Hydrox (breathing gas)
Precautions are necessary when using hydrox, since mixtures containing more than a few percent of both oxygen and hydrogen are explosive if ignited. Hydrogen is the lightest gas (half the weight of helium) but still has a narcotic potential and may cause hydrogen narcosis.
Although the first reported use of hydrogen seems to be by Antoine Lavoisier (1743–1794), who had guinea pigs breathe it, the actual first uses of this gas in diving are usually attributed to trials by the Swedish engineer, Arne Zetterström in 1945.
Zetterström showed that hydrogen was perfectly usable to great depths. Following a fault in using the surface equipment, he died during a demonstration dive. The study of hydrogen was not resumed until several years later by the United States Navy and by the Compagnie maritime d'expertises (Comex), initially during their Hydra I and Hydra II experiments, in 1968 and 1969. Comex subsequently developed procedures allowing dives between 500 and 700 metres (1650 to 2300 feet) in depth, while breathing gas mixtures based on hydrogen, called hydrox (hydrogen-oxygen) or hydreliox (hydrogen-helium-oxygen).
These studies scored a resounding success with a simulated dive to 701 metres (2,300 ft), by Theo Mavrostomos on 20 November 1990 at Toulon, during the COMEX Hydra X decompression chamber experiments. This dive made him "the deepest diver in the world".
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