2. LIQUID HELIUM
The most crucial operation with the SRM DEWAR is the refilling
of the reservoir with liquid helium. Before each refill this
section should be thoroughly reviewed and understood. The
attached summary chart lists the crucial step-by-step procedure
with references given to the appropriate paragraphs in this
manual for more detailed description.
Figures 1-1, 1-2, 1-3, and 1-4, given at the beginning of
section 1, will assist in understanding the filling and safety
procedures with the 2G magnetometer:
a) Measure the liquid helium remaining in the SRM reservoir.
b) Carefully check the 2G helium transfer LINE. Make certain that the LINE
will reach the bottom of the SUPPLY dewar and that the SRM
side is smooth and free of bends. Two LINE extensions are
provided. Both extensions are closed at the bottom with the
liquid pickup about one inch above the bottom to ensure that
solids are not picked up from the bottom of the SUPPLY. One
of the extensions must be used at all times. The SRM side
must be at least 14 inches long to fit into the ADAPTER and
seat in the fill port. If another LINE is used it must be
of equal or greater length. Have a supply (about 1000 psig)
of pure helium gas and a low pressure regulator available
with a 30 psig maximum output pressure.
c) Record the following operating “health readings" of the
dewar: Helium level, SQUID and shield temperature, cryocooler
temperatures, pressure on the safety oscillation damper gauge
(Ps), helium boil-off gas flow, and DC bias level of
each SQUID (reading of the 10 turn bias potentiometer). Figure
1-3 shows the dewar system with the three oscillation dampers
and the safety pressure gauge for measuring Ps.
d) Provide clear working space around the SRM dewar and the helium SUPPLY dewar.
e) Locate the helium SUPPLY dewar next to the SRM dewar so that the transfer
line can easily reach the FILL PORT.
f) Bleed the pressure on the SUPPLY dewar down to atmospheric
pressure. NOTE : this should be done very slowly to prevent
large flash-off loss of liquid. A “rule of thumb"
is to take about 5 minutes per one psi gauge pressure of positive
pressure that is in the SUPPLY dewar. Also keep the rate slow
enough so that the top plumbing of the SUPPLY dewar does not
become excessively frosty.
g) Make certain that the SRM dewar pressure as read on the safety gauge is about
two psig. The pressure should be increased, if necessary,
before proceeding with the transfer by adding pure helium
gas through the VENT port. If the SRM pressure is above two
psig as read on the safety gauge then the pressure should
be slowly reduced by bleeding gas off through the vent valve
(see figure 1-3 and 1-4 for the location of this valve). CLOSE
THE VENT VALVE.
h) Just prior to transferring it is advisable to close the needle valve , figure
1-4, in the vent popoff valve line. This will ensure that
air will not be drawn into the vent damper if the SRM pressure
falls below one atmosphere during the transfer (see back drafting
comments in section 2-2-e below) We have found that these
popoff valves will sometimes leak for very small negative
pressures. This leaking can allow sufficient air to enter
the damper volume and later to be cryopumped into the vent
line and cause a solid air plug.
i) Unscrew the hex brass nut with popoff valve from the fill port and immediately
close the port with a small black rubber stopper ( size #
00 ). When the hex brass fitting is removed a large flow of
helium gas will exit the port. It is important to seal this
port as quickly as possible. Always bring the stopper to the
port from the bottom side and rotate the stopper into the
port. If the stopper is moved horizontally into the port the
helium gas will very quickly freeze the stopper and your fingers.
If the fill port top is cold the stopper may be blown off
by the gas pressure. To prevent this hold the stopper in place
and warm the port to room temperature with the heat gun.
j) Attach the transfer coupling adapter to the SRM end of the helium
transfer line. Make certain that the rubber hoses are attached
to each end of the adapter and that the hose clamps are in
place to tighten around the fill port threads and around the
transfer line on the small side of this assembly. NOTE: it
is important to pre-adjust the clamp so that the hose is a
snug fit around the transfer line. When the transfer line
with the adapter is fitted over the threaded fill port connection
almost no gas will escape and the danger of freezing the hose
before the clamp can be tightened is greatly reduced. Close
the flip valve on the adapter.
