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.

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