X-Message-Number: 2042 From: whscad1!kqb (Kevin Q Brown +1 201 386 7344) Subject: CRYONICS Ettinger Replies Re: Cryogenic Insulation I just received a reply from Robert Ettinger to the CryoNet messages through Friday, March 19. (When he wrote his reply he had not yet seen the messages from March 20 onward.) Here are his comments. Kevin Q. Brown ----- March 26, 1993 From: Robert Ettinger To: Correspondents > Subject: Cryogenic Insulation; Other Comments I am still not on the net, but Kevin Brown has been kind enough to act as intermediary. My thanks to all those contributing interesting and potentially useful thoughts; to save space, I am not explicitly acknowledging most of them, and usually commenting only when it might be useful or when comment has been requested. 1. Cryonics Institute uses a foot of perlite, evacuated to less than 10 microns. In our two-patient cylindrical unit (HSSV-2), this gives a boiloff of less than 4 liters per day per patient. 1a. On the theory of heat transfer with vacuum insulation, which a couple of people asked about and a couple of others seemed slightly uncertain about. With "super-insulation" (multiple radiative barriers in very high vacuum), there is essentially no conductive or convective transfer, and the radiation barriers reduce the radiative transfer to a minimum; the aluminized mylar (or other) radiation barriers must not be pressed together. With evacuated perlite or other suitable powder, with a "soft" vacuum of well under one mm and preferably under 10 microns, the molecular mean free paths are longer than the distance between powder particles, hence the gas conduction/convection is small and the white powder particles keep radiative transfer rather low; but there is still some conduction through the powder, which is why we still get better insulation with thicker layers of powder. 2. Perlite is not used for mechanical support; it would be lovely if it were incompressible, but it isn't. In the cylindrical units the shape provides the support, except for weight at the bottom; in our rectangular units, we have to use solid braces between the inner and outer walls; see e.g. The Immortalist, March 1993. Perlite is used because it works, as note, with relatively soft vacuum (not requiring a diffusion pump), which means tiny leaks would not be important, and because it provides appreciable insulation even if vacuum is completely lost. 3. Evacuated perlite gives about a factor of 10 improvement as compared with unevacuated perlite. We come close to this over-all in our cylindrical units (HSSV). In our rectangular units, because of the conduction of the solid braces between the walls, the factor is only about four-- but the rectangular units can be made large enough for economies of scale. Unevacuated perlite is not as good an insulator as styrofoam, but is the same order of magnitude. 4. Foam (or powder), evacuated or not, has diminishing returns, as thickness increases, not only because of the dependence of the coefficient on temperature, but--more importantly--because of the geometry. As previously noted, the temperature gradient (to which conductive heat transfer is proportional) is much steeper near the cold inside than near the warm outside. This would not be true in the idealized (one-dimensional steady state) case of a wall of insulation infinite in two dimensions. Many textbooks offer the formulas. In the cylindrical case, steady state, it is easy to show that the gradient is inversely proportional to the radial distance. In general, as previously noted, there is little benefit in adding insulation if the insulation is already thick relative to the size of the cavity. Brian Wowk has already confirmed all this, but it may be worth reiterating. In commercial systems, unevacuated powder seems generally to be used for the largest systems, perlite with relatively soft vacuum for intermediate size systems, and high vacuum "super- insulation" for smaller systems. We (CI) prefer perlite even in our small systems because of the reliability factor. (Superinsulation systems not only are very sensitive to tiny leaks, and to slightly rough treatment, but also, as I understand it, demand periodic baking, as well as pumping and reinfusing with getters, to harden the vacuum, and this means moving the patients and sending the units back to the factory.) 5. Although Brian Wowk has offered intriguing ideas for simple/safe regulation of temperature in a not-so-cold room, it might be a good idea to wait a little before settling for foam. As previously noted, at least two new insulators may be available in the relatively near future. One involves evacuated panel modules for kitchen refrigerators, supposedly near commercial production and better than foam by a factor of at least two or three. The other is rigid open-cell foam. With this, one could get something like evacuated perlite efficacy in an incompressible medium--no internal bracing required for rectangular cryostats. This would greatly increase the size required before one would want to go to unevacuated foam or powder. The thing is, such (rigid, open-cell) foam exists, or has existed, but is not currently commercially available. It might take collective financing to pursue this. 6. In the discussion on the net I haven't seen mention of evacuated styrofoam. Styrofoam is mostly closed-cell, but partly open-cell, and even the closed cells aren't leak-proof. This means that, according to the literature (we haven't tried it), if you encase your styrofoam in a seal and pump it for a long time (months, depending on the details), you can improve insulation by a factor of about two. This might be worth while, even though pumping for a long time adds to the expense. It might also be possible to jigger the formula for styrofoam production to increase the fraction of open cells without sacrificing too much rigidity; then you wouldn't need as much pumping, or/and you would get better improvement on evacuation. 7. On another topic--cracking damage--I wonder if the following reasoning has any degree of validity. First, large cracks are less important than small cracks, because only a tiny fraction of most cells in a given volume can be affected by large cracks and there is a lot of redundancy in most brain systems. (The "fraction" mentioned, very roughly, would be on the order of the ratio of cell diameter or length to brain diameter.) Second, small cracks would be generally less difficult to repair, because it would be easier to see or infer the original connections. But if large cracks potentially confuse only a small fraction of the information and hence are not so serious, and small cracks less difficult to repair and hence also in effect not so serious, then cracks aren't really so serious. Needless to say, this in any case would not negate the concerns of Mike Darwin and many others, about trying not only to maximize the overall probability of revival, but also to minimize the time needed for revival technology. We have more influence and control while animate, and don't want to remain inanimate any longer than necesary. But these concerns have to be balanced against other considerations, including cost and asset allocation. 8. About convection in the proposed room-sized storage regions of Brian Wowk, Mike Darwin, and others: (1) I like Mr. Wowk's ideas, but if for some reason fans are preferred to Mr. Wowk's system, the fans could be outside the room, the air streams entering through relatively small tubes, hence the waste heat of the fans not reflected in boiloff. (2) It isn't clear to me that just increasing convection will automatically randomize air motion and homogenize the temperature. There still might be systematic effects and non-uniform temperatures, and I doubt that calculations will be reliable. (3) Any kind of convection, with fans or otherwise, will contribute some heat to the system, hence some additional boiloff. If Mr. Wowk's temperature- difference engine is used, a thermodynamic calculation--which I don't volunteer to make--will set the limits. 9. Repeating myself yet again for emphasis, I don't think we should go charging off in all directions, at least with any serious commitment of time or money, until we have some reasonably clear evidence that there will be a payoff in improving the patients' chances. ----- Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2042