“WE MAKE OUR DIVERS SAFER, SO THEY CAN STAY LONGER AND GO DEEPER.” — Master Diver (Retired) Samuel Huss SCIENCE [ THE NAVY EXPERIMENTAL DIVING UNIT TEXT BY MICHAEL MENDUNO | PHOTOS BY STEPHEN FRINK DEEP OF IN THE DIVING [ F our muscular Navy divers, all volunteers, file carefully through the first two interlocking pressure chambers on their way to “Charlie” chamber. From there they will climb down into a cylindrical, water-filled chamber — large enough to house a school bus — that constitutes the base of the U.S. Navy Experimental Diving Unit’s (NEDU) Ocean Simulation Facility. The divers, each designated by a number for the purpose of the experiment, wear Navy Mark 16 (MK-16) closed-circuit rebreathers equipped with full-face masks. The rebreathers are charged with either trimix 12/44 (12 percent oxygen, 44 percent helium, 44 percent nitrogen) or heliox 12/88 (12 percent oxygen, 88 percent helium) — the divers haven’t been told which mixture they have. Once they’re submerged in the wet pot, the dive watch supervisor in the control room will press the divers to 200 feet, where they’ll complete a 40-minute dive while pedaling cycle ergometers (stationary bikes). They will then decompress for nearly two hours according to the MK-16 trimix table, which is 15 minutes shorter than the corresponding decompression schedule for heliox and allows initial ascent to the first decompression stop at 70 feet (the first deco stop in the heliox schedule is at 90 feet). After surfacing, the divers will be monitored for signs and symptoms of decompression sickness (DCS). Because helium, which is nonnarcotic (unlike nitrogen), is believed to have faster tissue uptake and elimination than nitrogen, existing decompression models (including Albert Bühlmann’s algorithm, popular with technical divers) assign deeper stops and correspondingly longer decompressions the greater the fraction of helium in the breathing mix. This is sometimes referred to as the helium penalty. If the models are correct (that is, if decompression with trimix is more efficient than heliox for bounce dives), NEDU scientists would expect to see a higher incidence of DCS in the heliox dives in the study than in the trimix dives. But lead researcher David Doolette, Ph.D., who is also an underwater cave explorer, is not convinced that’s what they will find. NEDU researchers developed heliox diving in the 1930s as part of the command’s initial mission. Their goal was to find a way to limit the debilitating effects of nitrogen narcosis to make it possible to rescue crews from downed Navy submarines. They hypothesized that helium would require less decompression than nitrogen, but their early tests concluded otherwise. With the successful rescue of USS Squalus survivors in 1939, heliox became the Navy’s standard breathing mix for deep diving. In recent years the Royal Canadian Navy and others began trimix research programs, in part due to high helium costs, and invited the U.S. to participate. Doolette and colleagues Wayne Gerth, Ph.D., the head of NEDU’s decompression team, and Keith Gault, however, convinced their sponsor that the program would make sense only if trimix offered significantly reduced decompression times over heliox, a claim that had never been tested. They designed the experiment accordingly. The results? The four Navy test divers successfully completed their dives. Over the next nine weeks a total of 32 volunteers conducted 50 heliox dives without incident and 46 trimix dives with two diagnosed cases of DCS. Statistically that means that the researchers must 80 | SUMMER 2016
Alert Diver Summer 2016: Page 80
