Cryostats Status Report, June 2004
By Ben Best
Currently the Cryonics Institute has nine
cryostats in service for storage of cryonics patients in liquid nitrogen. One
cryostat is a capsule, three are rectangular and five are cylinders.
We call our custom-made fiberglass HSSVs
(Hard-Shell, Soft-Vacuum)
units cryostats to distinguish them from the HSHVs
(Hard-Shell, Hard-Vacuum) steel dewars manufactured by companies such as
Minnesota Valley Engineering (MVE, acquired by Chart Denver in
1999. http://www.cryenco.com
Dewars have a very high vacuum ("hard vacuum") in
an empty two-inch space between steel walls. Our cryostats have inner and outer
walls made of fiberglass-resin composite that is very much stronger than either
fiberglass or resin would be alone. The distance between the inner and outer
walls is about a foot for the entire circumference of a cylindrical cryostat (or
perimeter of a rectangular cryostat). Within that foot of space is perlite
insulation packed loosely enough that a soft vacuum could be applied.
Vacuum is measured in units of air pressure --
similar to mm Hg (millimeters of mercury) used for blood pressure, but orders of
magnitude lower -- zero microns Hg for a perfect vacuum. Some people use the
term hard vacuum to refer to a pressure of one-third or less
of atmospheric pressure (atmospheric pressure is
760 mm Hg), whereas soft vacuum is any pressure less than atmospheric, but
greater than hard-vacuum. Others (including cryonicists) restrict the term "hard
vacuum" to pressures of a few microns or less, and "soft vacuum" to pressures
greater than hard vacuum, but up-to but not greater-than a few orders of
magnitude higher.
A strong vacuum pump creates the hard vacuum for a
dewar at the time of manufacture -- a vacuum intended to last for 10 years. The
vacuum is reinforced by getters, chemically reactive metals (usually barium,
zirconium or their alloys), which react with oxygen, nitrogen, carbon dioxide
and water vapor to further, harden the vacuum and keep it hard. The vacuums in
our soft-vacuum cryostats are periodically reinforced by our Welsh-Sargent
DuoSeal Pumps.
http://www.welchvacuum.com
The perlite insulation of the cryostats provides a
backup for the soft vacuum. An armor-piercing bullet from a high-powered rifle
could travel through the entire diameter of a cryostat or dewar. But a pistol
bullet or forklift puncture would likely only put a hole in the outer wall of a
cryostat. For a dewar, such a puncture would be an emergency demanding immediate
removal of the patients. Even a dent can create a "hotspot" in a dewar. But the
loss of vacuum in a cryostat might not be much of a problem because of the
perlite insulation. There would be plenty of time to patch the fiberglass and
restore the vacuum.
Although we have some patients who are quite tall
and/or obese, we have not yet experienced any problem fitting six patients into
one of our cylinders. There would be even less problem in the rectangular units
where the patients lay flat and are simply stacked on top of each other 3 or 4
layers deep. In the cylinders the most crowding occurs in the area of the chest,
with general narrowing toward the feet (partly due to the variation of abdomen
and hip girth for men and women). There is plenty of legroom.
The cylinders and capsule are filled once weekly,
whereas the rectangular units are filled twice weekly. The depth of liquid
nitrogen ranges from 7.5 feet at the lowest to about 8 feet just after a refill.
The level of liquid nitrogen in the most efficient cylinders drops only a bit
more than 2 inches in a week (We will soon start filling these cylinders only
once every two weeks.) So in the cylinders our tallest patients, at about
six-and-a-half feet have at least a foot of liquid nitrogen above their toes at
all times. Should a disaster occur -- which has not happened since we began
service in 1976 -- the feet would be the first to suffer exposure and the head
the last. The first cryostat built is designated HSSV-2, signifying that it is a
Hard-Shell Soft-Vacuum unit holding two whole body patients. Hard-Shell means
that the shell is hard enough to maintain shape when a vacuum is applied (ie,
the walls do not collapse due to external or internal pressure).
A vacuum pump needs to be run on the cryostat or
else it will lose the vacuum required to slow liquid nitrogen boil-off, but the
pump is not so strong as to produce a Hard Vacuum in the sub-micron range, as is
done with Alcor's dewars. (Cryostats are like big vacuum thermos
bottles.)
HSSV-2, HSSV-R7, AND HSSV-R10, are the older model
cryostats.
HSSV-2 looks like a big gelatin capsule propped-up
at a 20-degree angle (or looks like a spaceship ready for launch). HSSV-2 is
tilted so that the patients' heads can be down, but it couldn't be built to be
vertical because the ceiling of the old building wasn't high enough. CI has
three rectangular cryostats, designated HSSV-R7, HSSV-R10 and HSSV-R14, which
hold 7, 10 and 14 whole-body patients respectively. HSSV-R7 is actually
soft-shelled rather than hard-shelled because it only maintains its shape under
vacuum due towooden supports between the walls.
