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If either the sample flow dewar leg or the shield unit is removed from the dewar, then
before putting it back check that the small holes in the bottom are clear of ice, using a
stiff wire such as a paper-clip.
It is not possible to remove the shield unit from the dewar top fitting when it is cold. To
dismantle the system when cold, remove the entire system including the dewar top fitting
from the dewar in one unit.
5.6
Evacuating the Outer Vacuum Chamber
(OVC)
The OVC of both the sample flow unit and shield unit have to be pumped to high vacuum
to make sure that they provide the required thermal insulation.
Both are pumped in the factory, but this needs to be repeated occasionally. When the
system is new, all of the materials inside the vacuum space are likely to outgas quickly, and
this will affect the quality of the vacuum. This does not mean that the system is leaking.
The OVC of the sample flow unit should be pumped if either of the following symptoms
appear:
•
The sample flow unit feels cold to the touch, or condensation or frost appears. Some
condensation or frost is normal on and just above the dewar top fitting.
•
The jet does not reach the desired temperature, or the voltage supplied to the heat
exchanger heater is lower than that given for the relevant temperature in the test
results appended to this manual.
If the Cryojet has not been used for a month or more and the dewar has warmed up, it is
often advisable to pump the vacuum in the sample flow unit before starting to use it
again.
The OVC of the shield unit should be pumped out whenever water or ice condenses on the
outside of the rigid dewar leg. Some condensation is normal on and just above the dewar
top fitting.
Both OVCs contain a sorb at the bottom of the dewar leg. Ideally they should be pumped
overnight, and during pumping the dewar leg should be heated to about 100 ºC to make
sure that the sorb outgasses. You can do this by setting the appropriate flow rate to 8
l/min (not more) with the dewar leg out of the dewar. You must support the dewar leg so
that the bottom end is not touching anything that could melt. Do
not
insulate it in any
way, as this could cause overheating. Before you set the flow rate you must over-ride the
low nitrogen level cutoff by removing the plug from the socket labelled “Low nitrogen
level cutoff” on the back of the controller. Remember to replace the plug afterwards.
Do not bake the flexible transfer tube or coldhead as these contain plastic components.
It is also possible to pump the OVC while the Cryojet is operating, although the vacuum
will not last as long in this case, as the sorb has not been outgassed.
