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> See Bennett and Elliot for some discussion of the "window", and other
> important issues of ogygen , inert gas, and deco. Don't pay even the
slightest
> attention to what the agencies say, or the deco weenies. - G
Although I consider myself a deco weenie, I'm going to comment anyway.
Basically, I think George is right, but I'll try to explain what he said
using the English language this time.
Your idea of establishing a high gradient for nitrogen and helium across the
alveolar membranes of the lungs by breathing argon is correct; however,
the gradient accross the alveolar membrane is not the only thing to
consider for decompression purposes.
The important thing to remember is that oxygen, when breathed at
sub-toxic levels, is generally not considered in decompression
calculations. The reasons are primarily: 1) a lot of oxygen is bound in
Hemoglobin, and thus not contributing to the total dissolved gas load;
and 2) oxygen is being metabolized ("burned up") by your body. The way
to think of it is that oxygen is a decompression "freebie".
As George mentioned, maximum allowable tissue tensions for decompression
is calculated by combining all of the non-oxygen gasses in solution
together. If you have loads of N2 and He in your tissues, and you start
breathing an argon-oxygen mixture, then N2 and He will come out fast,
but argon will be going *in* fast (there is essentially no argon in
your tissues to start with, so the argon gradient is big in the opposite
direction). At any given time, your total gas load is N2 + He + Ar. So, N2
and He are dropping over time because they are coming out through your
lungs. Argon is increasing over time because it's coming in. So, the
efficiency of decompression offgasing is measured by relative difference
in rates of N2+He loss minus Ar gain.
Now, He is a smaller molecule than N2, and N2 is smaller than Ar, so
looking only at diffusion rates, you could argue that the N2 and He are
coming out faster than the argon is going in. Thus, you would have a net
drop in total (combined) non-oxygen dissolved gas in your blood over time.
However, when you are breathing pure oxygen, your combined gas load drops
*MUCH* faster, because it is not off-set by the influx of argon (remember,
oxygen is a decompression "feebie"). The point is, from a deco
perspective, you want as much O2 as you can get.
A better argument for decompression with argon is to use it for
intermediate stops. For example, switch from He to N2 at 130 feet on
your way up, then switch from N2 to Ar at 60 feet (Ar is more narcotic
than N2, so you don't want to breathe it too deep), then O2 at 20 feet.
The theory is that, because Ar is a "slower" molecule, your net gas loss
would be greater during the 60-20 foot stops than it would be breathing
N2 for the same stops (only because N2 and He would be going out faster
than Ar would be coming in). However, when you look at almost any dive
profile, you'll see that the advantages (in terms of reducing deco times) of
switching to Ar on deco are pretty slim. The *disadvantages* of using Ar
for deco are primarily: 1) there are not enough data on the dynamics of
Ar and decompression to come up with numbers we can trust; 2) bubble
dynamics complicate the issue enormously (if bubble composition shifts
from N2/He to Ar, then the effects of DCS, if they do occur, may be more
difficult to treat); and of course, 3) Ar is more expensive.
In short, the theoretical advantages of using Ar for deco are small
compared to the practical disadvantages.
O.K., so maybe that wan't any more clear than G's explanation....
Aloha,
Rich
Richard Pyle
deepreef@bi*.bi*.ha*.or*
*******************************************************************
"WHATEVER happens to you when you willingly go underwater is
COMPLETELY and ENTIRELY your own responsibility! If you cannot
accept this responsibility, stay out of the water!"
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