on the same principles as a regular diving demand valve second stage. Like any other breathing apparatus, the dead space must be limited to minimise carbon dioxide buildup in the mask. In some cases the outlet suction must be limited and a back-pressure regulator may be required. This would usually be the case for use in a saturation system. Use for oxygen therapy and surface decompression on oxygen would not generally need a back-pressure regulator. When an externally vented BIBS is used at low chamber pressure, a vacuum assist may be necessary to keep the exhalation backpressure down to provide an acceptable work of breathing. The oro-nasal mask may be interchangeable for hygienic use by different people. Some models are rated for pressures up to 450 msw. The major application for this type of BIBS is supply of breathing gas with a different composition to the chamber atmosphere to occupants of a hyperbaric chamber where the chamber atmosphere is controlled, and contamination by the BIBS gas would be a problem. This is common in therapeutic decompression, and hyperbaric oxygen therapy, where a higher partial pressure of oxygen in the chamber would constitute an unacceptable fire hazard, and would require frequent ventilation of the chamber to keep the partial pressure within acceptable limits Frequent ventilation is noisy and expensive, but can be used in an emergency. It is also necessary that the BIBS gas is not contaminated by chamber gas, as this could adversely affect decompression. When this format of BIBS is fitted it can also be used for emergency breathing gas supply in the event of contaminated chamber atmosphere, though in those cases the contamination by exhaled BIBS gas would usually not be important. Locally vented BIBS When contamination of the internal atmosphere is not important, and where the external ambient pressure is higher than in the occupied space, exhaled gas is simply dumped into the internal volume, requiring no special flow control beyond a simple non-return valve. The delivery and exhaust mechanism of a BIBS demand valve for this application is the same as for a scuba or SCBA second stage regulator, and these can be used for this purpose with little or no modification. Applications Hyperbaric oxygen therapy The traditional type of hyperbaric chamber used for therapeutic recompression and hyperbaric oxygen therapy is a rigid shelled pressure vessel. Such chambers can be run at absolute pressures typically about