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Matt-

Damn engineers... ;)

The point is that often I see folks on this list delving into the obscure
theory behind this and that but miss the big picture. By the way you seem t=
o
be saying that helium is a good heat transfer medium in addition to being a
poor insulator, I really can't wrap my brain around this one. And can we
really use gases that get warm on expansion for diving? You worked up table=
s
for them yet?

Like I said, I am no physicist (or engineer) but have certainly felt the ic=
y
cold blast of air out of a regulator chill me to my bones. It feels good in
the summer and like death in the winter.

Hows this for an engineering project, take an ice diver with a frozen first
and the have him thaw it by breathing on it. Then compare the time it took
for the first to freeze and compare how long it took for it to thaw from hi=
m
breathing on it. At least you will get a dive out of it.

   Jim


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> From: mat.voss@t-*.de* (Matthias Voss)
> Organization: Harry Haller Memorial Fund
> Date: Wed, 21 Feb 2001 19:25:38 +0100
> To: Jim Cobb <cobber@ci*.co*>
> Cc: "moorea(uol)" <moorea@uo*.co*.br*>, techdiver@aquanaut.com
> Subject: Re: More about breathing Helium thermal loss
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> Jim,
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>> On a more practical note, when breathing open circuit, as the gas
>> expands from the tanks to the outlet of the second stage, it cools.
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> Some gases, like nitrogen, Oxygen, do. They do it within the limits of
> our diving environment. That is, bottle pressure 50 to 200bar, ambient
> pressure from 1 to 15 bar, ambient temperature from zero to 30 =B0C.
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> Other gases do not. It is rather fizzical, but there are gases which get
> warmer when they expand , in our kind of settings. In other settings ,
> any gas does. They just show a different behaviour ( cooling or heating)
> for different pessure/ temperature settings). I have to look it up how
> much helium is concerned here, but is definitely plays a role.
> For the temperature effect, remember that the heat capacity refers to
> mass, not to volume. We breathe volume, though, so please adjust the
> math to that.
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> The real issue about heat loss is, we do not have temperature sensors in
> our lungs ewhich tell us we should warm up a bit. This may lead to
> dangerous lowering of body temperature, apathy, drowsiness.
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>> This is the same principle that air conditioners work on.
> Well, yes. But these work with gases ( freon) which dangle near their
> point of condensation)
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> This would
>> be the primary source of heat loss rather than the composition of the
>> gas being breathed.
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> I feel the prime source is evaporation of water. Anybody care to do the
> calc?
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>> If you have ever tried a rebreather, one thing that immediately
>> strikes you is how warm and humid your breathing air is.
> This pertains to mix as well.
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> Another
>> comment is that thermal transfer infers that you are moving heat from
>> A to B, certainly not a factor when the heat has nowhere to go, i.e.
>> inside your lungs. While in a drysuit the transfer is from your body
>> to the water which surrounds you.
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> In a rebreather , B is the water surrounding the loop.
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>> As far as gas of various mixtures acting as a heat transport
>> mechanism, the expansion cooling effect has far more consequences on
>> heat loss on a diver than the composition of your breathing gas to the
>> point of rendering it inconsequential.
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> Could you prove that ?
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> Matthias
> ( no physicist, only an engineer)
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>> I am no physicist but this seems to be fairly obvious to me.
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>> Jim
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