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On Thu, 9 Mar 1995 bmk@ds*.bc*.ca* wrote: > Another pitch the people at BMD used was that fully closed circuit > rebreathers where very dangerous. They claimed that with thier unit > if you did not exceed the operational depth/time range of your mix > there was no way to be fatally affected by hypercapnia ( CO2 poisoning ), > hypoxia ( lack of O2 ) or Hyperoxia ( O2 poisoning ). I can believe > that thier unit is safer from this standpoint than the Prism, CIS-Lunar, or > BioMarine Seapack 1000. All of these units do have the potential for > the above problems. People on this list and else where have > state that break downs are common ( especially in the Seapack 1000 ) in > fully closed circuit units. BMD stated that many people have died using fully > closed circuit units. Does any one have any perspective on the safety and > reliability issue? I've noticed that one of the primary strategies of the various semi-closed rebreather companies is to bad-mouth fully-closed rigs unjustifiably. I can't speak for the BioMarine 1000 (Rod, wanna jump in here?), but from my experience with the Cis-Lunar prototype, and from what I know of the Oceanic Phibian (fully closed), these systems are NOT substantially more dangerous than the semi-closed designs. Yes, fully-closed systems have an increased potential for problems of hypoxia and O2 toxicity (I'm not sure I buy that they have an increased potential for hypercapnia). But if you define "dangerous" in terms of probability of an injury or fatality, then the fully-closed rigs are no more dangerous than the semi-closed. This is because they generally have MANY safegaurds against the aforementioned problems. Fully-closed systems have a notorious reputation because they rely more heavily on electronic control. Historically, the reliability of electronics underwater has been marginal at best (just ask any underwater photographer). But technology has come a long way. How many people have had dive computers fail on them underwater? A few, to be sure (I've had them fail on me twice), but failures are relatively rare. In the case of the Cis-Lunar rig, there are triple-redundant computers. How many people have had three computers simultaneously fail on them in a single dive? I'd be willing to bet none. Even if ALL electronics fail, the Cis-Lunar rig (and I imagine the Phibian and BioMarine) are not horrendously difficult to operate manually. Howard Hall (or maybe it was Bob Cranston) wrote in one of his earlier articles in Ocean Realm that the electronics failed on his Mk-15.5 at the beginning of the dive, and he continued in manual mode rather than aborting the dive. I think it's mostly not true that fully-closed systems are more vulnerable to a scrubber failure than semi-closed. In EITHER case, the dive needs to be aborted on open-circuit, so hat's the difference? BMD may claim that because of their 25% loop flush, their rig is less likely to cause a CO2 blackout. I've done intentional CO2 tests by over-using a scrubber charge (for training purposes), and in my experience, the symptoms of CO2 buildup (mostly shortness of breath) become apparent and obvious about an hour before they become a real problem. Hence, I can't imagine someone "suddenly" blacking out unexpectedly due to CO2. If CO2 symptoms are detected, fully-closed systems can always Simulate semi-closed systems in this regard by manually flushing a small amount of diluent periodically. To me, the two MAJOR advantages of closed-circuit over open circuit are increased gas efficiency and increased decompression efficiency. Semi-closed systems only provide a 3-5-fold increase in the former (compared with 20-100-fold for fully-closed), and a ZERO increase in the latter. To make-up for this shortcoming, the semi-closed camp seems to be trying to paint this picture of fully-closed systems as being death machines. They also try to make semi-closed systems sound as though they're MUCH more easy to use than fully-closed. Fact is, on a scale of operational complexity, semi-closed systems are a *LOT* closer to fully closed systems than they are to SCUBA. I've tried very hard not to promote the Cis-Lunar rig above other rigs in this message (I've referred to it only because I have a fair amount of experience with it). I'm also not trying to bad-mouth the semi-closed design concept. All of this equipment has a purpose; the different rebreathers are different tools for different purposes. I will probably end-up buying a semi-closed unit in addition to the Cis-Lunar rig. My gripe is with the misinformation & scare tactics the semi-closed camp has relied on to bad-mouth fully-closed. Sorry to have used-up so much bandwidth...this issue has been bugging me (and some others on this list) for a while now... Aloha, Rich deepreef@bi*.bi*.ha*.or*
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