Open circuit is the breathing system most commonly used for scuba diving. It is called open circuit because all the gas exhaled by the diver is expelled in the environment. A basic OC breathing apparatus is composed of a cylinder, a regulator first and second stage and a pressure gauge. The other systems are called CCR for closed circuit rebreather or SCR for semi-closed rebreather because they reuse in a loop all or part of the diver exhaled gas.
