Mailing List Archive Mailing List Archive
Pete Young wrote: > >Her profile: first dive to 45 ft for 35 minutes > > > > 2-1/2 hr surface interval > > > > second dive to 60 ft for 35 minutes > > > >So, why did she get bent? > > At the risk of labouring a point, the deepest dive should always be made > first. As I understand it (and of course I welcome correction from those > more knowledgeable) all the current decompression models rely on this. > > I suspect that this was a major contributory factor. I suspect not - at least not a *major* factor. All of the common models don't take this into account at all, though the instructions for tables may make it a rule or a suggestion for reasons not accounted for in the model, i.e. largely empirical or anecdotal evidence. The models are based on very simple diffusion theories which don't take subtle stuff like this into account. (Yes, I know there are some more advanced models out there.) It is true that it turns out that the models give you more bottom time on repetitive dives with the same or shorter surface intervals if you do deepest dives first. This is a consequence of doing your worst tissue loading first so you get the most theoretical benefit from your surface interval. (That is not a very good explanation. Just try figuring some profiles in different orders and see what I mean.) This rationale does not, by itself, preclude doing shallower dives first if you stay within the parameters of the model. It just penalizes you a bit as far as allowable profiles go. In any case, I don't believe that the profile above violates the spirit of the deepest dive first rule enough to worry about. We are talking about only a 15 foot difference in max depth, moderate (I would even say mild) exposures, and a nice long surface interval. On the USN tables she wouldn't have been very far over the limit with no surface interval at all. Her dives totaled 70 minutes. The tables allow 60 minutes at her max depth for the deeper dive. We aren't told if either dive had a sawtooth profile, which would be more cause for concern. If doing less than ideal profiles as far as order of ascents and descents was such a big risk factor there would be many more cave divers getting bent. Note that I am not being cavalier about the added risks of being forced by the environment to do profiles which are undesireable. But just because someone took an anomalous hit on a profile is not reason to get picky about a trivial violation of a guideline as questionable as deepest dive first. I agree with the rest of your comments. Bill Mayne
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]