X-Message-Number: 1899
Date: Fri, 5 Mar 93 12:39:00 CST
From: Brian Wowk <>
Subject: CRYONICS Cold Storage

        Several ideas for -130'C storage are now kicking about.
 
        With regard to the Ettinger floatation scheme, Steve Harris has 
correctly pointed out that convection would spoil the thermal gradient.  
(The cold inner wall of a dewar, if uninsulated, would create convection 
like crazy, and I hadn't considered that.)  The best solution, as Steve 
suggests, would be to encase the patient in a solid block of insulation 
and setup a deliberate and sharp thermal gradient (high heat flow) 
through it.  I don't like this scheme because it uses liquid nitrogen 
and space (as Mike Darwin points out) very inefficiently.
 
        Following an idea originally proposed by Jeff Soreff, Tim 
Freeman suggests using passive thermal switches to maintain patients at 
-130'C.  I must confess I do not understand how these switches work 
(either Soreff's or Freeman's).  For the switches to conduct heat a 
temperature difference (by definition) must exist between the two ends 
of the switch.  Now, which end controls whether the switch conducts?
 
        Steve Harris suggests a multi-wall dewar with LN2 sandwiched 
between two vacuum layers, and electrical heating of patients inside (to 
prevent them from reaching LN2 temperature).  This scheme would use LN2 
only slightly less efficiently than the highly efficient dewars we use 
today.  However the multi-wall dewars would be very expensive.
 
        How about this for an efficient storage system: Put your 
patients inside an ordinary dewar, and above them (still inside the 
dewar) put a small dewar with LN2 in it.  Release cold LN2 vapor from 
the small dewar under thermostatic control to maintain -130'C in the big 
dewar.  A small fan or passive electrical heat source at the bottom of 
the big dewar could create convection and minimize temperature 
stratification in the vapor.  Effects of stratification could be further 
minimized by encasing the patients in a thermally conductive material.  
(In fact, this is something that should be done in all schemes that 
store patients in a vapor.)  This scheme involves almost no new 
engineering, and would use LN2 more efficiently than any system yet 
devised.
 
        However I like Mike Darwin's idea best of all.  Nature abhors a 
vacuum, and this makes dewars inherently fragile and prone to 
catastrophic failure (vacuum loss).  The bigger you make a dewar (to 
achieve economies of scale) the more serious this risk becomes.  For 
this reason I don't believe the future of cryonics lies in dewars.  A 
large conventionally-insulated room with -130'C vapor inside is a much 
neater idea.  Consider for a moment how different the prognosis of such 
a room would be in the face of sabotage or earthquake compared to a 
vacuum dewar?  Even if the LN2 supply dewar inside the room failed, you 
just roll in another one and your problem is solved.  Compare that to 
the logistics of suddenly having to move a dozen patients out of a large 
failed dewar.
 
        Steve Harris has expressed concern that blocks of foam or 
similar materials tend to crack at low temperatures and lose their 
insulating value.  I do not think this is an obstacle to building a 
"Darwin room."  Many materials, such as foam pellets or powder, or even 
good old fiber glass pink could do the job quite nicely.
 
        Finally, could Mike please explain to us once again why his 
"cryosuit" experiment is necessary?  Rather than send humans into an 
environment resembling the moons of Jupiter (if they had an atmosphere), 
why not just use big electric heaters to quickly bring up the 
temperature in the room to, say, -50'C whenever you want to work inside.  
You temporarily shut down the fans (to prevent windchill), go in dressed 
like climbers on Everest, do your work and get out before your well-
insulated patients can budge from their -130' condition.
 
        The above proposal still requires good boots and mitts to 
protect against contact with -130' surfaces, but at least exposed skin-- 
and particularly your eyes --will no longer be at risk.  How will the 
proposed cryosuit protect your eyes?  It seems to me that any goggles or 
helmet would become hopelessly frosted up on the inside. 
 
                                                --- Brian Wowk 
        

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