In February 1988, four Comex divers and two French Navy divers successfully performed six days of work at 520 meters offshore Marseille breathing a hydrogen gas mixture. This operation was the culminating point of an extensive development program carried out by Comex since 1983 on hydrogen diving. The program involved physiological research, development of specific equipment and adaptation of a commercial diving support vessel for the safe use of hydrogen. It showed that hydrogen mixtures improve divers efficiency and comfort and allow to consider longer and deeper saturation dives. It appears as the ultimate, procedure for manned intervention on subsea production systems at depths ranging from 300 to 550 meters.
Deep divers are exposed to 3 types of environmental stresses:
the High Pressure Nervous Syndrome (HPNS), which is related to the effects of the pressure on the central nervous system. It appears at around 200 meters and increases with depth, causing psycho-motor disturbances that may impair the divers efficiency.
the density of the breathing gas, which also increases with depth, and induces ventilation efforts that reduce divers' work capacity.
long confinement in the saturation chambers (thermal stress, bad sleep, lack of appetite) that induces fatigue and body weight losses.
As diving uses gas mixtures, the scientists have tried to combine the properties of various diluents of the breathing gas to overcome these stresses.
The first deep dives were conducted using helium and oxygen, a gas mixture called Heliox. Helium has a low molecular weight but no anti-HPNS property and the pressurization must be very slow. In 1972, two Comex divers reached 610 meters during the Physalie VI dive after 7.5 days of compression on Heliox.
In 1975, the Duke University introduced the concept of "the Pressure Reversal Effect" (I), according to which the HPNS can be counterbalanced by a certain amount of a narcotic gas in the breathing mixture. Effectively, a reduction of HPNS symptoms was obtained by adding approximately 5% of nitrogen in the pressurization gas. The new mixture was called "Trimix" and permitted to perform the Comex Janus IV open sea dive to 500 meters, in 1977, and the Duke University Atlantis III record dive to 686 meters, in 1981.
However, with nitrogen, the reduction of HPNS is balanced by an increase in the breathing gas density. Because nitrogen is 7 times heavier than helium, a large amount of nitrogen cannot be used without affecting the diver's ventilatory function. The ideal deep diving gas should be both light and narcotic. It was found in the upper right corner of Mendeleiev's table: hydrogen.
Fire risk is the obvious concern in hydrogen handling and every one recalls the dramatic accident of the Hindenburg. In air, at atmospheric pressure, the lower explosion limit is 4% of hydrogen. However, under pressure and mixed with helium, little information was available on the risks related to hydrogen handling.
