Cryostats for Cryogenic Storage
by Ben Best
Cryonics patients are stored in liquid nitrogen either in cryostats (Cryonics
Institute) or dewars (Alcor). Cryostats and dewars are like big thermos bottles,
with liquid nitrogen in the middle rather than coffee. Because cryostats and
dewars are like thermos bottles, they are not dependent upon electricity. When
there is a blackout or power failure the cryonics patients remain at
liquid nitrogen temperature.
As of early 2009 the Cryonics Institute had thirteen cryostats in service
for storage of cryonics patients in liquid nitrogen. Three rectangular
and ten cylindrical. Cryogenic (a word often confused
with "cryonic") refers to temperatures less than −100ºC
(−150ºF). Liquid nitrogen temperature is −196ºC
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
in 1999). Dewars have a very high vacuum ("hard vacuum")
in a two-inch space between steel walls. A vacuum prevents heat transfer by
conduction and convection, but radiation can only be reduced by the use
of radiation barriers made of aluminum or aluminized mylar film.
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. I
have been told by a cryogenics fabrication company salesman/engineer that
steel dewars cannot possibly compete with fiberglass for efficiency, and
that steel dewars the size of our cryostats would have about twice the
liquid nitrogen boil-off. The
distance between the inner and outer walls of our cryostats 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 can be applied. There is no detectable
difference between room temperature and the outer walls of the cryostats, but
the cryostat lids are about 2ºC to 3ºC lower
temperature than room temperature.
is a non-corrosive, non-combustible, naturally-occurring
volcanic glass that can be used as an inexpensive insulator. Perlite ore is
a silicon-dioxide-rich volcanic glass containing 2 to 5 percent water. When
rapidly heated above 870ºC (1600ºF) the rapid vaporization of
the water makes the rock pop like popcorn to form countless tiny bubbles
that expand the volume up to 20 times — reducing the density and thermal
conductivity by a like amount.
Thermal conductivity at
low temperature and pressure
The resulting thermal conductivity for perlite at low temperature and
pressure and is about
0.0007 Watts per meter-Kelvin —
roughly one-thousandth the thermal conductivity of water or brick and
about one-fortieth the thermal conductivity of
extruded polystyrene foam boards, such as
Foamular®. Unlike extruded polystyrene, however, which
is chemically hydrophobic and impervious to moisture because of closed cells, the
expanded perlite has open cells which can be infiltrated with moisture. Moisture
must be maintained below 0.1 percent by weight or the insulating capability
is degraded — due to the much higher thermal conductivity of water. The Cryonics Institute
obtains its perlite from Grefco Minerals, Inc (HP 500 grade).
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 reinforced by our
Welsh-Sargent DuoSeal Pumps every two months.
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. (A
"hotspot" is paradoxically noticeable as a frosty-spot on
the outside — corresponding to a warm-spot
on the inner wall of the 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 (the patients are in sleeping
bags and are very buoyant in liquid nitrogen, so there is no crushing
weight or injury). 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 leg-room.
The cylinders are filled 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. 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 first cryostat built was designated HSSV−1, signifying that it
is a Hard-Shell Soft-Vacuum unit holding one whole body patient.
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). The HSSV−1 was taken out of service
in the mid-1990s, around the time when CI moved to its current
building in Clinton Township, Michigan. But the HSSV−2 (holding
two whole body patients) remained in service a decade longer.
HSSV−2 looked like a big gelatin capsule
propped-up at a 20-degree angle
(or looked like a spaceship ready for launch). HSSV−2 was 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. The HSSV−2 was removed from service in December, 2004 — with
the two patients it contained moved to one of our new HSSV−6 cryostats.
Click on this link: photographic essay
to see a "photographic essay" of the move.
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 to
wooden 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.
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 (11,000 liters), we are only
paying about 13 cents per liter
(just over 50 cents per gallon). A liquid nitrogen delivery truck
that looks like a gasoline tank truck fills the bulk tank with
2000 gallons approximately once every two weeks. A guage on the
tank reads liquid nitrogen levels in inches and centimeters. A fill
of 2100 gallons would raise the reading from about 40 inches
to about 130 inches — or about 105 centimeters to about
325 centimeters. It takes just over half an hour to
load the liquid nitrogen into the tank. From the bulk
tank the cryostats are filled with liquid nitrogen through
insulated pipes and hoses.
