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On9/4/00 11:49 AM, Paul Braunbehrens wrote: >Your argument makes a lot of sense, it would be nice to see how this >additional advantage (blood plasma transport of O2) offsets the >disadvantage of the air breaks. While all the theories are just >that, I feel a little uncomfortable with the idea of using O2 for a >shorter time than specified by the algorithms due to O2 breaks. Afer >all, those algorithms are what we have to use, I'm not going to >blatantly disregard them. Ideally, the algorithms would take into >account the blood plasma O2 effect. Paul - It really shouldn't come as a surprise that there are a lot of factors that Buhlmann's algorithm or any of the compartment based models fail to account for -- they were never intended to be complex. Take a look at John Lippman's book "Deeper into Diving" -- in a nutshell what Buhlman did is take Haldane/Spencer compartment halftimes, revise them, derive diffusion coefficients for Helium and Nitrogen, put them together in a fairly simple formula and start chamber testing. Doppler comes back with too many bubbles, revise the half times or coefficients. Test again. Keep revising until the test subject isn't bent or the doppler is acceptably clear. That's experimental validation. The resulting equation is an practical fudge that gets you out of the water the vast majority of the time in reasonably good condition -- and was never intended to account for the complexities of diving or even of decompression theory. If there's one important point I want to bring out in this discussion it's that divers -- all divers -- should question what they've been taught, not just new information. Why did IANTD recommend 80/20 instead of 100%? What software did IANTD or TDI use to derive their runtime tables and what assumptions are built into them? How were they validated? How does the deco software I'm using actually work? What physical factors affect my ability to off gas efficiently? People come into this list with the attitude: "What I've been taught is right. Why should I do something different?" When the first question they should be asking is "How do I know that what I've been taught is right?" The use of 80/20 is a good example of this. Is it a bad choice? Not at all. But it's not the best choice, and when you look behind the scenes at why it was adopted, you find out that it was a compromise put in place for students with questionable buoyancy control. Not the reason most of us choose our deco gasses. If you start instead with the idea that 80/20 is just one of many options, you can start to compare your choices and see which is best. >While all the theories are just >that, I feel a little uncomfortable with the idea of using O2 for a >shorter time than specified by the algorithms due to O2 breaks. After >all, those algorithms are what we have to use, I'm not going to >blatantly disregard them. By all means don't do anything you're uncomfortable with. One of the early points I made about deco algorithms and tables is that they don't just model decompression, they model risk. Think straight Bulhman is too risky or isn't as accurate as it should be, you can modify it with gradient factors, but you should at least know how they work. I don't blatantly disregard anything, but I do modify my tables intelligently based on theory, my own experience, and the experience of others that I have scrutinized and adopted. Dive safely. Best regards -- Bill -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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