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Hi all, First let me preface this discussion with a disclaimer. I've barely got a 3 digit number of dives, rarely entered the three digit depth range, only breathed helium in party balloons, and have never seen a rebreather in person. So, basically I don't know didley about tech diving and will gladly admit it. Despite this I would still like to make a few points that haven't been discussed in detail. IMHO Alpinist divers are either taking some pretty darn high risks or have unrealistic expectations regarding equipment reliability. First, lets put some numbers on risks. Lets say that over a diving career you perform a 1000 "serious" (ie, deco, deep, cave�.). Now over a "lifetime" what is an acceptable probability of something "bad" happening? 1 in 10 (kinda high for me), 1 in 100 (more like it), or 1 in a 1000? (pretty safe over a lifetime given the other bad shit that will probably do you in). Now which probability of bad you chose depends somewhat on what you define as bad. Moderate depths where failure would result a rather quick ascent with a tiny bit of omitted deco wouldn't be too bad at 1/10 over a lifetime. Longer/deeper with more omitted deco resulting in an IWD and flight to a nearby deco chamber perhaps 1/100. Yet deeper and still longer with a "crap I'm dead meat" ending should probably not exceed 1/1000 over a lifetime. Obviously amount /type of bailout, depth /time etc all contribute to the mix , but you get the picture. Now note that we are talking EQUIPMENT failure where you have to go to a bailout, which you either have enough of or you don't. Let us go with the 1/1000 lifetime failure rate. This means that with a 1000 dives the probability of a total failure PER DIVE needs to be in the 1 IN A MILLION. Rebreather failures can be placed in 3 categories: 1. SEEMS to be working fine but is actually hitting the grim reapers speed dial number. Unknown high or low PO2. Also starting with used/insufficient scubber that later fails when its late to abort safely. 2. Suffers some kind of partial failure that is detectable and bailout procedures USING the rebreather can be initiated. 3. System fails totally in a mechanical way, rendering it useless and a separate bailout system is the only choice. Loop failure, scrubber capacity exceed, broken loop valves, total loss of supply gas etc�.many SUBTLE ways to get this. Note that diver skill is really ONLY a consideration for type 2 failures. These type of failures can occur at a higher rate (1/1000 to 1/10,000 to 1/100,000) depending on the complexity of the bailout procedure and the likelihood of diver successfully executing it. However another form of diver skill is VERY important in type 1, and 3 errors. That skill is preparation, inspection, and maintenance of the rebreather. Only an anal retentive obsession with this skill will do, anything less and things get ugly fast. Type 1 failures CAN occur in ALL types of rebreathers! For electronic CCR it's the old crappy/condensation sensor story. Can ECCRs be made that reliable to 1 in a million per dive for type 1 failures? I think so. Are there any like that right now? I don't think so. In ANY rebreather, old/bad scrubber material can get you (remember the kitty litter incident?). ARE you sure its good and you put new stuff in? ARE you 1 in a MILION sure you changed it? ARE you THAT sure you did not mix up old and new stuff? Note that NON ELECTRONIC rebreathers have two additional modes that ECCRs don't. Using the wrong 02 mix for the drive gas. ARE you THAT sure the 02 mix is right? Another mechanical failure mode where gas dumping or addition isn't correct but still isn't detectable (seems unlikely but obviously better be pretty darn low since it is a type one failure). Note that type 1 failures need to be LOW with or without bailout REGARDLESS of bailout options due to their insidious nature (ok, bad scrubber is ok with bailout). Realize that a 1 in a MILLION failure rate is almost tantamount to saying IT CANT HAPPEN. Even 1 /10,000 is a lot lower than people realize. Again, preparation/methodology to prevent type 1 errors is always critical. Type 2 errors have been discussed previously, and their failure rates can probably realistically be achieved IF partial functioning rebreather bailout options are well planned and practiced by the user. If going the ALPINIST route, type 3 errors also need to be held to a 1 in a MILLION failure rate. Would a scrubber canister made out of metal meet this? YES. Would one made out of thin plastic and abused for a few years? Probably not. Are your loop hoses that reliable? Counter Lungs? Is your latch-on scrubber canister top THAT secure? Would a bolt on (multiple) one be? YES. What is the likelihood of maintaining a sufficient 02/gas supply? Is it as high as you think? Anyway, that's enough from me for now. Flame suite definitely on now. Buddy -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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