Twice each week the rectangular cryostats are topped-off,
and all cryostats are topped-off once per week. Topping-off
the rectangular cryostats results in a 20 cm drop in
the level of the bulk tank, whereas topping-off all cryostats
results in a 65 cm drop. At 36 liters per centimeter (cm)
the partial fill requires 720 liters and the full fill
requires 2340 liters of liquid nitrogen.
The bulk tank has pressure relief valves
of 175 PSI (Pounds per Square Inch)
and the line from the bulk tank into the building
has a pressure relief valve of 150 PSI (all other lines have
400 PSI relief valves). Pressure increases with time in
the bulk tank, but with two fillings per week, bulk tank pressure
is generally below 100 PSI. Just before a filling, a valve
on the bulk tank will be opened to release gas to
drop pressure below 50 PSI.
CRYOSTAT BOIL-OFF DATA (2008)
|Cryostat ||Boil-off (liters/day)
||Cost per patient per year|
The inefficiency of HSSV−6−10 and the relative
inefficiency of HSSV−6−9 can be explained by
the fact that they have had temporary lids while storing
HSSV−R10 requires a vacuum pump running about 16 hours
per day for at least 3 days every two months, during which
time the soft vacuum drops from nearly 200 microns Hg
to under 20 microns Hg. HSSV−R14 is more efficient
than HSSV−R10, with highs of 20 microns Hg and
lows of about 1 or 2 microns Hg after 2 or 3 (16-hour)
days of vacuum pumping. HSSV−R7 does the best job of holding
a vacuum of any of the rectangular cryostats, with highs not
over 12 microns Hg and only one or two days of
pumping required every two months.
For the HSSV−6 cylindrical cryostats
we typically pump vacuum for one or two 16-hour
days every two months. There does not seem to
be much variation in liquid nitrogen boil-off for any pressure
("partial vacuum") under 100 microns Hg.
The following table shows the maximum pressures
measured each two months before vacuum pumping,
and the maximum days required to pump the
cryostat to a reasonably low pressure
(well below 10 microns Hg and
generally very close to 1 microns Hg).
CRYOSTAT PRESSURE DATA (2008)
|Cryostat ||Maximum pressure
||Maximum days pumping|
|HSSV−6−1 ||55 microns Hg ||4|
|HSSV−6−2 ||195 microns Hg ||4|
|HSSV−6−3 ||45 microns Hg ||3|
|HSSV−6−4 ||34 microns Hg ||4|
|HSSV−6−5 ||32 microns Hg ||1|
|HSSV−6−6 ||109 microns Hg ||2|
|HSSV−6−7 ||55 microns Hg ||2|
|HSSV−6−8 ||29 microns Hg ||1|
|HSSV−6−9 ||69 microns Hg ||2|
|HSSV−6−10 ||42 microns Hg ||2|
HSSV−6−1 is less efficient because the
inner wall is supported to the outer wall with bracing
and thicker contracts (which conduct more heat) — in
contrast to the other cryostats for which the inner wall is
only connected to the outer wall by a circle at the top.
HSSV−6−2 is less efficient because it
was not as fully packed with perlite. Over time the maximum
pressure and the amount of pumping required for a cryostat
tends to decline because of "out-gassing" of water
and other volatiles from the perlite and fiberglass walls.
When we receive a new cryostat from the manufacturer
there is a fair amount of more work that must be done
at CI before the cryostat can be put into service,
including topping-off the perlite, making a bottom
and preparing a filter. The preparation process is
documented in detail in the WORD document
To test that the new cryostats have no cracks, we
first fill the bottom six inches
with liquid nitrogen and let it sit overnight. If there
has been no vacuum loss by the next morning the cryostat
will be filled over a ten hour period — about 20 cm per
hour. This is known to be a safe rate. We have not pushed
the limits to discover an unsafe rate guaranteed to cause
cracking. If there is a loss in vacuum during the filling
the liquid nitrogen is removed and the inner walls are
carefully examined for cracks that must be patched with
fiberglass/vinyl ester resin. In most cases the cryostat
can be filled with the full seven feet without the
occurrence of cracks (vacuum loss).
