2.3.3 Sample in Vessel 3
•
All taps and valves should be closed.
•
Open valves V2 and V3 along with tap C on Vessel 3 and freeze the sample
liquid by slowly raising a dewar of liquid nitrogen around it.
•
When it has been determined that the liquid has completely frozen switch the
pump to roughing mode and open valve V1.
•
Switch the pump to turbo mode and leave for several minutes. The pressure
within the system can be read from the baratron gauge on the pump.
•
When sufficient pumping has been done and the sample is ready to thaw, close
valve V1 and remove the dewar of liquid nitrogen from around Vessel 3.
•
The system must be ‘let-up’ to atmospheric pressure using helium gas by
opening valves V3 and V9. The pressure of gas in the system can be read
from the baratron gauge.
•
Tap D should be closed to prevent suck-back of the liquid as the solid thaws.
•
The sample can be warmed gently to aid the thawing process and when
complete close tap C.
2.4 Liquid transfers
The transfer of liquids within the system is best accomplished by using relative
differences in pressure brought about by cooling, evacuation or both.
2.4.1 Vessel 1 to sample cell
The transfer of liquid from Vessel 1 to the sample cell is brought about by the
difference in pressure between that in the Vessel and a vacuum in the cell. However,
in order to control the transfer of liquid the pressure difference should be as small as
possible but not so low that the liquid starts to boil.
•
Switch the pump to its roughing mode.
•
Open value V2 and tap A, and carefully open valve V1 until a suitable
pressure is obtained (~0.5 bar).
•
Close tap A and continue to evacuate the system and sample cell by opening
valves V3, V4, V5 and V6. Switch the pump to turbo mode.
•
When the required level of vacuum has been obtained, close valves V4 and V5
in order to isolate the sample cell.
•
Opening tap B on Vessel 1 will cause the liquid to move rapidly to fill the
volume between tap B and V5.
•
Valve V5 can be opened carefully in order to allow the liquid into the sample
cell. The movement of liquid can be monitored using the sight glasses
positioned either side of the sample cell.
•
When it is judged that sufficient liquid has been transferred to fill the sample
cell, close valve V5.
At this point, it must be remembered that the lower manifold contains liquid, which
must be removed before the system is evacuated.
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