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revisions: 5/17/00
WSG Notes on Coldhead exchange
(Please request 9/9/94 videotape for a visual demonstration)
Read through this entire procedure before starting any work!
Preparatory:
1. Have new coldhead ready to go:
a) remove from cylinder, slip ring compressor over 15 K piston ring,
and raise inner displacer piston on pivot pin.
b) cover with bag; have o-ring ready to go into place.
2. Prepare 3 slotted cleaning dowels with Techclean wipes (or equivalent);
have # 400 emory paper at hand as well as reagent grade alcohol
for use as a solvent.
3. Assemble heat gun and prepare Helium gas line for use during
heating. Use welding grade helium for heating and cleaning. Use
ultra high purity helium 99.9995% for all purging of lines and the
cold head.
4. Connect charging manifold to Ultra-high Purity (99.999%) Helium
gas cylinder, flush and charge to about 250 psi. Helium cylinder
volume should be approximately 290 cubic feet and should have at
least 2,000 psi available.
Removal and Replacement:
1. Disconnect supply and return hoses at old coldhead and bleed
down cold head using the special bleed valve. Do return side first
and then supply side. The cold head should be turned off the night
before and lines disconnected at that time.
2. Prepare Helium shroud (plastic bag and welding grade He gas cylinder
with Tygon hose) for use during coldhead removal.
3. Remove the six (6) socket head cap screws from the cylinder using
the 5/16" balldriver and cut-down 90¡ allen wrench. Loosen all screws
first and then use balldriver to remove quickly. Use the Helium
shroud and have a helper flush the underside of the drive displacer
with welding grade He gas as the coldhead is removed from the cylinder.
Pull fast and with significant force to be certain to get the displacer
out as fast as possible. This minimizes the time for air to condense
in the cylinder. This prevents condensates from being cryopumped
into the region and sticking the pistons to the cylinder walls.
Set the old coldhead aside.
NOTE WELL: DO NOT REMOVE THE BRASS 10-32 SCREWS AND NUTS FROM THE
VACUUM JACKET. REMOVAL OF THE CYLINDER FROM THE VACUUM JACKET WOULD
BREAK THE MAGNETOMETER HIGH VACUUM AND CAUSE SERIOUS INTERNAL PROBLEMS.
4. Connect commercial purity Helium gas supply to the heat gun and
warm the inside of the coldhead cylinder thoroughly. Set the red
switch to ~250 ¡C and the black switch to ~2.5. Heat for about two
minutes or until gas flowing out of the cylinder region is becoming
uncomfortably warm.
5. Use the wooden dowel swab sticks to thoroughly clean both sections
of the cylinder. Carefully inspect the cylinder for cleanliness
and for any scoring. Dress any scored areas with emory paper, re-clean
and re-inspect. Re-heat the cylinder as needed.
6. Make sure the cold head O-ring is in place and that it will stay
in position. Use tiny dabs of vacuum grease at 90¡ intervals on
the outside of the O-ring only if necessary. Push the 15K displacer
piston up on its pin to help align it with the cylinder during insertion.
Have a helper spray Helium gas into the cylinder as the new coldhead
is carefully inserted into the cylinder. Tighten the socket head
cap screws.
7. As quickly as possible, connect the supply line to supply side
of cold head using the purged and charged manifold. Connect the
pressure release valve to the return side of the coldhead and start
the Helium flowing. Connect the electrical cable from the compressor
to the cold head and turn on the compressor. Adjust the pressure
release valve until there is a good flow on the exhaust stroke and
also a small, but positive, flow on the intake stroke. Do not let
the flow go to zero at any stage of the cycle. Flush with ~1,000
to ~1,500 psi Ultra-high Purity Helium gas (~ 30 minutes).
8. Conclusion of purge. Remove the pressure release valve to stop
output flow. Turn off the compressor to stop cold head drive. Charge
cold head to 280 psig through manifold. Remove supply line from
manifold. Connect supply and return lines to cold head and turn
on compressor . This completes the exchange.
For a rebuilt cold-head:
Once procedure is completed, ship your used cold-head to:
Austin Scientific
500 Laurelwood Rd. #9
Santa Clara, CA 95054-2418
Att: Terry Watkins
Tel: (408) 496-0782
Please include the serial number of the rebuilt cold-head that you
received.
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