Buoyancy Control Device (BCD)

What a BCD does

Your buoyancy changes all through a dive, as explained in Archimedes' principle. Your suit compresses as you go down, your tank gets lighter as you breathe it, and air expands and shrinks with depth. The BCD is how you make up for those changes. It also holds the tank on your back, and at the surface it keeps you floating comfortably with your head out of the water.

The main styles

• Jacket BCDs wrap around the body and inflate around the sides and back. They are easy to float upright in at the surface, so they are common for training and rental. Under water they can squeeze the chest a little when full.
• Back-inflate BCDs put all the air behind you. They make it easier to hold a flat, horizontal position under water, which many divers prefer, but at the surface they tend to tip you forward.
• Wings are back-inflate air cells mounted on a backplate with a simple harness. They are popular in technical diving because they are adaptable and can carry twin tanks, and many recreational divers use them too.

Many BCDs have integrated weight pockets that replace a weight belt, with quick-release handles for an emergency.

The controls

The low-pressure inflator is a hose from your regulator first stage to the BCD. Press one button to add air from the tank and the other to let air out through the hose. You can also inflate by mouth through the same mouthpiece if the tank is empty, for example at the surface after a dive.

Dump valves let air out quickly from different parts of the BCD. One is usually at the shoulder, often opened by pulling the inflator hose, and another at the lower back. Air rises, so you vent from whichever valve is highest: the shoulder when you're upright and the lower back when you're head down.

An overpressure valve opens automatically if the BCD fills too far, so it can't burst.

Using it well

• Add air in small bursts and wait a moment for it to take effect before adding more.
• On the way down, add a little air as your suit compresses to stay in control.
• On the way up, let air out steadily, because expanding air in the BCD can turn a slow ascent into a fast one.
• Use your breathing for fine control and the BCD for bigger changes, as described in neutral buoyancy.
• Never use your BCD as a lift bag for heavy objects.

Choosing a BCD

Fit matters most. A BCD should be snug without squeezing your chest when full, and the tank should sit high enough that it doesn't hit your head or pull you over. Lift capacity should match your diving: a light travel BCD is fine for warm water with a thin suit, while cold-water divers with thick suits and more weight need more lift. Try it in the water before you buy if you can.

Care

Rinse the inside and outside with fresh water after diving. Fill the bladder partly with fresh water, slosh it around and drain it through the inflator hose, then let it dry part-inflated out of the sun. Have the inflator mechanism checked when your regulator is serviced.

Frequently asked questions