Diving BCDs

The 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

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
- 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 back plate 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 liter 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 barotraumas 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.