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> From: HeimannJ on Thu, Feb 25, 1993 2:15 PM > > [Under the conditions of his diving] I'm not sure I'd > ever go below 250 or so, even on mix. You may well not need mix or find it worthwhile, then, though 250 is pushing it on air, especially if you stay down for any length of time. On the other hand two years ago I would never have thought I'd do a lot of the things I've done in that time. You might surprise yourself some day. > The principal concern I have about mix diving is that it allows very little > flexibility. If I am diving deep on air, get tangled up inside the wreck and > exceed my planned bottom time, or need to ascend more slowly than planned, or I > can't find the anchor line, or my O2 stage malfunctions, I have contingencies > which allow me to work around all of these. Just overstaying your bottom time by a reasonable amount shouldn't put you in serious danger if you have planned a dive correctly whether you are using mixed gas or not. On the other hand if you aren't as narced you are more able to deal with getting tangled up or other unplanned events. You are right that if you can't find the anchor line and you haven't carried all your deco gas with you you would be in deep trouble. But in that case you would be in trouble even on air. Would you have enough air and, in your cold water environment, be able to endure the longer decompression not having your nitrox and O2 would require? On a cave dive in the range I do using air the 1/3 reserve would be sufficient and the water isn't that cold here. I don't know about a wreck dive where you are. I've taken a vow not to do any dive involving serious decompression without having nitrox and/or oxygen for the deco, by the way. Hence any leftover air counts as emergency gas. (For minor decompression I use it, being a cheapskate.) > The planning and logistics for mix > seem much more rigid. If I can't get at my nitrox or O2, I am bent. If I > deviate significantly from my plan, I may not have any way to recalculate my > deco schedule. Valid concerns. You can plan some contingencies. Once again I am reminded of how in some respects the cave diving environment is easier and safer. Unless I get too narced while deep or have some other problem which causes me to drown or they are stolen I know I'm not going to fail to find my decompression bottles. A regulator failure is of course possible. Generally the loss of access to one decompression bottle due to regulator or even burst disk failure will not prevent a team from finishing decompression safely. Supposedly you can even swap regulators under water (at least in fresh water) to get to a bottle with a failed regulator if you have to, provided the gas hasn't been lost. > For dives down to 200, air seems like a much more forgiving mix. > Beyond that, mix is probably a good idea, because narcosis and O2 tox make air > risky. Agreed. I have absolutely no problem going to 200 on air. If I get down and find I'm too narced I can always call the dive. Below 220 I start to get concerned with O2 toxicity, which frankly worries me much more than narcosis. Tom Mount and other veterans recommend using gas any time you go below 190 in an overhead environment, but don't always adhere to that. I think their standard is about right, even if don't live up to it in practice. > What disturbs me a little is that some people seem to be promoting mix for > intermediate depths (in my book, 150-200) without explaining that there are > associated complications and drawbacks. If people are doing that they shouldn't. But I don't think anyone is promoting mixed gas diving without training, nor that any training program doesn't cover the complications and drawbacks. Most of the mixed gas instructors and other veterans I know don't ordinarily use trimix above 190 and may choose to go even deeper on air sometimes. If I've appeared to promote gas in the 150-200 range we've just had a misunderstanding caused by vague words. When I say "deep" in this context I definitely mean 200+. Thanks for a good discussion so far. It is stimulating and useful to think about these issues. Considering cold water open ocean wreck diving also makes me realize how relatively easy I have it, and that I shouldn't get cocky. My hood is off to divers in more demanding environments. Bill Mayne
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