC during a descent, the gas bubble in the BC decreases in volume due to the increasing pressure, resulting in faster and faster descent with depth, until the diver hits the bottom. The same runaway phenomenon can happen during ascent, resulting in uncontrolled ascent, until a diver prematurely surfaces without a safety stop.
With experience, divers eventually learn to minimize this problem by minimizing the size of the "constant volume bubble" in their BC's. This requires learning the minimum weighting requirement needed for their system, at the beginning of the dive (see factors above). Somewhat complex automatic reflexes behaviors are also developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutral without having to think much about the issue. Experienced scuba divers may often be identified by the fact that they maintain neutrality without any fin use, as fish do.

# Orientation in the water
The orientation 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 diver typically wishes to be positioned face-down while under water, to be able to see and swim usefully, but face-up, to be able to breathe, when on the surface.
The orientation of a static and stable object in water, such as a diver, is determined by its centre of buoyancy and its centre of mass. At equilibrium, they will be lined up under gravity 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 much (see above-- a typical dive-resort "aluminum 80" tank at 200 bar contains about 2.8 kg of air or nitrox, of which about 2.3 kg is typically used in a dive), although a large change is possible if the weight belt is jettisoned or a heavy object is picked up; and any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure.
Generally, the diver has no control of the position of the buoyancy in the BC, only its quantity. By inflating the BC at the surface the conscious diver can easily float face-up. By deflating the BC underwater, the diver can easily be positioned facedown.  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 jettisoned to provide extra buoyancy in an emergency.  Current systems integrate the weights into the BCD providing substantial comfort and safety advantages as long as the BCD does not have to be removed from the body of the diver, for example in an emergency situation.
An unconscious diver can be made to float face up on the surface by placing 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 ABLJ always provides this orientation, but an inflated stab or old styles of wing BC generally floats the diver facedown because the centre of buoyancy is too far from the diver's front. Possible solutions to this problem are: fixing of quick release weights on or close to the diver's cylinder; using twins, large, steel, or high-density cylinders. All solutions move the centre of mass further behind the diver resulting a face-up orientation.
Many other factors, such as the number and position and density 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 influence each diver's orientation in the water.

# History
The ABLJ was developed by Maurice Fenzy in 1961. Early versions were inflated by mouth underwater. Later versions had their own air inflation cylinder. Some had carbon dioxide inflation cylinders, a development which was abandoned when valves that allowed divers to breathe from the BC's inflation bag were introduced. Since 1969 most modern BCs have used inflation gas from one of the diver's main gas cylinders, in addition to an oral inflation tube which is used at the surface to save gas. In 1971, Scubapro developed the Stabilizer Jacket, the first jacket-style BC, and in 1972 Watergill developed the Atpac wing.
More recent innovations for jacket BCs include, weight pouches to adjust attitude underwater, putting the weights on the BC rather than on a weightbelt, integrated regulators, heavily reinforced 1050 denier ballistic nylon. Innovations for wings include redundant bladders, stainless steel backplates, light weight soft nylon backplates, and 85lb lift bladders.
Dive Rite pioneered the first wing for diving twin cylinders in 1985. Competitors in tech diving include Ocean Management Systems. Other SCUBA manufacturers include Sherwood, Zeagle, scubapro and [ cressisub.

# Other buoyancy equipment
There are other types of equipment worn by divers that affect buoyancy:
- backplate
- diving cylinder
- diving suit
- diving weighting system