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At 10:12 AM 3/22/00 -0500, Brian McMillan wrote: Hello Brian, >In one of Max's rants over the past couple of days he made a comment about using multiple mixes for deco (including 80% O2 as opposed to 100%). I understand from a physiology standpoint why deco is more effective on 100% O2 then 80% O2. What I don't understand is why most people are opposed to using it. In models I have run deco on 80% works (mathematically) and in general doesn't extend deco time too much. > >I have been taking it on faith that I should be using 100% at 20' and above. Can anyone tell me the science behind it? There are a bunch of good reasons for using 100% O2 instead of 80%. Here are a few I can think of off the top of my head: 1. The oxygen-window is as open as possible with 100% O2 especially at the 20-foot stop. Now you're gonna ask what the heck is an oxygen-window, right? It's not a complicated subject, but I don't have time to do it justice now. I get asked this all the time, so it's probably worthwhile to write something on it. I'll do that in the next few days and email you a copy. For now, let's just say that 100% O2 will allow you to off-gas faster then straight Buhlmann (and intuition) says you should. In fairness, 80% does too, but it's disproportionately less efficient and has other important disadvantages. 2. If you have to bail out part-way through your decompression, breathing 100% O2 on the surface may allow you to walk away from it depending on how severely you violated your decompression and how good a shape your body and lungs are in. On most 'bounce' dives, you might skip your 30, 20, and 10-foot stops and still have a fair chance to get away with it if you breathe 100% O2 on the surface. Not so with 80%. Omitted stops can happen when diving in the ocean when weather or other circumstances require it. 3. If you do get DCS while breathing 100% O2, you're *already* administering the universally accepted treatment while awaiting transportation to a chamber. The last thing you want in your DCS treatment gas is 20% Nitrogen, the very gas that's hurting you! 4. While it's true that you won't get the full rated volume of your deco bottle when you fill it from typical O2 supply pressure, you actually end up saving money with 100% O2 because you'll never have to dump a partially-full 80% bottle to remix a new batch; you just top-off with O2 each time. Besides, there are better places than your life-support system to save money! 5. If you always use 100% O2, you can permanently mark your bottles and not have to wonder if the wrong analysis sticker got put on the bottle. A side benefit is that you can standardize your gases. How's that for starters? -Mike Rodriguez <mikey@ma*.co*> Pn(x) = (1/(2^n)n!)[d/dx]^n(x^2 - 1)^n -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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