SCUBA QUIZ: What is the purpose of a BCD?

Answer

BCD is an abbreviation for Buoyancy Control Device! Meaning that you can control your buoyancy (lightness) underwater with this device. By putting air from your tank into your BCD you can control your buoyancy. In short: Too much air will make you 'positively buoyant' which makes you go up to the surface, float. If you do not put enough air in your BCD you will go down, sink, 'negatively buoyant'.

You use a BCD to compensate / control your descent (going down) and ascent (going up). With a BCD you can hold and release air. The same goes for your lungs, with inhaling and exhaling. When you learn to SCUBA dive you learn to balance yourself underwater and control your buoyancy with the air in your lungs and your BCD combined. This is why you need to put air in your BCD in little bits and breath in between. Do NOT use a BCD as an elevator (inflate to go up) but swim slowly to the surface and release some air to make sure you do not go up too fast. This is to compensate expanding gases.

Jackets and wings are common types of BCD's.

Jacket BCD's (see image) you 'wear like a life jacket' and the air basically flows around the whole device, front and back. Most recreational dive centers will let you use a jacket. Nowadays jackets have integrated weight pockets with a quick release system so you do not need to wear a separate weight belt. Most jackets also have pockets with a zipper or velcro to carry small things.

Wings are less common in recreational diving but really comfortable to dive with and common for single dives with two tanks. The air tube is only on your back (like butterfly wings) and shaped around the tank. The tank is balanced on your back and in general you can move more freely. With some sort of harness you attach the wing BCD to your body. You can clip things on the outside instead of putting it in the pockets of a jacket BCD.