30
The temperature stays at room
temperature, even though the
sample flow is not set to zero.
Check that cable from the controller to the coldhead
is plugged in correctly, and that there is liquid
nitrogen in the dewar.
The heat exchanger could be blocked with moisture
from the atmosphere, if it was switched off while
cold. Set the shield flow to zero, and the sample flow
to 10 l/min. Use the back of your hand to feel
whether there is any flow through the inner nozzle.
(1) If there is no flow at all, and the temperature does
not start to fall, remove the whole assembly from the
dewar (sample flow unit, shield flow unit and dewar
top fitting), and purge it for at least two hours by
flowing dry gas through the nozzle. (If you attach the
gas supply to the outer nozzle, disconnect the shield
flow polythene tube and block the shield flow inlet to
the coldhead to prevent the gas escaping.)
Alternatively let the sample flow unit warm up nearly
to room temperature, and then purge it with air.
(2) If you can feel some flow with your hand at 10
l/min sample flow and 0 l/min shield flow, and the
temperature does drop a little, set the heat exchanger
to 300 K, with the sample flow at 10 l/min. Purge the
coldhead in this way for at least an hour. For the first
few minutes, check continually that there is some flow
through the inner nozzle, as the heat exchanger could
overheat otherwise.
If method (2) does not work, the sample flow dewar
leg could be blocked. Use method (1). See 0.
Water or ice condenses on the
sample flow unit or the shield
unit
Pump the vacuum (section 4.3). When the dewar is
refilled, the nitrogen gas venting from the dewar will
cool the dewar legs. This is normal, and does not
require action. If the vacuum valve becomes too cold,
however, the O-ring will not seal effectively, and the
vacuum will be lost. If this occurs, repump the
vacuum, and take steps to direct the vented nitrogen
away from the vacuum valves.
Cryostat OVC cannot be
pumped to high vacuum
Check the OVC for leaks using a mass spectrometer
leak detector if available.
If there is no leak there may be too much moisture in
the OVC and it should be pumped with a rotary pump
with the gas ballast valve open.
