5. CRYOCOOLER MAINTENANCE AND PROBLEMS :

            The CTI manual on the cryocooler should be read carefully. This is especially important regarding the tightening and removal of the quick connect fittings on the pressure lines.

5-1 Replacement of the Charcoal Adsorber:

            It is very important to replace the charcoal adsorber in the cryocooler compressor every 1000 hours (once per year). New adsorbers are available directly from CTI. This is the only routine maintenance that is required on the cryocooler.

5-2. Cold head sticking noises.

            A specific problem we have experienced is contamination in the cryocooler system that results in sticking of the displacer in the cold head component of the cryocooler. This sticking is evidenced by a stick-slip noise from the cold head and can be observed visually by inspecting the sight glass (if equipped) on the cold head. The rotating drive mechanism will be erratic in phase with the sticking noise. It is sometimes possible to remove the contaminant by purging the cold head with the SRM at helium temperatures. Please contact 2G before attempting to purge the cold head. The following procedure MUST be carefully performed in the exact sequence given. We have developed a procedure that uses the cold head as a heat engine, rather than a cooler, during the purge. To do a purge we connect the cryocooler electrical drive power so that the cold head runs in reverse direction. This causes the cold head to heat the cryocooler cylinder and to warm the frozen contaminant so that it is flushed out with the helium purge gas.

A special electrical adapter is available from 2G to change the cold head direction. This adapter connects to the compressor electrical connector marked “ Cold Head Power ” and the cold head drive cable connects to the adapter. With this adapter in line, the cold head will operate as a heat engine. To purge the cold head:

            5-2-1. Turn off the cold head and compressor and immediately disconnect both pressure lines from the cold head. Cap the ends of the lines and the fittings on the cold head. Let the system sit for at least 10 hours to warm the cold head.

            5-2-2. Have a standard 210 cubic foot cylinder of 99.9995% pure (ultra-high purity) helium available and charged to at least 1500 psi. Use a pressure regulator that has an output pressure of at least 270 psi. Normal purity helium, such as that used for welding, is not pure enough and will often cause further contamination during the purge procedure.

            5-2-3. Install the cold head direction-reversing adapter. If the reversing adaptor is not available the cryocooler cable can be modified as described below to reverse the direction: First, unplug the cable from the compressor. Remove the 2 screws and small clamp plates that secure the cable to the connector. Put the connector back onto the compressor to hold it while unscrewing the back of the connector (the part that the clamp screws went through). This back part is a snug fit to the rubber sleeve around the wires so try to hold the rubber sleeve to keep it from turning while you unscrew the connector. With the back of the connector slipped down on the electrical cable you will see the wires that go to the cold head. Looking into the front of the connector these wires are labeled A,B,C,D,E,and F going clockwise from the alignment slot on the connector. To reverse the motor direction you need to swap the 2 wires that go to pins E and B (or D). That is:

1. Remove the wire from pin E and remove the wire from pin B (it may be on pin D)
2. Leave the jumper from pin B to D.
3. Connect the E wire to B and the B wire to E.
4. Leave the housing off for now, but protect the wires (and you) from electrical shock with tape.

NOTE: It would be helpful to have a spare cable that is modified for this reversed direction operation.

            5-2-4 Have a 1/4 inch OD copper tube to connect the pressure regulator to the 1/4 inch “Gas Charge” FLARE fitting on the compressor. This fitting is located below the 220 volt power input box on the rear of the compressor.

            5-2-5. Connect the pressure regulator to the gas cylinder. Care is required to prevent contamination of the gas in the cylinder since the volume of the pressure regulator is filled with air, which will mix with the helium gas in the cylinder unless it is flushed during the connection process. FIRST turn the regulator pressure control in, i.e., so that gas flow will occur when pressure is supplied to the input side. SECOND connect the regulator to the cylinder then SLOWLY open the cylinder valve. A substantial flow of helium gas must immediately pass through the regulator thereby flushing the trapped air out rather than having it mix in the cylinder. After about 10 seconds turn off the regulator control valve and open the cylinder valve fully.

            5-2-6 Connect the 1/4 inch copper tube to the regulator with fittings adequate for the 270 psi charging pressure. Be certain that the tube is long enough to reach the compressor input and then flush the 1/4 inch tube with helium gas through the regulator for about 60 seconds. The gas input at the compressor is a 1/4 inch flare fitting just below the 220 V power connector box. It is about 5 cm from the CHARGE valve. Remove the 1/4 inch flare cap from this fitting and connect the 1/4 inch copper gas charging line. While gas is still flowing out the tube connect it to the compressor 1/4 inch flare fitting. Before fully tightening the flage nut at the compressor, but with helium gas still flowing through the charge line, open the compressor gas charge valve about 1/4 turn to let a small amount of helium from the compressor flush out the section of line between the charge valve and the flare fitting. Now tighten the connection and then increase the pressure on the regulator to 250 psig.

            5-2-7 Attach the purge valve supplied with the cryocooler tool kit to the cold head SUPPLY fitting. Tighten only a few turns at this time, and be certain that gas does not flow out the purge valve.

            5-2-8 Open the compressor charge valve and simultaneously turn on the compressor and connect the SUPPLY line to the cold head RETURN fitting AND turn on the cold head drive. These three operations are to be done in the order given and in rapid succession, but don't panic. Adjust the pressure regulator to keep the compressor pressure at 250 psig.

5-2-9 Now, carefully tighten the purge valve on the SUPPLY fitting, by hand, until gas flows out the valve. The objective is to get a good flow on the compression stroke, and still maintain some flow at all times. If the valve is tightened too much the compression stroke will give a burst of gas but then the flow will stop completely, making it possible for air to be drawn into the cold head. Continue to run the system as connected until about 1500 psi pressure drop occurs in the helium gas supply cylinder. Be certain to leave about 500 psig in the gas supply cylinder

            5-2-10. The cold head should run smoothly without any stick-slip sounds.

            5-2-11. To STOP PURGE, remove the purge valve and simultaneously turn off the compressor. Reconnect the supply line to the SUPPLY fitting on the cold head, and the return line to the RETURN fitting on the cold head. Remove the direction reversing adapter from the cold head drive circuit. Restart the compressor and after running for a few minutes add helium gas to achieve a compressor pressure of about 280 psig. Close the compressor charge input valve, reduce the pressure regulator pressure to zero and remove the charge line.

            5-2-12. This should have cleared the cold head of contaminants. It will now require about 24 hours for the SRM to return to equilibrium temperature and operation.

5-3. CHANGING THE COLD HEAD:

            The cold head may need to be changed if the above procedures do not solve the sticking problems. This may be done with the magnetometer at helium temperatures, or when the system is warmed to room temperature.

5-3-1: Cold exchange of the cold head:

            This procedure requires special tools and fixtures and should be done by a certified Cryogenic Technology Inc. (CTI) engineer or by a 2G engineer. Please contact CTI and 2G before this is done. Note, that the cold head cylinder is a PERMANENT part of the 2G helium dewar and is not to be removed with the cold head. To do an exchange, only the displacer part of the cold head is removed. Any loosening of the 10-32 bolts that hold the CTI cylinder to the 2G vacuum jacket can result in loss of dewar high vacuum and create a very dangerous and damaging situation.

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