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. In this application the exhaust gas is vented outside of the chamber. 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. 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. Function Externally vented BIBS These are systems used to supply breathing gas on demand in a chamber which is at a pressure greater than the ambient pressure outside the chamber. The pressure difference between chamber and external ambient pressure makes it possible to exhaust the exhaled gas to the external environment, but the flow must be controlled so that only exhaled gas is vented through the system, and it does not drain the contents of the chamber to the outside. This is achieved by using a controlled exhaust valve which opens when a slight over-pressure relative to the chamber pressure on the exhaust diaphragm moves the valve mechanism against a spring. When this over-pressure is dissipated by the gas flowing out through the exhaust hose, the spring returns this valve to the closed position, cutting off further flow, and conserving the chamber atmosphere. A negative or zero pressure difference over the exhaust diaphragm will keep it closed. The exhaust diaphragm is exposed to the chamber pressure on one side, and exhaled gas pressure in the oro-nasal mask on the other side. The supply of gas for inhalation is through a demand valve which works