From: mayne@pipe.cs.fsu.edu (William Mayne)
Subject: Re: Information about advanced deep diving course wanted
Reply-To: mayne@cs.fsu.edu
Organization: Florida State University Computer Science Department
Distribution: usa
Date: Thu, 3 Sep 92 19:00:08 GMT

In article <1992Sep3.162314.19311@axis.dsg.ti.com> pyron@skndiv.dseg.ti.com writes:
>[Discussion about nitrox and whether or not nitrox diving should be
>called mixed gas diving omitted.]
>>
>>[According to DAN statistics the biggest DCS risk is with repetitive
>>and multi-day ding.
>>
>>>  Now, consider the kind of diving most people
>>>	do on vacation (me too):
>>>	
>>>		4-5 days 
>>>		3-5 dives/day 
>>>		60-100fsw
>>>
>>>	That's repetitive, multiday, *and* deep (>60fsw).
>>
>>Well, not really deep in the context of physiology and decompression
>>considerations, just training at the beginning levels.
>
>The hazard I see is where people are not thinking about cumulative nitrogen
>levels.  I've been in the water with divers who made a late night dive (say in
>at 11pm) and thought of themselves as A divers at 9am.
>>
>>Bill Mayne
>>
>Gosh Bill, are we agreeing?  :-)

We are mostly agreeing. To get any further discussion out of this
I'll have to pick on the last point. Perhaps people aren't thinking
about cummulative nitrogen levels as much as they should, but even
if they were current theories and models (and tables) don't handle this
very well. According to "Sport Diver Open Water U.S. Navy Tables"
which I have in front of me if you finish that 11 P.M. night
dive by midnight as a K diver or better you will be an A diver
by 9 A.M. It is not easy for the average diver to be worse than
K. It is only in the 30 to 50 foot range that you can get to be an
L diver or worse on a no-decompression dive and your bottom time,
counting RNT, will have to be more than 90 minutes at 50 feet,
much longer at 30 or 40. You can't get to be worse than K if
you stay within the red line indicating recommended limits based
on ultrasound studies. Hence for most realistic profiles within
USN no-decompression limits divers calling themselves A divers
as you describe are in fact A divers. The problem may be with
the tables rather than people's use of them - other than failure
to be more conservative about padding.

You might think that Buhlmann, with its compartments up to 635 minutes,
would be more conservative here. It is not quite that simple. The worst
Buhlmann group you can reach in a no-decompression dive is H, which has
you a Buhlmann A diver in only 260 minutes. However a Buhlmann A represents
much more RNT. So this is not a good comparison. More significantly, for
any no-decompression dive deeper than 30 feet you would be G or better,
which has you to no group in 12 hours, just like USN. If you are Buhlmann F,
representing a maximum no-decompression dive to 60 feet, you would reach no
group status in only 8 hours. Hence Buhlmann could give you no RNT at all
earlier than 9 A.M. after your late night dive. And the deeper your night
dive the faster you reach no RNT status.

The PADI RDP has you with zero RNT in only 6 hours for any dive within its
limits, which is scary to me. Andy Schmidt recently posted a realistic
three dive profile without even having a very long surface interval which
required some decompression using USN, Hahn, or Buhlmann tables. RDP alone
allowed this profile within its NDLs. And as I recall its euphemistically
named "mandatory safety stops", even if called decompression stops, would
still not keep you within the limits of the other tables.

Bill Mayne

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