X-Message-Number: 1946
Newsgroups: sci.cryonics
From:  (Marvin Minsky)
Subject: Re: Storage Near -136 C (1/1)
Message-ID: <>
References: <>
Date: Thu, 11 Mar 1993 17:22:17 GMT


I couldn't figure out the attribution in that message (it appeared to
be from Mike Darwin), but it contains a "fatal error", if we can use
such an outdated term:

	 The temperature even a few millimeters (indeed even a millimeter)
	above the liquid nitrogen is greatly higher.  Physics tells us why
	this should be so, but rather than try to explain thermodynamics
	(which is by no means my forte) it is perhaps sufficient to point out
	that if the nitrogen vapor were the same temperature as the liquid
	nitrogen it would also be liquid nitrogen instead of a gas...  The
	degree and stability of temperature stratification in a cryogenic
	dewar refrigerated by liquid nitrogen is truly astounding.

This is all wrong.  The gas and liquid are at equilibrium at the
boiling point, and it takes a lot of energy to make the conversion.
If you are giving thermodynamic advice then you'd better run off and
learn something about it, really!

As for the gradient of temperature change ("even a few millimeters
above") this depends on the rate of heat flow through the system, in
analogy with Ohm's law; that is, the voltage (temperature) drop equals
the (current) heat-flow (current) times the resistance (reciprocal
heat comductivity.

Gaaah.

[ Message "Re: Storage Near -136 C (2/2)" contains Perry Metzger's
  correction (CRYOMSG 1805) to the above error (CRYOMSG 1791),
  which Mike Darwin acknowledges (CRYOMSG 1888). - KQB ]

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