Re: Dive Computers- jumping in.


Scuba Diving on the Great Escape Southern California Live-Aboard Dive Boat

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Posted by Kendall Raine on September 25, 2001 at 11:51:58:

In Reply to: Re: Dive Computers- jumping in. posted by AADIVER on September 25, 2001 at 11:19:08:

Hi Frank;

Personally, I think Russian Roulette is apt. I have heard anectdotally that injuries as a proportion of the diving population were higher in the "old days." Who knows? I have no confidence in the denominator today, much less before. I suppose if the aggregate number of bends cases in the 50's and 60's were similar to today, when there are far more divers, it would be reasonable to infer that proportionately, more people got hurt back then. Furthering this is the intutition that the ratio of reported cases to actual cases has improved over time with the advent of increased awareness, DAN, more chambers and a less macho diving crowd (on average :-)).

I guess one question I have is what would you conclude if you had an answer? If the ratio of bends hits to divers was higher before deco meters, does that mean that deco meters are "better" than tables? To me, the answer is more complicated because there are so many variables, as you point out. Certainly the slower ascent rates used by most computers is an improvement. The the 30 fpm ascent rate came from the Buhlmann tables and was integrated into the computer version, however, not the other way around. The 60 fpm ascent rate used in the Navy tables was an arbitrary rate determined more for operational and computational convenience than any understanding of gas kinetics. Something that gets little attention in the sport community is the issue of ascent rates. After all, what's a deco stop other than a way of altering the ascent rate? Bennett wrote an interesting piece in Alert Diver about 6 months back titled "Putting on the Brakes." In short, he concluded that doing a 100 fsw dive for 12 minutes, ascending to 15 fsw at 30 fpm and doing a 3 minute safety stop (well within Buhlmann/Navy/Spencer NDL's) would still leave you 12 to 15 minutes short of what your "true" (no bubble) deco should really be. Bennett suggested that, for that dive, one should take a 5 minute safety stop at 50 fsw before ascending to the 15. Somebody ran a simulation using VPM and came up with a similar profile. Another interesting question is adaptation and the possible connection of bends to compliment system activation.

It may interest you that based on DAN statistics, many, if not the majority, of bands hits reported are "undeserved" inasmuch as the victim was diving within the NDL's.

I've thought of bringing my Doppler on a multi day bug trip and using it on volunteers just for fun. No one has offered to pay my way yet...


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