On Thu, 9 Apr 1998, Tom Mount wrote:

> The statement was about a bulhman model I happened to agree with the answer
> Trout gave and showed the list why, by running bulhman model tables.
> Tom

Tom, let's go back and parse the question, since it's clear
the question was lost among all the posturing in this thread:

7.    Oxygen can be employed to increase safety regarding DCS or it
can be used to shorten decompression time by permitting the diver to
conduct the 10fsw stop at 20fsw.

True/False.              

OK first, "or" makes this two statements, making the entire question
false if either individual statement is false.  What we
get is:

[1] "Oxygen can be employed to increase safety regarding DCS"

Clearly, this is true.

[2] "it can be used to shorten decompression time by permitting the diver to
conduct the 10fsw stop at 20fsw."

The statement "it can be used to shorten decompression time"                   

is also obviously true, which is what I think you were
getting at with your deco examples.  Again, nobody is
going to argue with that part - obviously O2 can be
used for accellerated decompression.

The problem comes with the next part; "by permitting the diver to
conduct the 10fsw stop at 20fsw"  which describes HOW it is
that it can be used to shorten deco. 

IANTD uses Bulhmann tables in the manuals, and promotes
Buhlmann-based deco software packages.
I assume TDI does as well.  

The simple fact is that if you are using Buhlmann,
and you are breathing pure O2, conducting the 10fsw stop
at 20fsw will not make one second of difference in
your hang time.

And that makes the second part false, which makes
the entire statement false - which makes the answer "FALSE".

If somebody doesn't believe this, I suggest they
*try* it.  Run a profile with 100% 02 for deco
starting at 20ft, using Voyager, Decom, ZPlan, 
or whatever, which are all Buhlmann ZHL16-based 
software packages.  Most of these can combine 
your 10 and 20ft stops into one long stop.  

Run it both ways - you'll see there's virtually no
deco difference.

And to address the minless wanderings of this thread,
let me once again repeat:

This has nothing to do with whether or not Buhlmann
ZHL16 is correct in this regard.  It is the model
used to instruct students, and the model used
by 99% of recreational "technical" divers, and is 
therefore the basis with which we must answer this 
exam question.

And to further repeat: while there is some evidence
that higher PO2 by itself can be useful
in shortening deco times, there aren't any models
that explain why or exactly how.

Even the stuff about "oxygen window", (a very misused
term) does not explain it.  Oxygen Window refers
to a theory called "zero supersaturation ascent", which
is basically a play on total tissue saturation
that relies on the fact that due to metabolic
consumption, you always have an O2 pressure 
deficit in the deeper tissues, which is
normally 8-13% below ambient.   Your body
relies on this pressure differential to move the O2 from
blood to tissues.

You can pack this extra "room" below saturation with
excess N2 or He.  But it's not that simple,
as the saturation deficit increases linearly
with depth, and if you *use* that extra room,
you've got to vent off the N2 overload
in a conventional way before ascending, which
ultimately gets back to tissue half-times
and off-gassing gradients (recently modified
by the bubble theorists).

Anyway, the upshot is that "zero supersaturation"
and "oxygen windos" were popular themes with 
commercial dive theorists in the seventies, but
it does not by itself get around the problem
that an off-gassing gradient can't go higher 
than 100%, which is the core of the theoretical snag
when it comes to explaining how pure o2 is more
effective at depth, when it should be depth-independant,
since once you get on pure O2, your off-gassing gradient
is maxed, no matter how deep you are.

So the mystery is unexplained.

