Buoyancy Control Device

The Buoyancy Control Device (BCD) is also known by many divers as a Buoyancy Compensator (BC) or just a plain Jacket. It allows the diver to adjust buoyancy by increasing or releasing an amount of air, thereby allowing the diver to ascend or decend as required. It also allows divers to finetune their buoyancy while at the bottem, so that they can swim slightly above the sea bed rather than touching it, which disturbs sediment and damages marine life. At the beginning or end of a dive the BCD can be fully inflated to keep the head clear of the water while on the surface.

Modern BCDs designed for recreational diving incorporate a harness to secure the air cylinder, an air bladder to control buoyancy and a jacket with pockets. Many more recent models also include an integrated weight system.
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Weights

The buoyancy provided by the body and the additional buoyancy provided by the wetsuit means that a diver needs to wear additional weight in order to submerge. Traditional weight systems have been as simple as possible; a webbing belt with metal buckle onto which lead weights are threaded. Instead of solid lead weights, some belts have pockets into which bags of lead shot are inserted. This has the advantage of more comfort, but both weight belt systems can cause strain on the lower back, particulalrly if the amount of weight required is large.
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More recently, weight systems have been integrated into BCDs, with various mechanisms for releasing them in the case of a diving emergency. Due to the different type of mechanism that can be encountered, it is recommended that you familiarise yourself with your buddy's weight system before entering the water.