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in that message (it appeared to be from Mike Darwin), but it contains a "fatal error", if we can use such an outdated term: The temperature even a few millimeters (indeed even a millimeter) above the liquid nitrogen is greatly higher.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 ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1946