2. LIQUID HELIUM REFILLS:

            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:

2.1 PREPARATION:

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.

2.2 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 gas cylinder.

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 atmosphere.

j) Close the vent valve on the SRM dewar when the safety  pressure , Ps , reaches 2 psig.

2.3 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.

2.4 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.

2.5 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.