Built-in breathing system

A built-in breathing system is a source of breathing gas installed in a confined space where an alternative to the ambient gas may be required for medical treatment, emergency use, or to minimise a hazard. They are found in diving chambers, hyperbaric treatment chambers, and submarines.

The use in hyperbaric treatment chambers is usually to supply an oxygen rich treatment gas which if used as the chamber atmosphere, would constitute an unacceptable fire hazard.^[1][2] In this application the exhaust gas is vented outside of the chamber.^[1] In saturation diving chambers and surface decompression chamber the application is similar, but a further function is a supply of breathable gas in case of toxic contamination of the chamber atmosphere.^[1] This function does not require external venting, but the same equipment is typically used for supply of oxygen enriched gases, so they are generally vented to the exterior.

In submarines the function is to supply a breathable gas in an emergency, which may be contamination of the ambient internal atmosphere, or flooding. In this application venting to the interior is both acceptable and generally the only feasible option, as the exterior is typically at a higher pressure than the interior, and external venting is not possible by passive means.