From: Charlie_Gibbs@mindlink.bc.ca (Charlie Gibbs)
Newsgroups: rec.scuba
Subject: Re: Tanks and GA aircraft
Date: 6 Apr 92 19:14:55 GMT
Article-I.D.: mindlink.11277
Organization: MIND LINK! - British Columbia, Canada

In article <1992Apr6.120556.22605@axion.bt.co.uk>
pyoung@zaphod.axion.bt.co.uk (Peter Young) writes:

>From article <11222@mindlink.bc.ca>, by Charlie_Gibbs@mindlink.bc.ca (Charlie
>Gibbs):
>
>>      There's no problem at all.  Think of it in terms of relative
>> pressure.  Even at 10,000 feet the ambient pressure has only gone
>> down by about 5 psi.  A tank that's filled to 3000 psi won't even
>> notice the difference.
>
>I'm surprised to hear this. I thought there were regulations about flying
>with diving cylinders because technically they are pressure vessels.
>Perhaps this is only a concern when you are flying in a jet at 35,000+ feet?

     I think the concern is because we're dealing with pressure
vessels, as opposed to flying per se.  I'm not familiar with the
regulations, but I'm sure there are regulations which apply even
if you ship a pressure vessel by truck.  If a cylinder is damaged
the consequences can be serious, but this damage is most likely
to occur as a result of the cylinder coming loose and hitting
something.  Even if we were to fly out of the earth's atmosphere
completely (side note: how do astronauts store their air?), the
15-psi drop in ambient pressure would amount to only 0.5% of the
tank's rated pressure, while the tank is normally rated for a 10%
overfill and is tested to 50% above its rated pressure when it's
hydroed.

     Let's look at it another way.  Suppose you're shipping a tank
by truck to a distant location, and the truck has to cross a high
mountain pass to get there.  Is the tank any more likely to rupture
from being trucked over a 5000-foot pass than it would be from
being in an airplane at 5000 feet?  How about if that pass was
at 3000 feet?  2000 feet?  Would it be safe to even take your tank
up to an alpine lake for a dive?

     This isn't to say we shouldn't be careful when handling high-
pressure cylinders.  They deserve a lot of respect.  But it's not
because of ambient pressure; they're designed to take lots of
pressure.  You just don't want to damage them in any way, and
there's more of a risk of that when you're moving them around,
no matter how you're doing it.

>> In the absence of tables that specifically deal with flying
>> after diving (I don't know of any but I'd love to find one)
>
>The SAA (Sub Aqua Association) tables are based on work by Prof. Buhlman
>and include information about flying after diving, including the interval
>between surfacing and taking off. There is also a set of tables for diving
>at altitudes of 2,500ft .
>
>More info on request.

     Are the Buhlmann tables available (preferably in a form readily
usable for diving)?  The ideal thing would be to have a computer
that uses these tables, with printed tables as a backup.  But I'd
be quite happy with some sort of calculation that determines your
ceiling (much as in a ceiling for decompression stops) but doesn't
stop at sea level.  After all, if you fly after diving you'd have
to observe a ceiling much like the one you observe during a
decompression stop, because you are in fact decompressing yourself
below sea-level ambient pressure; it's just that instead of being
at 20 fsw, your ceiling might be at 5000 MSL or wherever.  Perhaps
it'd even be possible to have a table giving these maximum altitudes
in terms of the group letter you get from (e.g.) the PADI RDP.

Charlie_Gibbs@mindlink.bc.ca
"I'm cursed with hair from HELL!"  -- Night Court