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> I'm a Nordic CMAS *** diver. I do wreck dives in Baltic Sea in 40-50m
> region. Due to darkness in these depths we suffer from narcosis more
> than in clear "southern" waters.
> 
> 1) Since I'm not exposed to mixed gas diving: What in your opinion would
> be the optimum gas mixture to these depts?

Caves are dark, too. We bring bright lights and don't consider darkness
a big factor in narcosis. With experience handling narcosis it is doubtful
that you need trimix at only 50 m (about 165 feet, if my conversion is
close). If you use mix at all consider "poor man's mix", 17% helium
and the rest air. This has 17% oxygen, not that oxygen toxicity is
problem at your depths. An advantage is that it is very easy to make.
You put helium in your tanks, top off with air, and analyze for fraction
of oxygen. Actually an ideal mix would have more oxygen because that
would reduce your decompression. But you have to ask yourself if it is
worth the trouble, as well as whether you need mix at all.

If you do decide you need mix get training in it if you can.
A class should include how to pick the best mix. In practice you
have to go with something you have tables for unless you have a
computer program to make custom tables. The method to pick the
best mix is to decide what equivalent narcotic depth you can
tolerate. This tells how much nitrogen you can have in the mix,
and whether you need mix at all. Then decide what partial pressure
of oxygen you want. (Actually for deeper dives than you are considering
oxygen is the first consideration.) This should be as high as possible
without getting into the danger zone for toxicity, and depends on the
length of the dive and how hard you will be working. This tells you how
much oxygen to have in the mix. The balance is helium. Since helium is
the most expensive ingredient and is the worst for decompression for
dives less than about two hours you want as much oxygen and nitrogen
as possible in the mix. Diving in cold water the thermal properties
of helium should also be considered.

> 2) What kind of in-water precausions and equipment do _you_ use for
>    this type of dives? DSMB:s, ponies, deco tanks? These seem to 
>    differ alot around the world.

This is too much to go into here. Besides, requirements are quite
different for wreck diving so I won't comment. Hopefully some
wreck divers who use mix will tell us all how they do it.

Bill Mayne

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