The HSSV−6 cylindrical cryostats are coated with a fire retardant known as
which we selected after exhaustive research into the market for retardants.
Facilities Manager Andy Zawacki demonstrated that three coats
of FF88 could withstand a full minute of the hottest portion of a
blowtorch without penetration. The closest contender was
The older (rectangular) cryostats were constructed
with 10% antimony trioxide fire retardant incorporated into
the fiberglass-resin composite.
|HSSV−2, HSSV−R4 and HSSV−R7|
|Liquid Nitrogen Pipes|
Technical Details on Cryostat Resins
All cryostats are made from fiberglass/resin
composite material. The outer walls of the rectangular cryostats are made of a
composite of fiberglass & polyester resin, whereas the inner walls
are made of fiberglass & epoxy resin composite. The cylindrical
cryostats use fiberglass & vinyl ester resin for both inside
and outside walls.
In all cases fiberglass is saturated with a syrupy resin
mixture under conditions in which the resin monomers
polymerize ("cure") — hardening to form a very strong,
durable and corrosion-resistant composite. Polyester and vinyl
ester resins are cured using a catalyst which is not incorporated
into the structure, whereas for epoxy resins the hardener is
incorporated into the final cross-linked network.
An epoxide is a cyclic ether in which an oxygen atom is joined
to two carbon atoms. Epoxy resin monomers are typically large
aromatic-containing hydrocarbon molecules having epoxides ("epoxy groups")
at each end. The hardener is typically a diamine that causes nitrogens
to bind to the terminal carbon of the epoxy group, displacing the
oxygen, which accepts a hydrogen to becomes an alcohol group. The epoxy
resin used by CI was modified diglycidyl ether of bisphenol A, and the
hardener was modified aliphatic amine (triethylenetetramaine), both from
Tool Chemical Co, Inc..
Polyester resin is inexpensive and easy to use — it combines readily
with fiberglass.The polyester resin we used was 40-50% styrene
(Tool Chemical TCC-072). Epoxy resin is the most expensive, but it has the best
thermal properties. Unfortunately, epoxy resin is the most difficult
to work with. When applying resin-saturated fiberglass mats too
rapidly all the resins can produce too much heat during the curing —
resulting in bubbling, smoking & cracking. When applied too slowly
the layers don't adhere well enough to remove air pockets. Vinyl ester
resin is easier to work with than epoxy, is less expensive and has
better corrosion resistance. Like polyester resins, vinyl ester resins
shrink about twice as much as epoxy resins upon curing, which can
lead to internal cracks if care is not taken to prevent them.
CI's cylindrical cryostats contain a modified vinyl ester — Hetron 922
(Ashland Chemical) — which toughens
the monomer with carboxyl-terminated
butadiene-acrylonitrile copolymer. As with polyester resin, the curing
catalyst used is methyl ethyl ketone peroxide (2-butanone peroxide),
which initiates a
free-radical type of chain reaction
that does not incorporate the catalyst into the final network. Five milliliters
of MEK peroxide is used per pound of Hetron 922.
It is doubtful that modification of the resin will make much
difference in the thermal properties of CI's cylindrical cryostats,
which currently have a liquid nitrogen boiloff of less than $100 per patient
per year. There may be some room for improvement on vacuum maintenance,
however. The difference between 50 microns of pressure and a complete
vacuum in the cryostats makes no noticeable difference to boil-off. But when
the pressure in the walls rises to 100 microns of mercury, boiloff increases
noticeably. It takes about three weeks without running a vacuum pump for
the pressure to rise to 100 microns.
The two likely reasons why the pressure could rise are outgassing from
the resin and leakage from the valves. To deal with the former possibility
we are considering the idea of heating the cryostat shortly after delivery
to exhaust the outgassing capabilities of the resin. Concerning the latter,
we are currently using water valves rather than vacuum valves because the
only vacuum valves Andy has seen were too small for our purposes. I intend
to look more vigorously. Potentially we could improve performance to the
point that we would only need to run a pump every few months — or even years.
Those interested in exploring resin chemistry in greater depth might
find the following links to be of interest:
Resin Systems for Use
in Fibre-Reinforced Composite Materials
Resin Literature Review