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Date: Mon, 14 Jul 2003 19:48:34 -0400
From: Jsuw@ao*.co*
Subject: Re: What ppO2 for bottom mix?

Using standard mixes simplifies things.  As someone else mentioned, you begin
to know the deco for the dive with that mix because you use it often.  It also
helps your team of divers to have the same mix for a dive.  That way you are
all staying a team and doing the same deco.

The lower PO2 (e.g. 1.2) is to avoid both Central Nervous System Oxygen
Toxicity, but more importantly, to avoid seeing the respiratory degradation
which occurs with whole body oxygen toxicity.  The respiratory effects will
cause inefficient off-gassing on long exposures.  This is also the reason to
incorporate "back-gas" breaks during the deco.  This is where you'd switch to
the lowest PO2 mix breathable at a given depth during deco.  An example of this
for oxygen deco at 20' (6M) would be to breathe O2 for 12 minutes and then the
back gas for 6 and then O2 for 12, etc.  This helps prevent the respiratory

Most decompression tables and programs are based on Buhlmann algorithms for
decompression modeling.  Buhlmann assumed, without testing, that helium would
lengthen the decompression schedule.  Actual experience with helium mixtures
proves otherwise.  Helium on-gasses and off-gasses quickly, and can be
off-gassed very cleanly when doing "deep stops" and using slow ascent rates. 
Not only will you be coherent on dives with helium, but you will feel much
better, less tired (and less other symptoms "decompression stress"), etc. after
the dive.

You need at least 35% helium in the mix to make any real difference.  Less than
that is a token amount and not effective enough.

When I plan a dive using software, such as DecoPlanner, I use the table
provided by the program, add deep stops, add time at gas switches to have at
least 5 minutes at a switch, and then I trim a few minutes from the shallower
stops, following the shape of the decompression curve.

I suggest you spend some time reading the articles on the web
site, as well as the site.  I have some of these articles captured
on my own web site on the following page:

As far as solo diving, I encourage you to develop a group of safety minded
divers that are qualified to do the dives you wish to do.  Begin diving as a
team.  By team, I do not mean independent divers in the water at the same time.
Plan your dives together. Execute the dives together.  Support each other
during the dive.  Watch each other perform gas switches, etc.  It's safer and
more fun.

Jan Sitchin

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