Open-circuit diving involves the use of a traditional breathing apparatus, known as a regulator, in which no gas is recycled. Most divers use a gas mixture consisting of 21% oxygen, 78% nitrogen and 1% other gases (e.g. argon). Other divers use a gas mixture consisting of more than 21% oxygen. Regardless, in open-circuit diving, divers don’t recycle any of the gas they breathe.

Regulators used in open-circuit diving aren’t designed to recycle or reuse gas. The gas is stored in a tank. When you inhale through the regulator, the gas travel will travel from the tank to your regulator. When you exhale, the gas will typically be released into the surrounding sea. The only exception is if you are wearing a buoyancy control device (BCD), in which case the gas may be transferred to the wearable BCD to increase your buoyancy.

The defining characteristic of open-circuit diving is that no breathing gas is recycled. It goes from the tank into your lungs, and when you exhale, it’s either released into the surrounding sea or transferred to your BCD.