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> Can one rebreather sustain two divers for the ascent and decompression hang?? > How is buddy breathing off of one of those? The unit has to work not quite > twice as well as required for one diver. This would be really complicated, or near impossible, I would presume -- the rebreathers all use twin hose regulators which would make the passing of the mouthpiece alone very difficult. Also, the valve on the mouth piece would need to be closed prior to every pass, lest the scrubber get flooded. Then there is the problem of breath holding (as you or someone mentioned). But then again, a technical diver should not really be relying on a buddy for sustinence; a rebreather-equiped diver should be carrying some sort of backup gas supply (though as you point out later, that may be very difficult with current designs). These are some of the reasons I like the open circuit approach: the second stage still behaves like an ordinary second stage. While it would still be prudent to carry a pony (especially if the dilutent you are carrying is hypoxic), the tanks are still ordinary tanks with ordinary bands, and there is no large scrubber housing. Overall, I think operational complexity is still lower with open circuit than with "C2" -- largely because of the redundancy issues. It is simply easier and cheaper to provide for redundancy and bailout with open circuit gear, and I expect it to stay that way for some time. But I think the real issue is that providing redundancy becomes more difficult as you increase the functionality of the system -- perhaps "C2" should not stand for "closed circuit" as much as it should stand for "complex control". After all, an open-circuit system that manages pO2 is still subject to some of the more difficult redundancy issues: computer failure, for one. Consider just the case of deco in the event of computer failure: you have been running on a custom table generated on the fly, with non-constant pO2 and pN2 (or pHe), and now the computer dies as you are about to begin your hang. What is proper deco proceedure? No tables or other computers could possibly help you out, unless they were clued in to the gas parameters throughout your entire excursion. "In the beginning," with open circuit air systems and tables, redundancy was very simple: just add tanks, watches, lights, and an extra set of tables. With computers, it became more complex: add an extra computer? Or two? Wht proceedures do I follow to get out of the water alive if all fail? Full face masks also increase the problem of providing redundancy: now we must find a safe way to provide an airsupply to a diver with an obstructed face. With mixed gas, it is more complex still: how much extra of WHICH gas do I bring? This is, of course, comlicated by the issue of needing to consider alternative failure scenarios, or changes n the dive plan. Managed gas supply systems make the issue of redundancy more complex still: now we must cope with combined computer and gas supply failure. (And, as the computer controls the gas supply, statistics says that the gas supply is now less reliable than it was -- more parts!) Closed circuit systems take the redundancy problem to the limit: now we must cope with scrubber failure as well. Of course, all of these developments also brings advantages, both in terms of extending the limits of exploration, and in terms of safety. But it is important that we recognize that the new technologies, while increasing safety while they are functional, also may pose serious issues for how to maintain that safety in the face of failure. In terms of rebreathers, then, I have to ask: "sure, your rebreather makes it possible for you to do a 22 or 26 hour dive to 300+ fsw, but what is your plan if the computer craps out when you are 4 hours into the dive at 250 fsw in the cargo hold of the Doria?" Simply relying on the fact that when operational the rebreather greatly extends your air supply is not enough; to me, the rebreather must be measured in terms of its worst-case performance. -frank -- fhd@pa*.co* | [M]athematics is not the study of intangible Platonic 1 212 559 5534 | worlds, but of tangible formal systems which have arisen 1 917 992 2248 | from real human activities. 1 718 746 7061 | -- Saunders MacLane
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