X-Message-Number: 1901
Date: Fri, 5 Mar 93 17:15:10 EST
From: 
Subject: CRYONICS re: Message: #1893 - Maintaining constant temperature

Thanks very much.  Yes, I agree with you that your alternative thermal
switch looks like it will work.  I'm a little nervous about cold welding
between the metal surfaces as they touch and separate many time over the
decades.  Ideally I'd like some sort of thermal contact grease between
them to avoid this.  I don't know if anything acts like room temperature
grease at these temperatures, so another alternative to consider would
be a very thin layer of liquid on the bottom plate.

"high-temp" state
..............................................
...... #### ........................ #### ....
...... #### ........................ #### ....
...... #### ........................ #### ....
...... #### ........................ #### ....
       ####                          ####       gas gap
lllllll....llllllllllllllllllllllllll....lllll  liquid layer
..............................................
..............................................
..............................................

"low-temp" state
..............................................
...... #### ........................ #### ....
...... #### ........................ #### ....
...... #### ........................ #### ....
...... #### ........................ #### ....
lllllll....llllllllllllllllllllllllll....lllll  liquid layer
..............................................
..............................................
..............................................

I've modified the setup slightly to avoid pooling of the liquid in the
clearance around the parts of the thermally sensitive spacers in the
lower plate.

Another piece that this design requires is a mechanically compliant
thermally conductive link somewhere, so that neither the patient nor
the high-temperature or low-temperature heat sinks needs to move as the
switch expands and contracts.  Two plates separated by a liquid film
that is always present and some weak springs would be one example.

Another general approach to these things is to use pressure and volume
changes from liquid/gas phase changes to drive the switch.  These tend
to give larger volume changes, which loosens tolerances, but smaller
pressure changes, which might cause reliability problems.

A disadvantage of any scheme with liquid layers (thermally sensitive or
not) in them is possible leakage.  Fortunately all of these switches
can be treated as isolated components and sealed, but whether any seal
is reliable enough to hold an active component for decades is a problem.

                                                  -Jeffrey Soreff

standard disclaimer: I do not speak for my employer.

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