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 ]

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