Dating to 1966, this U.S. Navy training film highlights the Mark VI semi-closed circuit apparatus and shows how it works, how it is used, and how it should be maintained. The Mark VI is a chest-mounted, two- bag, semiclosed-circuit Mk VI unit that was used from the 1960s to the 1980s by Navy divers and U.S. Navy SEALs. It used a mixture of nitrogen and oxygen and was known as semi-enclosed because some gas had to be exhausted to the sea as the diver did not metabolize the nitrogen. The Mark VI was pioneered in SEALAB, the U.S. Navy’s experimental undersea habitat.

Semi-closed circuit rebreathers are less efficient than closed circuit, and are more mechanically complex than open circuit or closed circuit oxygen rebreathers. In a semi-closed rebreather the loop mix depends on a combination of factors:

* the type of gas addition system and its setting, combined with the gas mixture in use, which control the rate of oxygen added.

*work rate, and therefore the oxygen consumption rate, which controls the rate of oxygen depletion, and therefore the resulting oxygen fraction.

*depth, which has the usual effect of increasing partial pressure in proportion to ambient pressure and oxygen fraction.

Rebreather diving is underwater diving using rebreathers, which recirculate the air already used by the diver after replacing oxygen used by the diver and removing the carbon dioxide metabolic product. Rebreather diving is used by recreational, military and scientific divers where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable.

Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface supplied diving.

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This film is part of the Periscope Film LLC archive, one of the largest historic military, transportation, and aviation stock footage collections in the USA. Entirely film backed, this material is available for licensing in 24p HD and 2k. For more information visit http://www.PeriscopeFilm.com