A comparison of open-circuit scuba diving to closed-circuit (“rebreather”) diving was conducted while collecting fishery data on black coral beds in Hawaii. Both methodologies used mixed gas from the same ship-based support system. The comparison was based on a series of eight dives, four open-circuit and four closed-circuit. These were used to make a direct-comparison of the gear in a square dive profile, a multilevel profile and two dives of varying profiles. Four general criteria were considered: time requirements for topside equipment preparation and maintenance, consumption of expendables, decompression efficiency, and potential dive durations and bailout capabilities for each of the two technologies. The open-circuit divers required 4 times as much topside equipment preparation as the rebreather divers, consumed 17 times as much gas, and cost 7 times more in expendables. The open-circuit divers incurred 42% more decompression time for the square profile dives and 70% more decompression time for the multilevel profile dives than the closed-circuit dive team. Most of the decompression advantage for the closed-circuit team is from the benefit of real-time decompression calculations, but some benefit comes from the breathing gas optimization inherent to rebreathers. For a given mass of equipment, the rebreathers allow for as much as 7.7 times more bottom time, or emergency bailout capability (depending on the chosen depth of the dive), compared with the open-circuit system.
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