A buoyancy compensator (BC) or buoyancy compensating device (BCD) is a jacket that helps divers maintain neutral buoyancy during a dive.
It is positively buoyant if an object underwater rises, on the other hand if it sinks it is said to be negatively buoyant. If it does neither than it have achieved neutral buoyancy and this is the goal of all scuba divers.
To fully enjoy investigating the differences of the undersea world from dry land you don’t want to be constantly sinking and rising, bobbing up and down. Thus when a diver achieves neutral buoyancy it allows the diver to control the dive and move around freely.
Most scuba diving courses will teach several different methods for maintaining neutral buoyancy.
The method used by buoyancy compensators is simple at least in principle. A buoyancy compensator operates by filling bladders in the device with air. The more air you put in the bladders the more upward force you will have. It will be positively buoyant since air is less dense than water. That is of course if everything else is equal.
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). If 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.
The diver can control how much or how little buoyancy force it provides by simply inflating or deflating the buoyancy compensator.
Buoyancy compensators are not only for making a dive convenient and enjoyable but they also have a safety function. If the diver happens to become unconsciousness he can float safely to the surface just by using the buoyancy compensator.
If a diver doesn’t use the buoyancy properly, the diver may incur two problems.
First problem if the diver rises to the surface too quickly he may suffer decompression sickness.
Second problem is even once the diver reaches the surface it is possible he can drown if the buoyancy compensator faces the diver down rather than up.