-Will



> -----Original Message-----
> From: RMC <brownies@ne*.ne*>
> To: Tom Mount <TOM.MOUNT@wo*.at*.ne*>
> Cc: William M. Smithers <will@tr*.co*>; Jesse Armantrout
> <armantrout@wo*.at*.ne*>; Tech Diver <techdiver@aquanaut.com>;
> cavers@ww*.ge*.co* <cavers@ww*.ge*.co*>
> Date: Thursday, April 09, 1998 11:18 AM
> Subject: Re: Lovely
> 
> 
> >Tom,
> >
> >You could just do it our way and run a fixed gas (30/30) for the 100/100
> >bottom time and hit O2 for 15 minutes at 20 feet with a ascent time in
> >excess of 8 minutes to get to 20 feet from the bottom. As you know this
> >is one of our favorite weekly Fort Lauderdale dives during the summer
> >and we routinely utilize the triple reef lines to make our deco more
> >enjoyable. The most effective runs to date have been scootering accross
> >the 60-80 reef for 10 minutes on the way to the shallower reefs where we
> >grab our last few lobster's in 30 feet and get out. We have even done
> >this on bottom mix with slightly longer shallow times.
> >
> >You should join us this summer and see how simple our "tek" dives have
> >become. I have three spare Gavins charged at all times in addition to my
> >old Gavin. I really think this is the most fun you can have in single
> >Saturday dive.
> >
> >Have a good weekend,
> >
> >RMC
> >
> >Tom Mount wrote:
> >>
> >
> >> 5 run a constant po2 of 1.2 and deco on 1.4 the runtime is 134 minutes
> >> Tom
> >
> >
> >> -----Original Message-----
> >> From: William M. Smithers <will@tr*.co*>
> >> To: Jesse Armantrout <armantrout@wo*.at*.ne*>
> >> Cc: Tech Diver <techdiver@aquanaut.com>; cavers@ww*.ge*.co*
> >> <cavers@ww*.ge*.co*>
> >> Date: Wednesday, April 08, 1998 11:30 PM
> >> Subject: Re: Lovely
> >>
> >> >
> >> >On Tue, 7 Apr 1998, Jesse Armantrout wrote:
> >> >> >From the 1996-1997 TDI Exams Book - Decompression Procedures Exam -
> page
> >> >> 1 - question 7:
> >> >>
> >> >> 7.    Oxygen can be employed to increase safety regarding DCS or it
> can
> >> >> be used to shorten decompression time by permitting the diver to
> conduct
> >> >> the 10fsw stop at 20fsw.
> >> >> True/False.
> >> >>
> >> >> TDI Answer: False
> >> >>
> >> >> The track record speaks for itself.
> >> >
> >> >I didn't do my tech training with TDI, so I can't
> >> >speak for their record or training first hand,
> >> >but actually, the answer to the above question
> >> >is "False" according to most deco models, including
> >> >Bulhmann, which is the predominant deco model
> >> >used for decompression today (WKPP secret funky homebrew aside).
> >> >
> >> >According to the Buhlmann model, there is
> >> >absolutely no deco difference between
> >> >breathing O2 at 20, 10, or 60ft - depth won't
> >> >shorten your hang time.
> >> >
> >> >Why? Well, under Buhlmann, the only thing
> >> >that makes a difference in your deco time
> >> >is the partial pressure of N2 and/or He in your
> >> >deco mix, as this affects the relative
> >> >gradient between the gas dissolved in your tissues
> >> >and the partial pressure of the gas it is
> >> >working against (what you are breathing).  And *that*
> >> >affects your off-gassing rate.
> >> >
> >> >Your PN2 will vary with  depth, so as long as you are
> >> >breathing a mix that contains *any* N2 or He, depth
> >> >matters, but as soon as you go onto 100% O2, there is no
> >> >diluent in the mix, so depth makes absolutely no
> >> >difference - you are at your maximum off-gassing gradient, and
> >> >because there's no He or N2 in the mix, the
> >> >partial pressure of these gasses can't increase.
> >> >
> >> >-Will
> >> >
> >> >
> >> >
> >
> >--
> >Robert M. Carmichael
> >
> >http://www.browniedive.com SSA, Mixed Gas & Tank Fill Compressors
> >http://www.halcyon.net Rebreathers, Harness Backplate & Wings
> >http://www.selfrescue.com Responsible Diver Self Rescue P.S.A.
> >
> >"Simplify diving and restore the ADVENTURE.................Do it Right"
> 
> 
--
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