BEGIN HELIUM TRANSFER:
a) Inspect the fill adapter coupling that is now on the helium transfer line
from step 2-1-i) above. Make certain that the adapter flip
valve is open and that the clamp is pre-adjusted to be a snug
fit to the transfer line. Check that a closed end transfer
line extension is fitted to the storage dewar side of the
line. This extension should have a hole cut in its side
about one inch above the bottom. The purpose of this extension
is to prevent the transfer of solid contaminants from the
bottom of the storage dewar into the SRM.
b) Flush helium gas through the transfer line for about one minute to remove
all traces of air. Keep the line as an inverted U during and
after this flushing.
c) Slowly insert the helium transfer line into the SUPPLY dewar -keeping the
SUPPLY dewar pressure at atmospheric - until the line
is fully inserted. Now connect the helium gas supply to the
SUPPLY and apply about 1/4 psig gas pressure. This will cause
helium gas to flow through the transfer line and further purge
it of any air. Before this exit gas starts to cool quickly
remove the rubber stopper from the fill port and insert the
line into the port until the large rubber hose of the adapter
fits over the fill port threads. NOTE, it is very important
that the line not be precooled further as in a more conventional
helium transfer. This is because the frost that will accumulate
on the outside tip of the line may get trapped in the fill
port and be cryopumped down to the inner vapor cooled shield
and cause a solid air plug. The LINE only needs to be inserted
about 4 inches into the fill port at this stage. Now tighten
the hose clamp on the large hose and push the line in until
it is felt to hit bottom in the fill port. This will leave
about 2 inches of the 3/8 inch diameter fill line exposed.
Tighten the hose clamp around the LINE and then increase the
helium gas pressure in the SUPPLY dewar to one psig. Pull
the line back out about 1/4 inch to permit rapid precool.
The flip valve on the SRM dewar
FILL ADAPTER was opened at the beginning of the transfer to
precool the transfer line and thereby prevent warm helium
gas from entering the SRM reservoir. Let helium gas vent out
of this valve until the valve just gets frosty. Open and close
the flip valve as it cools to prevent the valve stem from
freezing open. Close the flip valve when the line is precooled.
Notice that the adapter pressure gauge will oscillate by +/-
1 psig when the line is properly precooled. Push the LINE
fully into the fill port, and make certain that helium gas
is not leaking around the hose to adapter fitting.
d) The SRM dewar vent valve should be closed from step 2-1-g)
above. Leave this vent valve closed and slowly increase the
transfer gas pressure on the SUPPLY dewar to 3.0 psig (over
about a one minute period) by using pressure from the helium
Monitor the safety gauge pressure,
Ps, continuously during this step and as soon as it rises
to 2.0 (two) psig open the vent valve. NOTE: connect a short
rubber hose to the vent valve hose barb to direct the very
cold helium gas away from the dewar top plate fittings and
to above the instrument. If possible this gas should exhaust
outside the room so it does not produce a large background
concentration of helium in the atmosphere.
The open end of the vent hose
must be accessible since it may need to be sealed with a rubber
stopper during the initial phase of the transfer. See the
following step e).
e) Relatively warm helium gas will now be entering the cold reservoir and the
boil off will increase until the transfer line has been cooled
to liquid helium temperatures. This will require about 3 to
5 minutes if the transfer pressure is maintained at 3 psig.
CAUTION: The most crucial
part of the helium refill will occur at this time. As soon
as liquid helium from the SUPPLY dewar starts to enter the
SRM dewar reservoir it will abruptly cool the gas in the reservoir
and in some cases can produce a pressure in the reservoir
lower than ambient. This reduced pressure can result
in air being drawn into the vent port (back drafting). This
air will rapidly condense inside the vent line and can cause
a solid air plug to form in the vent line in a few seconds.
EXTREME CAUTION MUST BE EXERCISED TO PREVENT THIS. Continuously
monitor the safety gauge pressure Ps , during this part of
the transfer and if it begins to fall below 1.0 (one) psig
close the vent valve. This valve may be frozen and
very difficult to turn so insert a rubber stopper into the
open end of the vent hose then warm the vent valve with the
heat gun until it can be closed.
f) Maintain a constant transfer gas pressure of 3 psig until the safety pressure
on the SRM dewar rises to about 2 psig, then open the vent
valve again. This back drafting problem should not reoccur
during the refill process. Now, slowly increase the transfer
pressure to 3 to 4 psig. Maintain the pressure at 3 to 4 psig.
g) Read the helium level gauge every minute to monitor the transfer rate. Liquid
should collect at a rate of 3 liters per minute or roughly
5% per minute on the level gauge. It is undesirable to read
the level gauge more frequently than once a minute since it
adds to the helium boil off.
h) Continue the transfer of liquid helium maintaining a constant transfer gas
pressure. Much more efficient transfers are possible if all
of these procedures are done slowly and smoothly.
i) If the transfer pressure is held constant it will be very easy to tell when
the SUPPLY dewar is empty or the SRM dewar reservoir is full.
