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At 01:35 AM 9/23/99 -0400, Jeff wrote:
>Carl wrote:
>>
>> Dave DeBarger wrote:
>> > if one completes two shallow dives in the morning,
>> > (buddied with an OW-certified relative, say) would a deep afternoon
>> > dive then be an ill-advised reverse profile -- assuming a surface
>> > interval between morning and afternoon dives of three hours or
>> > more?  (For the purposes of this question let's assume typical
>> > recreational dives, not bounce dives.)
>
>> How deep is deep?  Personally, with a 3 hour SI, I wouldn't think
>> twice about doing a "deep" afternoon dive.
>
>I would concur, with the clarification that you're using a computer as
>most of us do these days.  Depending on the features of your computer,
>the "time-to-fly" if supplied is a worse-case scenario of your slowest
>imaginable compartments.
 
(SNIP)
 
Jeff, the N2 loading was not the issue Dave was asking or worrying about.
It's the REVERSE PROFILE, one that the old Cockroach computer tweaked
to the extreme -- adding penalty points for unsportsmanship conduct
every time a dive with a max depth X is followed by one with max depth
Y, where Y > X, until the computer eventually croaks WITHOUT EXCEPTION
on all of MY liveaboard dives.  :-)
 
The "reverse profile" no-no is based on some mumbo-jumbo THEORY of
how little bubbles and big bubbles play ball with microbubbles ...
in which there are some truth to it in the lab and ideal theoretical
environment, but to date, has had absolutely no empirical or
scientific DATA (based on controlled experiments on scuba dives) to
quantify the adverse effect of "reverse profile" which is, just
like "a deep dive", a very ill-defined concept itself.
 
On liveaboard diving (4-5 dives a day), there is hardly a single
day I DON'T do a reverse profile.  Nobody goes out of his way to do
a reverse profile.  :-)   But if a great WALL comes along on the
3rd dive of the day (quite common), I'll go "deep" in a heart beat
Am I still alive?  Hmmmmm ... let me check ... yes.  :-)
 
To Dave I would say, don't worry about your "deep" second dive,
which is probably just a "deeper shallow".  :-)  Worry about the
fundamentals of good diving skills -- slow ascents, safety stops,
no popping to the surface from 15 ft after a safety stop (a
rather common BAD practice by divers, I've noticed, especially in
Cozumel drift dives when the boat is right above -- the last 10
ft of ascent should be VERY VERY SLOW, if at all possible) ...
and you might live to be as OLD and NASTY as Feesh one of these
days.  :-)
 
-- Bob.