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To: <boock@wr*.co*>
To: techdiver@opal.com
Subject: Re: bent in Amvergris Caye
From: Carl Heinzl <cgh@ma*.ai*.mi*.ed*>
Cc: cgh@ma*.ai*.mi*.ed*
Date: Tue, 21 Mar 95 10:54:47 -0500
Lori wrote:

>It seems to me that the technicial used her computer and ignored what
>was learned in basic open water. i.e. may deepest dive first, followed
>by next deepest... So following a 21 ft. dive, even if it was sort of
>like decompression, with a 87 ft. dive was not a good profile. I was
>taught in my first open water class the a computer will allow this, but
>it still shouldn't be done.

Robert Mullins quoted from Discover Diving:

>"That morning I had done a 100 foot+ dive, surfacing with plenty of time
>allowed by the computer.  My second dive of the morning was a mere 21
>foot dive, so I actually spent that time decompressing from the first.
>After lunch I went back out and made a third dive to 87 feet.  After
>each of the deeper dives, I had ascended to much shallower depths for
>the last fifteen minutes as a safety stop.  When I surfaced after my
>last dive, the computer said I still had approximately fifteen to twenty
>minutes of bottom time left.  According to the computer's information, I
>thought I was being careful.  Yet I still got bent!"

We can determine very little from the given scenario.  If this statement
was an exact quote (i.e. unedited) we can determine that their
"explanation" of the situation most likely belies a very poor
understanding of computers and how they work.  The statement "When I
surfaced after my last dive the computer said I still had approximately
fifteen to twenty minutes of bottom time left" is so poorly constructed
that it's laughable.  Anyone on this list knows that remaining "bottom
time" depends on the depth (e.g. many computers will scroll the NDL
times for each of the depths from 30 to 150').  I would *assume* that
they meant that they had about 15-20 minutes of NDL time at their
"shallow safety stop", which most likely means that they were indeed
fairly heavily loaded, but this may be a bad assumption.

We don't know the profiles or the surface intervals and, possibly even
more importantly, we don't know the physiological state of the diver at
the time this occured (i.e. how dehydrated, how tired, any alcohol
remaining, any other drugs, etc).  We don't even know what the symptoms
were that this person had (i.e. if they *really* were bent or if it was
something else).  I'm not trying to trivialize the situation, just
saying that, as stated we as divers cannot learn anything from this
(except to describe your situation more exactly using facts, profiles,
times, etc).

Personally I have done much more aggressive diving than this with no
problem.  There is always the real possibility that this diver is one of
those with PFO.  I know that we went over this here a couple months
back, but, I wonder if it wouldn't be a good idea to include a
description of PFO, what it is, how you can be tested, possible
consequences, in *every* first level of scuba class, especially since
it's quite likely that there are a percentage of divers out there for
whom the tables as currently used are NOT safe because of this.

Lori listened well in her basic open water class.  The thing that I have
not seen ANY open water class teach is... How long to wait before you
can dive deep again!!!  Do you wait until you're "totally clear", do you
wait until you're an "A" diver, or what?  Computer won't tell you this,
but, neither will the tables, AND, if you're diving square profiles,
computers are quite often more conservative than the tables!

An interesting discussion recently came up on rec.scuba regarding the
algorithms of some computers, in particular the Cochran Nemesis.  I
thought I'd mention it here in case others are interested.  It seems
that the Nemesis penalizes you (i.e. reduces your NDL times) for
subsequent dives done during the same "dive day", where a dive day only
ends when you're totally clear (thus a dive day could be a week or more
long!).  The penalty increases on the 2nd, 3rd, and 4th dives, then
stays constant.  The 4+ penalty reduces NDLs on all dives in the 75+'
range.  I don't have the information as to HOW much it reduces them for
the 3rd and 4th (and subsequent dives), but the *1st level* of
compensation only starts at 140 ft and is reduced as follows
	No comp		1st level comp
140ft.    9 min		8 min
150ft.    8 min		5 min
160ft.    7 min		2 min

Now, while this seems pretty innocuous (after all, 140' is pretty deep
to begin with), remember, that the 4+ dive penalty affects all
dives at the 75'+ level.

-Carl-

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