High Quality Content by WIKIPEDIA articles! The breathing set used in the Apollo lunar landing missions had an emergency bailout in case the main supply failed. This bailout was provided by an oxygen purge system (OPS) similar to an open-circuit scuba system. Like the PLSS, the OPS also maintained suit pressure and removed heat and water vapor. When activated, the OPS provided oxygen to a separate inlet on the pressure suit. A vent valve on a separate suit outlet was manually opened to establish a steady, once-through flow to space, removing CO2, heat and water vapor. Like scuba, the OPS used gas far less efficiently than the PLSS, but as an emergency backup simplicity and reliability were paramount. The OPS was a separate unit mounted on top of the PLSS backpack, immediately behind the astronaut's helmet. The OPS was also used as a backup on tethered EVAs where a spacecraft provided oxygen to the astronaut through an umbilical cable.

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