From: mayne@rho.cs.fsu.edu (William Mayne)
Subject: Re: Nitrox (Was Re: Thermodynamic Decompression)
Reply-To: mayne@cs.fsu.edu
Organization: Florida State University Computer Science Department
Date: Fri, 21 Aug 92 22:08:29 GMT

In article <1992Aug20.100013.24851@dcs.glasgow.ac.uk> kehler@dcs.glasgow.ac.uk (Carsten Kehler Holst) writes:
>
>DISCLAIMER: I "know" nothing about Nitrox. This is just my naive 
>understanding of the matter.

I'm not an expert either, though I have looked into this in some
depth. I play one on the net partly because I expect someone will
correct me if I'm wrong, and I would like to know.

>mayne@ds1.scri.fsu.edu (Bill Mayne) writes:
>
>>... From what I understand there are
>>physiological and safety advantages to nitrox even if used with equivalent
>>air depth for calculations. These include:
>
>>(1) Slight reduction in narcosis - not likely to be a big factor at the
>>    relatively shallow depths where nitrox is used.
>
>>(2) Staying in safer regions of the tables, with better resulting groups
>>    for repetitive diving, if the dive drives the decompression schedule
>>    rather than the other way around.
>
>>(3) Faster nitrogen elimination during decompression or safety stops,
>>    which is a benefit ignored by the air tables and the NOAA
>>    nitrox tables...
>
>>(4) Direct benefit to using higher percentage of oxygen as opposed to
>>    the primary benefit of reducing the percentage of nitrogen.
>
>1) My understanding is that when diving on Nitrox, say 32% O_2, 68% N_2,
>   the partial pressure of N_2 is reduced 68/79 = 0.86, so being at depth
>   d on Nitrox is like being on depth D = d*0.86-1.4 on normal air.

You've got the idea right. I don't know how you got the 1.4. That term is
a correction for the difference in partial pressure of N_2 at the surface
when using nitrox. The actual formula, with .68 substituted for the fraction
of N_2 is:

IAD=d*(.68/.79)-33*(1-.68/.79)
   =d*0.86-4.62

Hence, for example, the IAD at 100 feet on this mix is 90.62 feet,
close enough for me to call it a 90 foot dive.

>This makes (1) and (2) clear, but doesn't explain claim (3). I don't 
>understand claim (4) either. Are there something I have missed? What 
>I'm asking is: Are there any factor (bubble size, bubble formation) 
>that explain claim (3)?

No. Claim (3) is explained entirely by conventional theory. Tables
assume you are breathing air at 10 feet = 43 fsw total pressure =
43*.79 fsw N_2 pressure. If your actual breathing mix is only 68% N_2
the actual N_2 pressure is 43*.68. Lower ambient partial pressure of
N_2 yields faster elimination of excess N_2. In almost any theory,
other things being equal, a lower ambient partial pressure of N_2
means slower absorption or faster elimination of N_2 from tissues.
In fact the benefit may be less than predicted by the simple
conventional theory, but it is still an advantage.

>2) Please explain the term Nitrox decompression. Does it "just" mean diving
>   on a Nitrox blend (thereby allowing you to use the reduced equivalent
>   depth described above when doing decompression). Or does it, as I
>   have been (mis)lead to understand mean diving on air and then switch
>   to Nitrox blend when doing decompression stop. Because unless my 
>   understanding is wrong that seems to be the completely wrong thing
>   to do - though it sounds a little like claim (3).

You are correct that it is exactly like claim (3) in most cases.
Conventional theory would say that you could actually shorten
decompression using nitrox. NOAA nitrox tables don't do this. It would
be risky unless you had custom tables computed by experts.

Why do you believe that switching to nitrox during decompression from
an air dive would be the completely wrong thing to do? You certainly may
get better results using nitrox for the whole dive, provided you can
do that safely. But switching to nitrox, or to a more enriched blend, as
your depth allows during ascent won't hurt and may help. A common
application is doing a deep dive on air, where nitrox could not be used
because of the risk of oxygen toxicity, but switching to nitrox after
ascending to nitrox-safe depths. This isn't just my speculation. Dick
Rutkowski (sp?), the IAND nitrox guru, lists decompression from deep air
dives as an application. (Ref. IAND Nitrox manual.) Of course if you
aren't spending a significant amount of time in decompression the benefit
to switching will be negligible. Another scenario is that even if you dive
is shallow enough you may not have enough nitrox for the whole dive, and
choose to use your limited supply only in a decompression bottle.

As for claim (4) (direct benefit to the higher fraction of oxygen),
there are at least a couple of factors. One is that if the partial
pressure of O_2 is around 1 ATA or more the rate of inert gas exchange
is reduced. You would thus absorb more slowly at depth than theory
predicts. You also off-gas slower than theory predicts, but still faster
than if you were breathing air instead (as nitrox tables or schedules
using standard air tables with equivalent air depths assume). Second is
the therapeutic value of oxygen, just as you'd use O_2 for first aid if
your respiratory or circulatory systems were compromised. Even if you
aren't experiencing DCS symptoms it is possible you could have some silent
bubbles or other problems reducing circulation in capillaries. In a more
oxygen rich environment it is easier to get oxygen where it needs to
go, to your cells. Supposedly breathing a gas richer in oxygen is also
good for CO_2 headaches and hangovers. I couldn't say since I'm not sure
whether or not I've experienced the former.

Bill Mayne
Newsgroups: rec.scuba