Mixed gas rebreathers

All rebreathers other than oxygen rebreathers may be considered mixed gas rebreathers, as the breathing gas is a mixture of oxygen and metabolically inactive diluent gas. These can be divided into semi-closed circuit, where the supply gas is a breathable mixture containing oxygen and inert diluents, usually nitrogen and helium, and which is replenished by adding more of the mixture as the oxygen is used up, sufficient to maintain a breathable partial pressure of oxygen in the loop, and closed circuit rebreathers, where two parallel gas supplies are used: the diluent, to provide the bulk of the gas, and which is recycled, and oxygen, which is metabolically expended. Carbon dioxide is considered a waste product, and in a correctly functioning rebreather, is effectively removed when the gas passes through the scrubber.