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 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1992