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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 Not too difficult - already been answered. >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). Ok, let me stick my neck *WAY* out here and hope no one has a meat cleaver :^)... I've already discussed this in private email with one kind individual so I don't think I've grossly missed somthing. The problem of breath holding seems to be very greatly exaggerated in this scenario. Let's take the case of a failure at 165' necessitating a buddy breathing ascent. If you do not take lung fulls of air (and virtually no one who is a seasoned scuba diver does due to the bouyancy issues), then there is quite a bit of "room". Let's take the example of a lung that is 3/4 full of air and use 165' (that's exactly 6 ata). In order to fully inflate your lungs you'd have to rise up to 4.5 ata (or 115.5 feet) meaning that you could "hold your breath" while you're going up 50 feet. If your lungs were 1/2 full you could rise all the way to 3 ata (66 feet - 91.5 feet of ascent!!!). Now, I'm not advocating this, however the scenario of holding your breath while slowly ascending and buddy breathing from a rebreather seems to be safe (given that we do not take lungfuls with each breath). >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. Ponies are alway a good idea. I've thought that *2* 40 cu ft bottles might be easier to carry than one 80 (more well balanced, more redundancy). Open circuit, however, will not provide the biggest benefit of closed circuit, and that is the extremely long bottom times available. >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. Operational complexity is less with an abicus (sp?) than with a computer too, but that doesn't stop people from designing virutally everything on this planet on computers. It's largely a user interface issue. One possibility is to look at the CC rebreather system as merely replacing the tri-mix or bottom gas and still have the backup/deco setup ready with open circuit scuba - then you have removed all the extra tanks used for extended deep dives. Now, if all goes as planned, you won't even need the other deco bottles and as such the cost will be less (no refill on all those tanks). >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. Well, as far as computers go, it would not be difficult to have a dual (or even tri) computer setup. The older models of CIS Lunar rebreathers were *fully* redundant but from what I understand they went to a single unit because they just weren't having any failures! Granted, 15k is still a good chunk of change, but what if these units were, say 10k, 7.5k, or 5k? >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. All the more reason for the type of computer I mentioned in one of my other posts - one that is not necessarily "connected" to the rebreather but merely monitors the pp02 (and has sensors on the deco tanks) so it can keep track of all scenarios. Of course you would still need the rebreathers "control" computer system, but the one I'm talking about would act as a backup and allow you to fall off of the rebreater and onto open circuit while maintaining complete information on *your* dive. This would be a redundant computer and would virtually never be necessary, except in the event of a rebreather failure. >"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 Have you ever seen how many tanks some of the deeper/more technical dives require? I would call that anything but simple. >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 If everything fails - at some point, you're just f**ked (pardon my french) but this is true in many cases and in all walks of everyday life. >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 Not necessarily so. There are other probabilies to take into account. After all, what happens if you have custom designed tables and you deviate from the dive plan? It's much more difficult to attempt to correct via this method (and much more prone to error I suspect since you would have to think under a highly stressful situation within a very limited time period than having a computer do it. Remember, you are comparing two systems and as such you should not be comparing the rebreather to the ideal, no failure scenario. >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. This is not a fair measure of performance. Worst case performance for a car would be to have the wheel fall off at 60 mph just as your rounding a mountain curve, but even though things like wheels do fall off (very rarely) no one plans for the .0000001% case. Sometimes, shit just happens. What must be ascertained is - what is their failure rate in reality under a wide variety of conditions. If the individual design is so good that the failure rate is extremely low, then this risk may be deemed acceptable. Anyone who says that *any* risk is unacceptable shouldn't be diving. Bill Stone/Rich Nordstrom has have had rebreathers under development for several years now and has thousands of hours of field tests. They are currently being used in Mexico to do some cave exploration that would have been prohibitive had they had to lug all the tanks necessary for this endeavor to the site. I *would* like to see the rebreather manufacturers publish their statistics on failures/etc (but I doubt that this will happen). Whew - sorry for such a long post. Good discussion though - can't wait for my aquaCorps to show up :^) -Carl-
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