X-Message-Number: 1893
Subject: CRYONICS Maintaining constant temperature 
Date: Thu, 04 Mar 93 17:32:10 EST
From: 


Recently there was discussion here about how to economically maintain
a patient's temperature at some steady value warmer than LN2.  There
was an idea presented in cryomsg 862 by Jeff Soreff about how to do
this: concoct a substance that is liquid below the temperature you
want and gas above that temperature.  Since liquids conduct heat
better than gasses, you can put both a heat conductor connected to the
patient and a heat conductor connected to room temperature in a resevoir
of this concocted gas, and the patient's equilibrium temperature will
be no less than the condensation point of the gas.  Jeff called this
a "passive thermal switch".  He said there were other ways to make
them too.

Here are some other variations on the idea:

Connect room temperature to a thermal switch to the patient, and
connect the patient to a thermal switch to the LN2.  Except for the
heat conductors forming these connections, insulate the patient, the
outside world, and the LN2 from each other.  In the limit as the heat
insulators and heat conductors in the system become perfect, and
assuming that the warmer thermal switch is set to a slightly higher
temperature than the cooler one, the thermal regulation scheme doesn't
have to increase the rate that heat is lost from the LN2 to the
outside world.

At one point I was trying to reconstruct Jeff's idea in my head, and I
came up with something else that might also work.  Imagine two kinds
of metal.  One, called "#", expands quickly as the temperature
increases.  The other, called ".", expands more slowly.  Imagine a
configuration like this:

..............................................
.......####..........................####.....
.......####..........................####.....
       ####                          ####
.......####..........................####.....
.......####..........................####.....
..............................................

Assuming that the size of the gap is chosen correctly and that no two
of the pieces of metal are welded together, at a lower temperature the
configuration will look like this:

..............................................
.......    ..........................    .....
.......####..........................####.....
.......####..........................####.....
.......####..........................####.....
.......####..........................####.....
..............................................

The greater surface contact area of the low-temperature configuration
conducts more heat from the top of the diagram to the bottom, so we
have another passive thermal switch.  I haven't plugged any numbers
into this to see if parts could be machined to the right tolerance, or
we would have to worry about things like metal plates sagging under
gravity or dust preventing good thermal contact.

Does anyone have any other ideas?  Does anyone have the enthusiasm to
plug numbers for real metals into the scheme I outlined above to see
if it is anywhere close to manufacturable?

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.

P.S.  Garret Smyth lied.  I am Claressa Wells.  :-)

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