Mailing List Archive

Mailing List: techdiver

Message Display

Upon further thought I believe I should post a correction to
something I wrote earlier. In response to a question of the
best mix selection from a diver anticipating dives of 40-50m
in cold water I questioned whether trimix is even needed at
that depth, but suggested considering "poor man's mix", 17% He
and the balance air (yielding 17% O2) because it is easier to
make. I pointed out that the low percentage of oxygen is not
ideal, but that you must ask yourself if it is worth complicating
the mix by adding oxygen. I stand by the first part, that trimix
may not be needed at only 50m, certainly not at 40m. But having
thought about it a bit I believe that bringing the fraction of
oxygen up so that the partial pressure of O2 at depth is as high as 
1.4 and possibly 1.5 would be worthwhile for anything but a short
dive with minimal decompression. This corresponds to a fraction of
oxygen of 23.3% or 25%, i.e. a slightly hyperoxic mix. For a dive
of any length this would give a significant decompression advantage
over 17%, which may be even more important in cold water.

The much smaller advantage of adding such a small amount of oxygen to
air for use at that depth to air might not be worth it, but when the
presense of helium in the mix changes things. Besides, if you are set
up to make trimix at all adding some oxygen to the mix doesn't really
make it that much harder. Anyone doing trimix should have the ability
to mix in some O2 anyway.

Once you start adding oxygen you can consider going to higher
fraction of helium to get the equivalent narcotic depth down to
whatever is acceptable. This principle is known as "dial a high".
A narcotic depth of 165 feet is fine with me, which is why I wouldn't
use trimix at all at that depth.

Remember the cautions about the added complexity of using trimix in
a recent discussion in this mailing list. It would not be worth it
for me. Your mileage may vary.

Bill Mayne

Navigate by Author: [Previous] [Next] [Author Search Index]
Navigate by Subject: [Previous] [Next] [Subject Search Index]

[Send Reply] [Send Message with New Topic]

[Search Selection] [Mailing List Home] [Home]