From: mayne@pipe.cs.fsu.edu (William Mayne)
Newsgroups: rec.scuba
Subject: Re: Nitrox (Was Re: Thermodynamic Decompression)
Date: 26 Aug 92 14:30:34 GMT
Reply-To: mayne@cs.fsu.edu
Organization: Florida State University Computer Science Department

In article <1992Aug26.042322.25885@scifi.uucp> njs@scifi.uucp (Nicholas J. Simicich) writes:
>In article <1992Aug24.234734.19448@mailer.cc.fsu.edu> mayne@cs.fsu.edu writes:
>
>>This is standard agency advice in a country like the U.S.A. which has
>>far too many lawyers. It doesn't make sense, though. What matters is
>>partial pressure of O2, which is lower when the tank is initially
>>partly filled with 100% O2 than after it has been topped off with air
>>and the O2 diluted to 40%...
>
>Bill, I think you are wrong on this one.  One counter example:
>Hydrox, the Hydrogen/Oxygen mix that is used by Comex on the deep
>dives has a PP of O2 that is comparable with 1 ATA normoxic PP O2, yet
>doesn't support combustion (or it would, since the hydrogen is quite
>flammable).
>
>I suspect that the other gases in the mix do exert a buffering effect
>on combustion. 

I believe that this is a different situation. The percentage of O2 is
very low in non-flammable hydrogen/oxygen mixtures. As with most gas laws
the laws break down some at extreme limits. Also reactions with hydrogen
which is mixed with the O2 may be different than with hydrocarbon
residues on a tank surface. I don't believe that 60% nitrogen is
going to inhibit combustion. Hence concern about hydrocarbon residues
in uncleaned tanks may be valid, experience in the rather exceptional
Florida cave diving community notwithstanding. I just doubt that it makes
much (if any) difference whether you mix by putting in O2 and immediately
topping off with air or you fill with ready-mixed nitrox. If a tank is not
safe by the former method I wouldn't trust it by the latter, either.
But I am not a chemist and I welcome informed corrections. Perhaps I
should ask about this in sci.chemistry.

I realize that the "experts" say you must have cleaned tanks to mix
by the oxygen-then-air method. But the general recommendation to have
dedicated nitrox tanks should eliminate hydrocarbon risk, anyway.
They may know something they aren't explaining. Or maybe they are just
being cautious out of fear of liability. Someone putting O2 into a
tank might be inattentive and put in more than they are supposed to,
giving a higher partial pressure than the intended final mix, for one
thing. I know experience is that people use normal tanks all the time
and get by with it. This doesn't mean it is safe, however.

> Acetylene-Air cannot reach the same flame temperatures
>as Acetylene-Oxygen.

Aren't you thinking of C2H2 being burned at atmospheric pressure?
In that case certainly it burns hotter with pure O2, since the partial
pressure of O2 is higher. I don't know how it would work in air at 5 ATA.
I suspect it would be a bit cooler than with oxygen at 1 ATA just because
it has to heat up all that N2, but still hotter than burning in air at
1 ATA. Also if there is any buffering effect 79% N2 would be more effective
than 60%. Someone probably has experience with this working in pressurized
tunnels. Does anybody know if special fire precautions are required for
compressed air workers? Can they smoke on the job?

Bill Mayne
Newsgroups: rec.scuba