The safety pressure will abruptly rise about one psig when
either the SRM is full or when the SUPPLY dewar is empty-in
either case it is time to stop the transfer. Also, you should
watch the gas plume from the vent line - the gas flow will
increase by about 2 to 1 when either the SUPPLY dewar is empty
of the SRM is full. The helium level gauge readings will stop
rising, then begin to fall but this is more difficult to observe
quickly than the above two events. When these events occur
release the transfer pressure by opening the SUPPLY dewar
vent valve and let the SUPPLY dewar pressure reduce to one
j) Close the vent valve on the SRM dewar when the safety pressure , Ps
, reaches 2 psig.
STOP HELIUM TRANSFER:
a) The SRM dewar reservoir pressure, Ps, should be allowed to reach about 2.0
psig as read on the safety gauge, with the gas transfer pressure
released as described above in 2-2-i and -j and the vent valve
should be closed.
b) Loosen the hose clamp on the large adapter hose and remove the transfer line
and adapter from the fill port. Immediately plug the
fill port with a solid rubber stopper.
USING A SECOND SUPPLY DEWAR (IF NEEDED):
a) Remove the transfer line from the first SUPPLY dewar when it is empty. Be
certain to close the top valve on the SUPPLY dewar to prevent
air from being cryopumped into the SUPPLY dewar reservoir.
Hold the transfer line in an inverted "U" and flow
room temperature helium gas through the line until in exits
at room temperature. It is not advisable to try to move the
cold line directly to a second supply dewar, to save a little
helium. The danger of getting air or moisture into the fill
line is too great.
b) Insert the transfer line into the next SUPPLY dewar to be used. Insert the
line all the way to the bottom of the SUPPLY dewar, apply
a 1/4 psig transfer pressure, watch the exit of the transfer
line until helium gas flow is observed then quickly insert
the line and adapter onto the fill port coupling being careful
not to freeze the rubber hoses. NOTE: it is much safer and
more efficient on the average to waste a little helium by
not precooling the transfer line than to risk getting air
into the fill port or freezing the hose couplings and having
to restart the transfer. Follow the procedure described above
in sections 2-2-c to 2-3 to partially precool the LINE.
c) When the transfer line is inserted, sealed and partially precooled, slowly
increase the transfer pressure to 3 to 4 psig. Monitor the
safety pressure and as soon as it starts to rise above two
psig open the vent valve (note: this valve was closed in step
2-2-j above). Continuously monitor this safety pressure for
the first 5 minutes of this second transfer process. It is
very unlikely that the system will back-draft as described
in section 2-2-e above, but caution is still a necessity.
d) Continue the transfer as described above until this SUPPLY is empty or the
SRM is full.
e) Stop the transfer as described in section 2-2-i&j and section 2-3.
CONCLUDING THE TRANSFER:
a) Leave the line in the SUPPLY dewar and remove the transfer line and adapter
assembly from the fill port and insert the rubber stopper
in the fill port. Make certain that the stopper is snug, and
warm the fill port to room temperature, REMOVE THE STOPPER
, and quickly fit the hex head brass cap with its popoff to
the fill port. The requirement when removing the transfer
line from the fill port is to prevent the port from freezing
thereby making it difficult to seal the port with the rubber
stopper. Further, and most important, it is crucial that the
line be removed and the port sealed without any opportunity
for air to condense in the SRM dewar fill port.
b) As soon as the transfer line is removed from the fill port and the fill and
vent ports are closed all vent gas will go through
the oscillation damper and out the one way pop-off valve.
This valve is now closed with the needle valve between the
pop off and the oscillation damper (see section 2-1-h above).
The reservoir pressure will usually decrease to about one
atmosphere (zero on the gauge) within 10 minutes after the
transfer is stopped. The pressure will then slowly rise,
over the next few hours, until it reaches the setting of the
pop-off valve. Continue to monitor the safety pressure until
it reaches a positive pressure of about 1/2 psig then open
the needle valve (section 2-1-h above).
c) Remove the transfer line from the SUPPLY dewar and close the SUPPLY dewar
top valve. Store the transfer line in a safe place. Measure
the helium level in the SUPPLY dewar if desired.
d) When the transfer is completed and all equipment has been put away it is
advisable to repeat the system "health readings"
taken in section 2-1-c above. These readings can be very helpful
in analyzing any problem that may develop.
e) It will take one or two hours for the SRM to stabilize after a helium refill,
and as long as one day to reach its optimum noise levels.