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On Wed, 7 Sep 1994, Ronnie Bell wrote: > well here is a different perspective. > When making extremely deep dives make a rapid ascent of 60 feet per minute > up to your first deco stop. > Why do this? It gets you out from underneath all that ongassing pressure. > If you think about it, what drives the mechanism and the accompanying dcs? > It's the pressure DIFFERENTIAL. In other words going from 363 feet to 165 > feet in one minute is the same as going from 33 feet to the surface in terms > of pressure differential. Making a "rapid" ascent to your first stop is not > inherently dangerous (imho)! If you cut tables that assume you are > ascending at 30 fpm and you ascend 10 fpm then which way are you > "biasing" your dissolved gasses? Well I think you end up with MORE gas > than what the table "thinks" you have so that when you end up doing your > deco it MAY be inadequate. > I would post a little matrix showing what ascent depths involve in terms of > percentage of pressure redution per foot but I know we don't need that. > > SO - minimize ongassing (less time at depth i.e. rapid ascent) > maximize gas pressure differentials on deco - high O2 low N2 > do your deco as prescribed in the table (or use DECOM and ongass > the extra time you are going to spend at that depth so that you > totally account for all gas activity) Yeah, I know all about the pressure differential at different depths (very important thing to consider when trying to bring up little reef fishes with swim bladders alive). Stated simply, a given increment of ascent at depth results in a proportionally smaller drop in ambient pressure than the same ascent increment in shallow water. Nevertheless, our experiences ARE our experiences, regardless of what we hypothesize SHOULD happen (given our knowledge of diving physics & physiology). The only "rational" explanation I can come up with is that microbubbles may form on initial fast ascents (even if those ascents occur between depths of 300 & 150 feet), which could possibly grow during the subsequent decompression. The greatest problems are encountered is a scond dive is conducted. The patterned we've observed is as follows (with lots of variation, of course, but this seems to be an underlying pattern): 1 deep dive only with SLOW deep ascent - no problem 1 deep dive only with FAST deep ascent - occassional problems 1 deep dive with SLOW deep ascent, followed by second medium-depth dive with slow ascent - no problems 1 deep dive with FAST deep ascent, followed by second medium-depth dive with slow ascent - frequent problems. In other words, if we ascend relatively fast on a deep dive, we have more problems than if we ascend slow (by fast deep ascent, I mean ~60 fpm from 200 feet to 100 feet). But more importantly, a fast deep ascent on a deep dive causes FREQUENT problems if we do a second dive to moderate depth (60-100 feet). To me, this means that the fast deep ascent is setting us up to get into trouble from subsequent exposures. Clearly, there's lots we still don't understand. Aloha, Rich deepreef@bi*.bi*.ha*.or*
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