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
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
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
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