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 ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1790