BCD (Buoyancy Control Device)

An inflatable jacket or wing that lets a scuba diver add or release air to control buoyancy at depth and float comfortably at the surface.

In scuba diving, the BCD is the inflatable harness that holds your tank and lets you fine tune how much you float or sink. Add a little air and you rise, vent a little and you descend. At the surface a fully inflated BCD keeps you floating without effort, and at depth small adjustments keep you neutrally buoyant so you neither crash into the reef nor drift toward the surface.

The BCD connects to the regulator first stage through a low-pressure inflator hose, so you inflate it with tank gas at the press of a button and deflate it through the inflator or a dump valve. Because your wetsuit compresses as you descend, you lose buoyancy with depth and must add air to compensate, then vent that same air on the way back up to keep your ascent rate under control.

Common styles are the jacket BCD used in most recreational training, the back-inflate BCD, and the backplate and wing favored by technical divers. Whichever you use, checking the inflator and dump valves is a standard part of every buddy check, and mastering the BCD is the heart of good buoyancy control.