The OptistatCF works on a continuous flow principle using an oil-free pump to draw liquid helium from a storage dewar, along a transfer tube, to the heat exchanger ("pull" mode).
The cryogen is regulated by a needle valve on the transfer tube.
If the noise and vibration from the pump are undesirable then helium liquid can be pushed through the heat exchanger by pressurising the storage vessel ("push" mode).
The advantage of the pull mode operation is that the storage dewar pressure does not need to be monitored (since it remains at 1 atmosphere), the cryostat can reach a lower base temperature and the helium flow stability is improved. The advantage of the push mode operation is that the need for a gas flow pump is removed thus saving cost and eliminating the noise and vibration generated by pressurising the storage dewar.
Temperature control is achieved by a combination of manual helium flow control and power dissipated in an electrical heater, regulated using a temperature controller. The temperature is monitored by a rhodium iron temperature sensor fitted on the heat exchanger. To monitor the temperature at the sample position, an extra temperature sensor can be fitted at the sample position.
Changing the sample simply involves removing the sample rod, maintaining over-pressure of the exchange gas, replacing the sample and inserting the sample rod back into the cryostat. There is no need to break the insulating vacuum or warm the cryostat up. The resulting sample change times are very short, typically few minutes.