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).
In July 2012, after about a year of preparation and planning, members of the Swedish Historical Diving Society and the Royal Institute of Technology Diving Club, performed a series of hydrox dives in memory of Arne Zetterström, who was accidentally killed during the ascent from his record dive using hydrox in August 1945. The memorial dives were performed using the same breathing mixture of 96% hydrogen and 4% oxygen as was developed and tested by Zetterström in the 1940s. The dives were made to a depth of 40 metres (130 ft), just deep enough to be able to use the oxygen-lean gas mixture. Project Leader Ola Lindh commented that in order to repeat Zetterström's record the team would need to make a dive to 160 metres (520 ft), and even today a dive to that depth is considered extreme.
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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