Microstat He Manual
6
System Operation
6.1
Preparations
The following procedure assumes that you are using liquid Helium with the system. The system can also be used with liquid Nitrogen but
some of the techniques are different. Please see the section at the end of the next chapter for more details.
Before you start to use liquid Helium make sure that you are aware of the precautions that are necessary to ensure
your safety. Refer to
Safety Matters
for more information.
Ensure that the cryostat’s insulating vacuum and the transfer tube have recently been pumped to high vacuum. Connect the system
together and prepare it as described previously.
6.2
Cooling the system
Close the needle valve on the transfer siphon fully, then open it by six turns. If the transfer siphon has been supplied with the cryostat it
should fit the system without any modification. If not, you may need to remove a PTFE washer from the transfer tube arm.
Fully open the needle valve on the VC-U gas flow controller and connect the “from cryostat” connection to the transfer siphon using the
polythene tube.
Open the exhaust valve of the liquid Helium dewar to release any pressure, keeping your hands and face away. Remove the plug in the
transfer siphon entry fitting. Slowly lower the dewar leg of the siphon into the dewar, engaging the nut on the syphon with the dewar fitting.
Switch on the GF4 pump. Some liquid will be used to cool the leg, and the dewar exhaust must be open to allow the boil-off to escape. If
you try to cool the leg too quickly a large amount of liquid will be wasted, and there is a risk of being burnt by the cold gas.
Once the siphon leg has been loaded into the Helium dewar, monitor the pressure gauge on the VC-U until you see the flow of Helium,
then turn off the pump and disconnect the polythene tube from the syphon. Push the other end of the siphon into the cryostat entry arm
and engage the nut on the siphon with the thread on the cryostat arm. Take care not to over-tighten the nut. Now connect the polythene
tube from the VC-U “from cryostat” connection to the christmas tree fitting on the rear of the transfer siphon. Turn the GF4 pump back on.
The cryostat heat exchanger and sample should now cool steadily, typically cooling to 4.2K within 10 minutes.
6.3
Cooling below 4.2K
Temperatures below 4.2K are achieved by lowering the pressure in the heat exchanger. Since the pumping speed of any pump is limited,
this can only be achieved by limiting the rate at which Helium is supplied, using the needle valve in the transfer tube.
© 2017 Oxford Instruments Nanoscience
MAN-MHE-1.1.0 (28ff3d8) Sep 2017
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