A buoyancy compensator (BC) or buoyancy compensating device (BCD) is a jacket that helps divers maintain neutral buoyancy during a dive.
When an object underwater rises, it’s positively buoyant. If it sinks, it’s negatively buoyant. When it remains stationary, divers say it has achieved ‘neutral buoyancy’ – the goal for all divers.
The purpose of diving is to enjoy investigating the undersea world so different from dry land. That’s difficult if you’re constantly sinking or rising. Neutral buoyancy allows a diver to control the dive and move around freely. Hence, divers are taught early on in scuba diving courses several methods for maintaining neutral buoyancy.
In principle, the method used by buoyancy compensators is simple. A buoyancy compensator works by filling bladders in the device with air. Since air is less dense than water, it’s positively buoyant. The attached jacket forces the diver up. All other things being equal the more air, the more upward force. But not all things are equal. As the diver rises, the pressure decreases, expanding the volume of the air in accordance with Boyle’s law (Pressure x Volume = a constant, at a given temperature). Similarly, as the diver sinks the pressure on the jacket increases, lowering the volume of air it contains. That reduced volume causes the buoyant force upward to lessen, leading to an even faster descent.
By inflating or deflating the buoyancy compensator, the diver can control how much or how little buoyant force it provides. Working alongside (or with some designs, inside) the buoyancy compensator are weights that counteract the buoyancy.
Weight belts are often used around the waist and/or ankles to maintain the desired level of buoyancy. The ankle weights are used to compensate for positively buoyant fins. In many contemporary designs, the weights will be integrated into the jacket, but in such a way that they can be quickly released during an emergency.
Balancing out all those forces to maintain neutral buoyancy is a dynamic process for both the jacket and the diver. Part of diver training in any scuba diving certification course will involve education in how to deal with those inescapable physical facts.
Beyond making a dive convenient and pleasant, buoyancy compensators have a safety function. A diver that loses motor function or consciousness can be floated safely to the surface using the buoyancy compensator alone.
Using the buoyancy compensator properly is imperative, though, in order to avoid two potential problems. (1) Rising to the surface too quickly can produce decompression sickness. (2) Even once the diver reaches the surface it’s possible to drown if the buoyancy compensator faces the diver down rather than up, as many contemporary styles do.
Divers should always dive with a buddy and preferably with at least one topside observer in the boat. Anyone who bobs to the surface can then be quickly retrieved and administered first aid, if necessary.