|pictured: Mares Vector 1000 AT scuba BCD|
Scuba BCDs (buoyancy compensation devices, or buoyancy control devices) are part of the scuba gear which gives the scuba diver control over buoyancy. A simple BCD is merely an inflatable jacket or vest and is nothing more than a fancy life jacket and will commonly be used by swimming instructors or lifeguards. However, a scuba BCD is much more than that! The scuba BCD is integrated with a harness to strap the tank on your back, pockets and straps for your gauges and octopus, it is an inflatable vest and backpack in one; it is the wearable item to which all of the other gear is secured.
Just like the divers who wear them, scuba BCDs come in many different styles. The most common BCD are the "Jacket" or "Vest" style, which is worn like an inflatable vest with buckles in the front. The vest style is distinguished by the way it wraps around your front, with side panels containing inflatable bladders. In this way, the vest resembles a typical "life vest" used by boaters. The tank straps onto the back and there are extra straps and pockets on either side to secure other gear, like the regulator hose, octopus (spare regulator), gauges and other sundry items. Another style of scuba BCD is the "Wing" style, which is conspicuously missing the side panels, and instead has conspicuous "balloons" either extending from the sides or alongside the tank support. These are intended to balance the buoyancy in the back, where the greatest weight is carried (being the tanks), and to make it easier to assume a face-down swimming position in the water.
Scuba BCDs are rated for lift capacity; the amount of weight they can keep afloat. To test lift capacity, a BCD is laid flat on the surface of the water and weights are suspended and added to the BCD until it sinks. The lift capacity of a scuba BCD is a factor you can consider when choosing a BCD to buy or rent. Due to size, body fat, muscle mass, bone density and lung capacity some people are naturally more buoyant than others. The least buoyant people will be large men with high muscle mass and low body fat. A less buoyant person (someone who you would call a "sinker") will need a larger lift capacity from their BCD.
But remember, a scuba BCD offers variable buoyancy control. By inflating or deflating your BCD, you can adjust your buoyancy to suit your own body, weight, depth, compression and density. The only time a diver should be concerned with lift capacity is if they are a "sinker" or if they are in the habit of carrying an extra 40 pounds of camera equipment on their dive!
Just as importantly, a scuba BCD should fit properly. On the surface your BCD will fit like a jacket, not too tightly under the arms and snug across your belly. When you strap on the scuba BCD it should be a little tight, but not uncomfortable. Women should choose a BCD designed for a woman's body, which every manufacturer produces in dozens of styles.
More expensive (and decadent) scuba BCDs are those which come with an integrated weight system; these are weighted and ballasted to render your weight belt unnecessary. Some scuba systems are so integrated that the BCD, weights and breathing system are all in one pre-assembled piece.
Whatever style of scuba BCD you choose make sure your BCD is a good one. Here are some things to consider: