Buoyancy compensator

A buoyancy compensator (or buoyancy control device, BC or BCD) is a piece of SCUBA Equipment worn by divers to provide:

• life saving emergency buoyancy both underwater and on the surface
• a platform for the breathing equipment, such as diving cylinders, the diver must carry
• the ability to adjust and control the overall buoyancy of the diver and his or her heavy equipment

BCs allow the diver to descend or ascend while underwater by inflating or venting the BC.

Features

BC's can have a any of the following features:

• A low pressure direct feed that transports gas from the 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.
• 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 backpack to support the diving cylinder
• Pockets for carrying reels, buoys and decompression tables
• 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, is a dangerous gas to have in a BC because the gas from a BC can be inhaled by the diver.

Types

There are main three types of BC to choose from:

• Wings consist of inflatable bladders worn behind and to the side of the diver. They are a recent development and often used in technical diving. The diver is strapped to a back plate on to which the wings are attached. The spacious location of the bladders allows their volume and therefore their buoyancy to be high: 30 litre wings are not uncommon. Heavy equipment such as diving cylinders can be fixed to or slung from the back plate. A problem with wings is their tendency to float the diver face-down at the surface, which could be lethal in the event of the diver being incapacitated.

• Stab jacket or vest BCs are inflatable vests worn by the diver around the upper torso. They typically provide up to 25 litres of buoyancy and are fairly comfortable. They may float an unconscious casualty face-down.

• Adjustable Buoyancy Life Jackets, ABLJs or horsecollar BCs: are worn around the neck with straps around the waist and 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 aqua-Lung.

Attitude in the water

The attitude the submerged diver is influenced by the BC and by other buoyancy and weight components contributed 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 attitude 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, although it is possible if the weight belt is jettisoned or a heavy object is picked up.

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 face-down. 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.

It is possible to make an unconscious diver float face up on the surface by placing buoyancy and weights so that 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 invariably provides this attitude. On the other hand, an inflated stab or wings BC generally floats the diver face-down because the centre of buoyancy is not close enough to the diver's head. Potential solutions to this problem are: fixed weights on the diver's cylinder or the use of large, high density cylinders such as a 300 bar twinset. Both solutions move the centre of mass further behind the diver resulting a face-up attitude.

Many other factors, such as the number, 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 individual diver's attitude in the water,

Other buoyancy equipment

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

• diving suit
• diving weighting system