X-Message-Number: 860
Date: 28 May 92 05:52:43 EDT
From: "Steven B. Harris" <>
Subject: High Temp Patient Storage

Dear Fellow Cryonicists:

   Thanks for the Alcor New York News in electronic form.  I was
interested in the discussions of higher temperature patient
storage, and in response to the invitation for thoughts on this
matter I hereby offer some ideas that I came up with some time
ago, and have been kicking around with a few cryonicists ever
since.

   First, let me point out that for thermal stability reasons,
storage at any temperature is made much easier by addition of a
thermal ballast.  A thermal ballast is something that undergoes a
phase change at the temperature you want, so that addition or
subtraction of heat doesn't change the temperature much (it only
changes the phase).  Examples of such ballasts are the ice in
your drink which keeps it at a constant 0 C. until all the ice is
gone, and the liquid nitrogen currently used for cryonics
storage, which boils when heat is added but doesn't change
temperature.

   It's a bit difficult to identify ballast materials in the
range we want, but in 1987 I did test some candidate materials at
the Alcor lab (then in Fullerton) and discovered that out of a
number of candidate compounds, ethyl chloride (chloroethane,
C2H5Cl) in particular forms nice crystals at its melting point of
-135 C and seems to be generally well behaved, with an adequate
temperature "knee" as the phase changes.  Moreover, ethyl
chloride in industrial purity is cheap, as Michael Paulle's
recent research has demonstrated (thanks, Michael!).  It's the
best candidate compound I've identified so far.

   The system I envision works like this:  patients are suspended
in sleeping bags in standard dewars, hanging in dry nitrogen at
-135 C. (Dewars will need to be sealed to keep out moisture). 
The cooling unit which hangs along-side the patient is a fan and
a long hermetically sealed cylinder of ethyl chloride, at the
heart of which lies a multiply coiled heat exchanger tube, into
which is introduced liquid nitrogen.  In this system, when the
outside of the cooling cylinder begins to warm, indicating that
most or all of the ethyl chloride "ice" has melted, a thermostat
triggers a valve which admits liquid nitrogen under pressure from
a standard LS-160 liquid nitrogen tank at a fixed flow rate,
slowly freezing the ethyl chloride until a sudden temperature
drop at the reservoir surface indicates that all the liquid is
again frozen.  At this point the nitrogen flow is stopped until
the cycle begins again.  It may be possible to supplement the
system with fiberoptic ice sensors.  Power outage protection need
consist of only enough battery backup to run the sensor, valve,
and computer (not much), since cooling (as today) will be done by
liquid nitrogen.

   Such a system will not be completely free of temperature
swings, of course, but at its single design temperature it may be
nearly as good as much more expensive helium cryo-refrigeration
systems.  To make the system _really_ thermally stable at the
patient level, the "electric blanket" idea already discussed as
part of an alternative cooling system, might be used.  I believe
that such a thermal blanket would be totally impractical in the
suggested use of shielding a patient at -125 (say) from liquid
nitrogen at -196 C (unless the patient was in a second nested
dewar), but it might well be very useful for shielding a patient
at -125 from swings in air temp from -140 to -130 C which could
occur as an ethyl chloride ballast was being recharged.  A
workable system, then, might well be a hybrid of those proposed
in the Alcor NY news, with the exception that our familiar liquid
nitrogen could be used in the ballasted coldfinger, instead of
helium.  
                                  -- Steve Harris

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