Semi-closed circuit rebreathers

SCRs are almost exclusively used for underwater diving, as they are bulkier, heavier, and more complex than closed circuit oxygen rebreathers, and applications at atmospheric pressure and below do not require the oxygen to be diluted to avoid acute toxicity. Military and recreational divers use these because they provide better underwater duration than open circuit, have a deeper maximum operating depth than oxygen rebreathers and can be fairly simple and cheap. They do not rely on electronics for control of gas composition, but may use electronic monitoring for improved safety and more efficient decompression. An alternative term for this technology is "gas extender".

Semi-closed circuit equipment generally supplies one breathing gas such as nitrox or trimix at a time. The gas is injected into the loop at a rate to replenish oxygen consumed from the loop by the diver. Excess gas must be vented from the loop as necessary to make space for fresh, oxygen-rich gas. As some oxygen remains in the vented gas, semi-closed circuit is wasteful of both oxygen and inert components.

A gas mix which has a maximum operating depth that is safe for the depth of the dive being planned, and which will provide a breathable mixture at the surface must be used, or it will be necessary to change mixtures during the dive. As the amount of oxygen required by the diver increases with work rate, the gas injection rate must be carefully chosen and controlled to prevent unconsciousness in the diver due to hypoxia. A higher gas addition rate reduces the likelihood of hypoxia and provides a more stable loop gas composition, but wastes more gas.