17
Cryojet dewar calibration
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Depth of liquid, cm
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Figure 9 Cryojet dewar calibration
5.5
Warming up the system
The system should be switched off whenever it is not in use. To prevent the remote
possibility of ice or water building up in the coldhead (which is designed to prevent this
happening),
either
a)
Block the inner nozzle with a rubber stopper. The cold head can then be left to warm
up naturally.
Or
b)
Set the temperature to 300 K and allow the displayed temperature to reach room
temperature before switching off. To speed this up, reduce the sample flow to about 3
l/min.
If you do not use the Cryojet for several days, the dewar can warm up. It is then possible
for water to collect at the bottom. This could block the system when it freezes, although
the dewar legs have been designed to prevent this. This can be prevented in several ways:
(a)
Before allowing the dewar to warm up, raise both dewar legs. To do this, loosen the
three small screws in the dewar top fitting. Raise the shield unit and the sample flow
unit dewar leg by a centimetre or two, then clamp them in place by tightening the
three small screws. Leave them in place when you later refill the dewar. (To maximise
the available time between refills, you may want to lower the legs to the bottom
later, but it is easier to do this when the dewar top fitting is no longer cold from the
filling process.); or
(b)
Before refilling a warm dewar, attach a dry air or nitrogen supply to the outer nozzle.
Gas will then flow backwards through both the sample flow unit and the shield flow
unit, blowing out any water from the holes at the bottom.