Like HSSV-2, all of the rectangular units were
built by CI facilities manager Andy Zawacki using epoxy fiberglass for the inner
walls, polyester fiberglass for the outer walls and wood for structural support.
The HSSV-R10 and HSSV-R14 were built in such a way as to avoid the use of wood
between the walls -- because wood conducts heat. The HSSV-R14 unit ("the largest
cryostat in the world") took Andy two years to build. He was too busy with the
pressures and projects of running the CI facility and he was having problems
with rashes from the epoxy fiberglass, which is needed to hold the liquid
nitrogen. So it was decided that it would be necessary to contract with a
manufacturer to build fiberglass cryostats.
Robert Ettinger favored an upright cylindrical
design for units that would hold up to six patients. The first upright
cylindrical unit, the HSSV-6-1, had structural defects. First Andy found a hole
which he had to plug in order for the unit to hold a vacuum. When he put liquid
nitrogen into the unit, it cracked -- forcing him to reline the inside with
fiberglass.
The thick top conducts too much heat. Another
manufacturer had to be found. The second manufacturer uses a type of fiberglass
resin which is the same as one they use for liquid nitrogen testing of cruise
missiles.
They gave CI good warranties on the quality of
their work, which has been (for the most part) very good. Between the inner and
outer walls of the cryostats -- in the area which takes the vacuum -- there is
perlite, a non-corrosive, noncombustible, naturally-occurring volcanic glass
used as an inexpensive insulator.
On HSSV-6-2 -- the first unit from the second
manufacturer -- Andy did not notice that there was not enough perlite in the
walls near the top until the unit was already being used for patients. The
perlite had settled during shipment from the manufacturer. So HSSV-6-2 has poor
insulation near the top. Andy took care to add an adequate amount of perlite
before storing patients for the last three units received -- HSSV-6-3, HSSV-6-4
and HSSV-6-5, which are our most efficient cryostats. (HSSV-6-4 is currently
only about as efficient as HSSV-6-1 and HSSV-6-2 because of the temporary lid,
which will soon be replaced.) HSSV-6 series seen above.
Prior to getting the bulk liquid nitrogen tank we
were paying 50 cents per liter for liquid nitrogen. But with the bulk tank,
which holds 3000 gallons, we have only been paying 13.5 cents per liter. A
liquid nitrogen delivery truck that looks like a
gasoline tank truck periodically fills the bulk tank, from which the cryostats
are filled with liquid nitrogen through insulated pipes and hoses. On
the first of July the price of liquid nitrogen
rises to 15.3 cents per liter. Using this price, I constructed the following
table:
Cryostat Boil-off (liters/day) Cost per patient per
year
CRYOSTAT DATA
HSSV-2 11.5 $319
HSSV-R7 58 $460
HSSV-R10 68 $380
HSSV-R14 61 $243
HSSV-6-1 19.5 $173
HSSV-6-2 17 $160
HSSV-6-3 10.5 $98
HSSV-6-4 17.5 $164
HSSV-6-5 10 $94
The most uncertain estimate is for the HSSV-2
which, because of its shape and positioning, makes it too difficult to calculate
boil-off on the basis of drop in height. The estimate comes from information on
page 4 of the June 1988 issue of THE IMMORTALIST. At that time a vacuum
pump was running on the unit at all times and it was being filled from cylinders
of known volume. Of all the units, HSSV-2 does the worst job of holding a
vacuum.
Andy runs the vacuum pump on HSSV-2 constantly
while he is in the building, but does not do so on his days off because of the
risk of a machine failure that could result in complete loss of vacuum with no
one present to notice. The worse the vacuum, the greater the boil-off.
HSSV-R10 & HSSV-R14 have a vacuum pump running
about 16 hours per day, 4 days per week. Pressure after Andy returns from his
days off work will be up to 40 microns Hg. HSSV-R7 does the best job of holding
vacuum -- the pump only needs to be run about one or two days per month and
pressure is rarely over 20 microns Hg. The HSSV-6 cylindrical cryostats are vacuum pumped about one day every two weeks,
with vacuum as high as 40 microns Hg. Although the HSSV-R7 is the most efficient
at holding a vacuum, the wood in the walls causes heat loss.
The HSSV-2 cryostat has the most sentimental value
and is the most photogenic, but it is a pain to maintain (including checking
liquid nitrogen levels, which is done daily for all the cryostats). HSSV-2 is
the first cryostat I would replace. The future looks bright for the HSSV-6s,
however, because of their wonderful efficiency.
Above: HSSV-2, HSSV-R7 and HSSV-R10