Beginner 8 minutes
The BCD: much more than a float
Jacket, back-inflate, and backplate-and-wing: what actually differs between them
The BCD has two jobs that look like the same thing but aren't: keeping you comfortable with your head out of the water at the surface, and letting you sit perfectly still at fifteen meters. The different designs out there are, at bottom, different answers to which of those two jobs to prioritize.
In this article you'll see how a BCD works, what types exist and how they differ, how to use its valves correctly, and what the lift capacity in the specs actually means.
The BCD is the piece of gear that determines your position in the water and your comfort throughout the dive. Choosing and adjusting it well changes the experience more than almost any other purchase.
How it works
The principle is Archimedes', which you saw in the buoyancy article: adding air increases the volume you displace without adding weight, and therefore increases upward force. The BCD is simply a variable-volume container that lets you adjust that force at will.
Its components are few. The bladder or air cell, which is the inflatable volume. The inflator, connected by a low-pressure hose to the first stage, with one button to add air and another to dump it. The dump valves, usually two or three, located at different points. And the harness, with its buckles, straps, and rings.
One detail that confuses a lot of new divers: dump valves work by gravity. Air rises, so it only escapes through a valve that's at the highest point of the air cell at that moment. To dump through the shoulder valve you need to be more or less vertical; to use the lower one, you need to be horizontal or hips-up. Dumping without the right posture is the most common cause of the "the air won't come out" frustration.
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