X-Message-Number: 1992
Date: Fri, 19 Mar 93 13:11:04 CST
From: Brian Wowk <>
Subject: CRYONICS Thermal Conductivity

Tim Freeman:
 
> Surely this is dealt with in thermodynamics textbooks.  Does anyone
> here remember their thermodynamics?  (This isn't sarcasm; I don't
> remember my thermodynamics.)  Gasses are fairly simple, and I would
> expect the thermal conductivity to simply be proportional to pressure.
 
        The comments on dewar construction that you attributed to Mike 
Darwin were mine.  The thermal conductivity of a gas *is* independent 
of pressure.  If you don't believe me, go look it up.  It is an 
elementary (though admittedly surprising) result of kinetic theory.
 
        The soft vacuum system of the Cryonics Institute (CI) utilizes 
about a meter of perlite powder within the vacuum space.  If soft 
vacuum improves the performance of this system, it may be because the 
mean free path of air molecules is increased to become comparable to 
the size of microscopic air spaces between the perlite granules.  On 
the other hand, the reduced air pressure may just be reducing 
convective (not conductive) losses.  I would really like to know how 
their boiloff compares to what you would get by just replacing the 
perlite with good foam insulation without vacuum.
 
                                                --- Brian Wowk

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