X-Message-Number: 1790
Date: Sun, 21 Feb 93 20:39:38 CST
From: Brian Wowk <>
Subject: CRYONICS -130'C Storage

        Robert Ettinger has suggested achieving stable -130'C storage by 
floating a patient above liquid nitrogen (LN2) on "pontoons."  
Respondents have expressed concerns that the pontoons might leak.  This 
concern is not justified since the pontoons can be made of foam 
materials which are impervious to leaks.  It has also been pointed out 
that this method requires a temperature gradient in the gas above the 
LN2.  This is a more important detail that needs further scrutiny.    
 
        Will heat loss through the gradient be excessive?  The thermal 
conductivity of nitrogen gas at 77'K is 0.013 Watts per meter per 'K.  
As an approximation, consider this conductivity to be constant between 
77'K and 137'K (-136'C).  Assume the patient floats 0.5 meters above the 
LN2.  This will require a temperature gradient of 120'K per meter 
between the LN2 surface and the patient.  According to the Fourier heat 
conduction equation, the product of the temperature gradient and thermal 
conductivity gives the heat flow per unit area between the patient and 
LN2.  This works out to 1.56 Watts per square meter.  It takes about 
175000 Joules to vaporize one litre of LN2, so the boiloff works out to 
a bit less than one litre per day per square meter of LN2 surface.  This 
is in the ball park of normal losses for cryogenic dewars.  Therefore 
this scheme does not require dramatic increases in LN2 consumption.
 
        HOWEVER, there is the question of gradient maintenance.  The LN2 
surface provides a stable heat sink at fixed temperature and fixed 
distance below the patient.  But what about the heat source above?  A 
fixed temperature gradient requires a fixed temperature at fixed 
distance *above* as well as below the patient.  I do not see how this 
requirement can be met as the LN2 level changes between fillings. 
 
                                                   --- Brian Wowk
 
P.S. Kevin, will you be forwarding these comments to Mr. Ettinger?

[ Brian, good idea.  Unless someone objects to their reply being
  forwarded, I will send all the accumulated replies to Ettinger
  soon. - KQB ]

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