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On Mon, 1 Aug 1994, Roger Carlson wrote:
> If I could remeber any pchem or physics, I could make more sense of this.
> I think there is a cloud or a fog forming in your masks, which happens
> whenever the temperature of a gas decends below the dew point of the gas for
> its water content.
[stuff deleted]
> Anyway, for some reason, the gas in your masks momentarily crosses the
> dew point. Could be that the gasses are literally at different temps, which I
> think is somewhat unlikely, could be some kind of stochiometric cooling due to
> mixing, could be some kind of cooling due to the introduction of a gas with a
> different rate of heat transfer to the outside, could be all those things. The
> answer's in there somewhere. Like I said, wish I could remember physics.
I wondered about this myself, but this post got me to go look.
Having a chemistry text (Chemistry? Physics? it is all the same...)
handy, I checked the entropy of mixing (dS), which is
dS= -nR{xln(x)+yln(y)}
where x and y are the mole fractions of the gases one and two, R is the ideal
gas constant, and n is the number of moles that one is mixing.
Basically, the deviation from ideality would have to be huge for one to
see fogging, as the order of magnitude is about 6kJ/mole of gas mixed for
typical
air/ bottom mixes, and thus the change in temperature is almost nothing. This
is due to
two effects;
1) in terms of moles, we have very few moles, even at 100 fsw.
2) the effect per mole is small.
It sounded nice...
Peter David
david@la*.st*.ed*
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