X-Message-Number: 2256 Date: 16 May 93 05:09:06 EDT From: Paul Wakfer <> Subject: CRYONICS: dewars in coldroom I wish to comment on Mark Volker's comments re: storing dewars in the cold room. With all due respect, Mark's comments reflect a lack of real-world experience with cryogenic patient storage dewars. First, plumbing is NOT in place to rapidly pump out LN2 and this is NOT a simple problem. The current system used to empty dewars is a water sump pump which is lowered by crane other ungainly means into the dewar. Care is taken to a) not immerse the motor, b) never shut off the pump so that its oil lubricated bearing doesn't freeze, and c) not to punch a hole in the side of the dewar. The pump is not left in place for several reasons (other than its bearing freezing): a) it is large, b) there is no room for anything extra in the dewars as they are currently designed, c) any pipe, plumbing, etc which is large enough to pump a significant quantity of liquid through in a TIMELY fashion is an enormous heat leak. Indeed, the Bigfoots have only low diameter nylon fill lines which penetrate only the foam neck plug for just this reason. Brian is not quite right about the amount of heat one of these monsters would such out, but he is close. Failed superinsulation does provide some insulation, and it is certainly true that backing things up with foam will help a lot. However, things are considerably more complicated than just these considerations. Bigfoot dewars were built without vacuum bursting disks. A vacuum burst disk is a piece of what amounts to thick aluminum foil incorporated into a holder in the outer can of the dewar with another holder holding a mouse-toothed can opener pointed at the foil. In the event of an internal can failure (the most common kind in general and almost certainly the failure mode during seizmic disturbance or mishanding (the most likely modes of sudden failure)) the LN2 rushes (is sucked!) into the vacuum space where it very rapidly boils and EXPANDS. If a burst disk is present this instantly bows the foil out so the mouse-tooth shears it open and in seconds there is a 2-3" gaping hole for gas to escape. If there is no burst disk or other way for pressure to be relieved a disaster occurs: first the inner can which is comparatively thin is pressure-wrapped around the contents of the dewar. This is very nasty and it takes someone with a very good saw, a plasma torch, or some other good cutting tool an eon to cut out whatever the inner can has "shrink-wrapped" itself around. Hugh Hixon and I actually had such a wide mouthed dewar in our posession which Hugh did surgery on to get the expensive sample racks out of (incidentally, the poor SOB's samples which probably comprised months or years of work were still rotting inside). The second thing that happens is that the dewar outer can usually explodes. A small 20 liter dewar can tear a man's arm off or kill him. I saw photos of the aftermath of such a small dewar explosion when I visted Andonian cryogenics in Boston. Now, instead of bursting disks the Alcor Bigfoots use another system which I personally don't like. The vacuum port (where the dewar is evacuated at) has a plug which is inserted at the end of evacuation. This plug has an O-ring seal which is lubricated with silicone grease and then the the plug is covered in more silicone grease or a material called Duc-Seal and the fitting covered with a caplug. Now, if you cool this assembly to a low subzero temperture what happens is that the plug GETS FROZEN IN PLACE. It will not come out in the event of an inner can failure (and the inner can is thin as a dime!). You have a disaster on your hands. The other reason I do not like this system is that it is very possible for the fitting holding the plug to get a small nick or to be slightly deformed by something banging against it. This will pin the plug in place and cause the same catastrophic failure mode. While such failure modes are very rare, they have both occurred and killed and maimed people. The tragedy Isaw while in Boston was due to the evacuation plug/port being chilled during filling (the LN2 ran over the top of the dewar and down the outside). This froze the plug in place and set the situation up for an accident. The point of this dissertation is that high-vaccum dewars are not designed to work at -130 ambient and re-engineering them with burst disks will be logistically and otherwise very costly. Furthermore, the reason burst disks were not used on these dewars was because THEY TEND TO LEAK and thus shorten vacuum life. Since the dewars are to operate at ambient temperature and the evacuation plug-relief valve technique is considered safe, this design was chosen by the engineers over my objections. As to other matters, the savings in LN2 will not be so great as the hassle and inconveience of designing the room around big foot dewars. There are many design tradeoffs that would have to me made to accomodate bigfoots which will compromise coldroom design. Regretably, I cannot discuss these since they are proprietary and relate to patents we are currently pursuing. Mark is certainly right when he indicates that there will be some time for transfers. However it needs pointing out that patients will probably be stored close to the craking point and that in any event the acceptable tolerances of temperature swing will probably be 1*C or LESS. Cooling below this point will not, as Mark states, result in devitrification. Devitrification would occur only if patients were WARMED significantly above TG. Rather, cooling below the safe temperature will result in fracturing of patients. In the event of failure of a Bigfoot it will be necessary to remove the patients by crane and lower them into another Bigfoot. There is very little margin of safety for the patients as they are currently packaged. I wanted to line the pods with open celled urethane foam to hold liquid if the patient needed to removed from the dewar and transferred. This idea was ko'd because it would add too much weight to the pods -- weight in excess of the ability of Alcor's current marginal hoist set up to handle. Patient's are thus packed inside two sleeping bags with dacron wool insulation. Unfortunately, dacron wool does not stay wet with LN2 and the patient's will begin to warm within minutes from LN2 temperature. This means that the best way to handle an emeregency is to have a backup dewar roughly half-filled with LN2 when you are ready to do the transfer. At that time the two Bigfoots are rolled next to each other and the patient's are raised from one and lowered into another. Forgive me if I have made any of this seem easy. It is NOT. During my tenure at Alcor I repeatedly suggested that the Bigfoot dewars be wrapped in fiberglass batting or better still, boxed with heavy wooden structures and foam to both protect them from seismic injury and provide fail-safe capability for more routine vacuum failure. This was not done for a variety of reasons. Today, five years later, seismic and other protection for WB dewars consists of some pieces of wood interdigitated between the feet of the dewars. I have asked several times why better protection has not been put in place and have been told that "no one can agree on what to do." This is a management, not a technical problem. Over the last several months many excellent ideas hsave been put forth with enthusiam to create a cold room. I would suggest that the same degree of energy and enthusiasm be put forth to come up with a scheme, interim or permanent, to protect Alcor's WB patients. Several years have elapsed since I was given the excuse that "we will be moving soon so we don't want to spend the money...." In several more years the activity of two adjacent crustal plates may make such considerations moot. It is my opinion that Alcor's officers, directors and personnel should properly apply themselves to this goal before expending one microjoule of energy on coldroom designs. Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2256