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Ken, You wrote: <I have a question, which maybe someone can answer for me. < <Two weeks ago, after a rather lengthy dive, I noticed a friends <Solution wanted to keep him in the water 10 minutes longer then my <Sherwood Source on the same dive profile. I got to wondering about <this, and put on my investigators hat. What I found out was that the <schedule for the dive we did on the Navy tables would have had us out <of the water about 20 minutes before we got out and my computer was <ready to pull us out 10 minutes before my friends Solution. < <Last night we dive which we decided to test the theory that Navy <tables pull you out of the water quicker. We were a three person <team, myself diving my Source, my friend Kevin diving his Suunto, and <another friend (Todd) diving the tables. < <The profile was 98' for 57 minutes. Average depth was about 95'. < <The schedule for the Navy tables was 20'-9m, 10'-28m (tbt=37m). Todd <added 5 minutes as an added safety stop and got out 42 minutes after <deco started. < <My Sherwood kept me at 20' for 10 minutes, then 10' for 45 minutes. I <got out of the water 55 minutes after deco started. When I left, <Kevin still had 15 minutes to go, even though at the start of deco <his computer said he only had 39 minutes total deco time. He decided <to violate his computer (I will not get into a debate condoning or <condemning his action) when I left based on the tables and my <computer. < <So the question(s) are: < <1) Why? I realize that the advantage of a computer is for multilevel <diving, and the majority of that dive was a square profile, but we <were not pushing the tables totally to the edge (depth wise). < <2) I have noticed that it only happens on very long relatively square <deco dives (we were staging and wearing doubles). Has anyone else <seen this same thing happen? Basic consensus is that the "old" US Navy tables were notoriously poor for decompression diving on air. In fact, even Ed Thalmann at EDU documented this (see Bove and Davis, Diving Medicine, 2nd edition, pages 43-45), showing that long decom dives at moderate depths (ie, 80ft for 120 minutes) had estimated DCS risks of 10-16%, and required triple the decompression times to bring the DCS risk into acceptable levels; shorter but deeper decom dives ( ie 40 at 150fsw or 30 at 190fsw) had DCS estimated risks of 5% and required double the decompression times to bring the risks into acceptable levels. Longer deeper divers, (ie 60 at 150fsw) had estimated DCS risks of 9-15%, and even tripling the decom times did not reduce the risk into acceptable ranges. Several studies have shown that computers based on haldanian models (or Buhlmann's algorithm) produce shorted ndl's, and longer decoms on the first dive, especially on dives that approximate square wave profiles. In fact, the profile that you used as an example, looking at the DCIEM air decompression tables calls for 6min at 30fsw, 9min at 20fsw, and 43min at 10fsw, for a total decom time of 58 minutes. Bottom line is that comparing decompression dives with computers to the "old" US Navy tables is like comparing apples to oranges, not a real valid comparison. Hope this helps explain the results you are seeing with your dive computers. John (johncrea@de*.co*) Submariner Research, Ltd.
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