X-Message-Number: 2192
From:  (Will Dye)
Newsgroups: sci.cryonics
Subject: Re: LN2 Backup
Date: 1 May 1993 19:57:13 GMT
Message-ID: <>
References: <> <>

Brian Wowk <> writes:

[Brian describes a system to use -196'C liquid nitrogen to 
maintain a -130'C cold room.  ]

>insulation is calculated (and later fine tuned) to conduct 500 watts 
>between a -130'C ambient temperature and the -196'C inside.  By moving 
>room air past the tank you will then maintain -130'C with perfect 
>stability and zero effort.  You just refill the tank once a day.

I shy away from the word 'perfect'.  Insulation can change with time, 
especially with earthquakes, maintenance personnel clunking around, 
etc.  Also, the interior of the room will change somewhat as new 
patients are added.  In other words, you may have to re-tune your 
insulation periodically.  That could be a real mess, since the 
only way I can think of involves 1) switching off power & letting 
LN2 take over, 2) taking LOTS of measurements all over the room 
to see if 3) any part of the insulation is getting things too cold 
or too hot.  Even after you solve the measurement problems, letting 
things get too cold/too hot is bad for the patients.

Maybe I'm being too paranoid.  I just have these fears of turbulent 
airflow & heatflow creating semi-stable, cyclic structures (like 
the Great Red Spot of Jupiter) that introduce temperature cycling 
on the patients.  Sure, the temperature differences would be small, 
but how small is small enough?  

Like I said, maybe I'm being too paranoid.  I'd just feel better
if we had better ways to control (or even measure) small-scale 
temperature fluctuations. 

>...  The cold LN2 reservoir in only one heat exchange 
>cell (compared to the warm opposing cell) will form a thermo-siphon 
>that will mildly drive the over/under air circulation even if the fans
>fail.
>        This phenomenon is too beautiful to be an accident.  I take it 
>as a sign from God that this is the way He intended a Cold Room to be 
>built.  ...

I'm also a big fan of design elegance.  Is there a way to extend this 
sort of thing to soothe my above-mentioned fears?  "Inherently safe" 
nuke plants (under development) use fuel that expands when heated.  
If heated near the point of meltdown, the fuel expands to the point 
where the chain reaction slows down.  Thus, the physical property of 
expansion acts as an 'inherent' control rod.

Can we use a similar mechanism to control temperature?  In a sense, 
we already use it.  If liquid nitrogen gets too hot, it boils away, 
thus leaving the remaining nitrogen at the proper temperature.  
Is there some physical property that changes at -130'C?  
Can we rig something to respond to the change by introducing 
more/less heat at the point of the change?  I already know we 
can't just find a liquid of the right boiling point, but is there 
something else we could use?  

--Will

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