Microstat He Manual
control.
4.2
Continuous flow cryostats
Continuous flow cryostats do not have an internal reservoir to store a supply of cryogen. The liquid is supplied from a separate storage
vessel through an insulated transfer tube. The transfer tube delivers the liquid Helium to a heat exchanger close to the sample space.
The gas returning from the heat exchanger then cools the radiation shield and flows out of the cryostat. A thermometer and heater are
mounted on the heat exchanger, and these can be used with a temperature controller to balance the cooling power of the cryogen and to
control the temperature of the sample.
Either liquid Helium or liquid Nitrogen can be used in these cryostats. Temperatures down to about 77K can be reached using liquid
Nitrogen, but even at these temperatures better temperature stability can be achieved using liquid Helium. If liquid Helium is used, it
is possible to maintain a temperature below 4.2K continuously using the standard gas flow pump (GF4) and controller (VC-U). Lower
temperatures can be achieved using a larger pump such as the EPS40 rotary pump.
4.3
The cryogen transfer siphon
The LLT transfer siphon is designed for ultra low loss performance. The cold exhaust gas from the cryostat flows along the tube, and the
enthalpy of the gas is used to shield the flow of liquid from the room temperature surroundings. The LLT 600 and 700 are used for manual
control, and the LLT 650 and 750 are automated versions, which allow the gas flow rate to be optimised automatically using the Mercury
iTC Temperature Controller.
To operate the needle valve manually, set the gas flow control on the Mercury iTC to Manual. To close the needle valve, set the gas flow
to 0%. To open the needle valve, set the gas flow to 100%.
4.4
The gas flow pump and flow controller
The Oxford Instruments GF4 gas flow pump is used to promote the flow through the cryostat. It is an oil-free, twin-piston pump with a
nominal displacement of 42 litres per minute. The air leak rate is guaranteed to be less than 10cm
3
/min. This pump is described fully in a
separate manual.
The VC-U gas flow controller is used to control the flow of gas through the cryostat. It includes a flow meter (calibrated for Helium gas)
and a pressure gauge, so that the flow can be monitored.
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