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SCUBA-L  September 1999, Week 4

SCUBA-L September 1999, Week 4

Subject:

Re: Reverse Profile?

From:

Lee Bell <[log in to unmask]>

Reply-To:

Lee Bell <[log in to unmask]>

Date:

Fri, 24 Sep 1999 09:54:35 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (81 lines)

I've been followig this thread for a while, considering adding a couple of
cents worth of opinion.  It has not been easy because I agree and disagree
with everyone.  This takes some explanation, so here it is:
 
The original question was "Should my more experienced friends worry about
doing beginner dives in the AM and more advanced, deeper dives in the
afternoon?"  My answer to this is "If they're experienced enough to make
their own risk decisions and particularly if they're more experienced (user
defined terms) than I am, then it's their worry and not mine."  The best
think I could say to them is "I'm going on the shallow trip in the morning.
You're more than welcome to go too, but understand that you may prefer to do
dives you find more interesting."
 
To continue the discussion, I have to move on to what I think about when
making decisions about my own diving.  This is a different question, one I
have both a right and responsibility to answer.  For context, I use the
strictest definition of reverse profile.  I consider any dive that starts at
one level and includes descent to a deeper level part way into the dive and
any in a series of dives which is even an inch deeper than the previous dive
to be a reverse profile.  I find that this very strict definition allows me
to focus on the risk issue without adding a variable definition to my
thinking.
 
I believe that doing deeper dives first is lower risk than doing deeper
dives second.  The logic (coloquial definition, not technical) is consistent
with my opinion that it's better to load at depth and offgas during a long,
gradual ascent.  This does not mean that every reverse profile increases
risk enough to trigger the computer I carry between my ears.  My dives last
weekend are a good example of reverse profiles which I considered and still
consider safe enough for me.  I went to 59 feet on the first and 65 on the
second.  Further, both dives, individually, included an element of reverse
profile.  These were drift dives in low visibility conditions.  I'm still
trying to get used to new equipment and am temporarily something less than a
reliable buddy.  In low visibility conditions, it does not take much of a
distraction to lose track of buddies.  On both dives, I got separated and
surfaced to locate the towed buoy, swam to it, and descended to rejoin the
group.   The graphs of the dives (Hyper Aqualand) shows them to be closer to
a square profile than my "normal dive" and both show that the deepest point
of the each dive occurred just after my surface trip back to the buoy.  In
the strictest sense of the word, both dives were reverse profiles during the
dive and between the dives.  In effect, I made 4 dives, each one deeper than
the previous.  Still, these profiles were well within my risk limits.
Interestingly, while these dives were done with a computer, they fell within
the tables (unusual).  They were done on EAN36 and ended at an "I" (eye)
pressure group.  This may help some decide whether they would have felt as
comfortable with this as I did/do.
 
As additional comment, I agree with Bob's statements to the effect that
there are no conclusive scientific studies of reverse profiles and no
uniform definition of what a reverse profile is.  To the best of my
knowledge, advise against reverse profiles predates my diving experience,
and I've been diving for a very long time.  It certainly predates the multi
day, multi dive trips so common today and the very sophisticated (by the
standards of equipment I started with) dive computers.  I believe it's
another recommendation developed for commercial and/or military divers
which, unlike others recommendations (safety divers, on site recompression
chamber, etc.) has been adopted by the recreational diving community.  In
this case, what I think started as a recommendation has taken on the
appearance of a "rule."  Like "never hold your breath" and "always dive with
a buddy", this recommendation has grown beyond it's original significance
and context.  In context, these are all good things to consider.  Out of
context, they have little validity.
 
To head off the flames, I'l add a bit.  Solo diving has been discussed to
death.  Most would agree that it is an acceptable risk under at least some
conditions given suficient experience to understand the risks and deal with
them.  The breathing "rule" is a good way to get new divers to think about
the risk, but can clearly be violated safely when a) the diver is descending
or at constant depth and b) when the lungs are not full and the ascent is
not sufficient to fill them to the limit of their tissues (no, I'm not going
to define how far is too far).  Both are good ideas, neither is a valid
"rule".
 
My personal answer to the entire issue is that profiles are a personal risk
decision.  I do not recommend them, but neither do I avoid those that I
consider safe enough for me.  I'll consider all I know and listen to all
opinions on the subject, but reserve the right to decide what is right for
me for myself and support the right of other divers to do the same.
 
Lee

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