X-Message-Number: 1986 Subject: CRYONICS High vacuum needed in dewars? Date: Thu, 18 Mar 93 17:27:59 EST From: In a past post, Mike Darwin said that only a high vacuum helps to insulate a dewar. This seems to contradict Ettinger's success with a soft vacuum. Also, Mike said that the mean free path of the gas molecules in the near vacuum has to be comparable to the distance between the walls of the dewar for the near vacuum to act as an insulator. This doesn't make sense: suppose our gas molecules have a mean free path of 1 inch. If Mike is right, a one-inch wide void filled with this low pressure gas will insulate better than a ten-inch wide void filled with the same gas. I would expect a thick insulator to be better than a thin one. The observation Mike mentioned to support this is that the quality of insulation of the dewar goes up dramatically as a high vacuum is reached. If there is superinsulation in the void, we can explain this without having to assume that the gas behaves strangely. When we have a soft vacuum, the superinsulation is the element of the system that provides most of the insulation. Since the soft vaccuum doesn't provide much of the insulation, small improvements in the quality of the vacuum don't improve the total insulation much. Once we have a high vacuum, though, the vacuum provides most of the insulation, so small improvements in the vacuum correspond to measurable improvements in the quality of the insulation. 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. Tim Freeman <> When they took the fourth amendment, I was silent because I don't deal drugs. When they took the sixth amendment, I kept quiet because I know I'm innocent. When they took the second amendment, I said nothing because I don't own a gun. Now they've come for the first amendment, and I can't say anything at all. [ Tim, I think that you may be referring to Brian Wowk's message #1970 on "Dewar Construction" rather than a posting by Mike Darwin. - KQB ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1986