At shallow depths, a person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in (4%–5% of the inspired volume). The remaining oxygen is exhaled along with nitrogen and carbon dioxide. As the diver goes deeper, much the same quantity of oxygen is used, which represents an increasingly smaller fraction of the inhaled gas. Since only a small part of the oxygen, and virtually none of the inert gas is consumed, every exhaled breath from an open-circuit scuba set represents at least 95% wasted potentially-useful gas volume, which has to be replaced from the breathing gas supply.
The rebreather recirculates the exhaled gas for re-use and does not discharge it to the atmosphere or water. The unused oxygen is kept for reuse, and the rebreather adds gas, to replace the oxygen that was consumed, and to compensate for compression when depth increases, and scrubs the carbon dioxide. Thus, the gas in the rebreather’s circuit remains breathable and supports life and the diver needs only carry a fraction of the gas that would be needed for an open-circuit system. The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent.
Though the number of users is still small, rebreather technology has greatly expanded the tech diver’s underwater envelope. It’s also been a boon to photographers/videographers and the early adopters among scientific and recreational divers, as evidenced by the conversation in Forum sessions chaired by explorer and instructor trainer Martin Robson.
Dives that would be logistically difficult or even impossible on open-circuit are routinely completed using rebreathers, and some explorers like Robson, Richard Harris and others are now pushing the limits of human physiology. During a Friday afternoon session, Harris detailed his team’s exploration dives to 680 feet (210m) at the Pearse River Resurgence (caves) in the South Island of New Zealand where divers are hitting up against the limits of “respiratory sufficiency”, and arguably surface-based diving.
However, as David Conlin, Chief of Submerged Resources Center for the National Park Service explained to the assembly, “The real value of rebreathers is not deep diving at all, but staying longer at 70-100 feet (20-30m). You can work at those depths nearly all day long when the conditions are good.” Conlin reported that rebreathers have increased Park Service diver productivity by nearly 40 percent. “We gain nearly one day for every three days we’re in the field,” he told the assembly.
There are safety concerns with rebreathers:
Though no one knows the actual risks, worldwide there have been more than 200 reported rebreather fatalities since 1998, averaging approximately 10 per year prior to 2005 and about 20 per year since. To put these numbers in perspective, on average there are about 100 to 120 scuba diving fatalities annually in the US, Canada, UK and Europe combined, which represents the majority of the worldwide market. Given that there are millions of open circuit divers compared to at most tens of thousands of rebreather divers, the fatality rate for rebreather diving is evidently much higher than its open circuit counterpart, as industry insiders are all too well aware.
During one of the opening sessions, Dr. Andrew Fock, head of hyperbaric medicine at The Albert Hospital in Melbourne, Australia asked for a show of hands in response to this question: “How many people in this room believe that the current rebreather safety record is acceptable?”
No one raised a hand
Read more about the safety concerns for rebreathers here
There are roughly 10-15 thousand rebreather divers worldwide. If you are a rebreather diver, or have had any experiences with them please share your experiences with us in the comments area below.
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