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I wrote: >>> >>is still more likely to fail than OC.<< > Dave Sutton wrote: > > How? It has one moving part. No, two: A first stage regulator and > a second stage regulator. It actually is exactly as complex as > a single hose regulator (with -one- hose) attached to a single > tank. << I disagree. In addition to the gas addition system, you have a scrubber, bag, and non-return valves to consider also. In the spirit of disclosure, I'll say that _all_ of the failures I have seen (one exception)with those components were due to operator error (assembly), or worn equipment (ie, dry-rotted bags) that should have been caught during pre-dive checks (again, operator error). But, we can say the same about most equipment, can't we? (The one exception was, I recall, an indeterminate case of CO2 intoxication - the canister seemed OK, and the diver wasn't really overexerting himself. We never did figure out what happened). Failures of only the gas addition system _seemed_ to happen as rarely as similarly maintained OC gear. One caveat, though - the demand regulator on the Draeger required more frequent adjustments than an OC regulator, to keep it in tune. Now, for the money maker - HOW much more reliable is OC? Well, I'll back off on this, to say I'm not an engineer, so I can't quantify it. If you want to make the judgement, that the decreased reliability is so minute, that it is offset by the system's advantages (for deco), well, I won't dispute that judgement. I may not make the same one myself, (or hell, who knows? maybe I will) but I won't dispute yours. Still, oxygen doesn't cost _that_ much - and the decreased weight of the Draeger is offset by it's decreased robustness (or have you replaced that flimsy fiberglass shell?). I would think that an aluminum tank & OC reg could take alot more abuse than the rebreather could, but then again, I'm not an engineer - that's just an educated guess. As for nitrogen off-gassing, decreasing the FO2 during deco, I'd be interested if someone out there who is math-smart (rules _me_ out) could approximate its effect. Is the amount of dissolved N2 in the body so insignificant, when back in gas phase, as to not really change the FO2 that much? Or is it so little that a user should just re-purge every five minutes or so? Does anyone know the _rate_ of elimination, in units that are not simply time expressions? Rich L ----- Original Message ----- From: Dave Sutton <pilots@na*.ne*> To: Rich Lesperance <richl@uf*.ed*>; <techdiver@aquanaut.com> Sent: Sunday, October 24, 1999 1:16 PM Subject: Re: 02 rebreathers for deco > > >>>>As for the advantages of a Draeger for deco, which you described below, > I > >>agree, but my one reservation is that, even though it is a good system, > it > >>is still more likely to fail than OC.<< > > > How? It has one moving part. No, two: A first stage regulator and > a second stage regulator. It actually is exactly as complex as > a single hose regulator (with -one- hose) attached to a single > tank. Open the valve, gas to a first stage, to a second stage > regulator, with a purge valve. That's it. The demand diaphram is > the auto add valve. The purge button is the manual add push-button > which depresses the diaphram exactly like a second stage. > > > >Just be sure & practice procedures for free-flow! The Draeger _is_ > >wonderfully reliable, but it does have occasional failures (just like OC). > >Whether the failures are more common than OC, I can't really answer, but > >they take more skill to resolve. The most common I've seen are demand valve > >failures - either too high a cracking pressure (as you noted - that's what > >the bypass is for), or a free flow. Free flow seems to be less common, but > >takes alot more skill to resolve. > > > Err.... how hard is it to turn off a tank valve mounted at yer belly button > and then crack it open when you need to refill the bag every 5 minutes > or so? That's the -entire- emergency freeflow checklist for the thing. > > > More skill to resolve than open circuit? Hmmm......... Not so sure. > > Still, hanging a hose from the boat seems best. > > > Dave Sutton > > > > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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