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D4. OPERATION
a. Open the regulator to 0.5 bar to start a flow of gas through the system. Check that flow is coming
from the system outlet pipe. Allow the gas flow to continue through the system for at least 2 minutes.
This will purge the system of air to avoid formation of ice during cooling.
b. Place the coil in the dewar and fill the dewar with liquid nitrogen. Always maintain at least 25 mm
of liquid above the top coil.
c. Increase the pressure to a maximum of 2 bar to start the cool-down cycle.
d. Liquid nitrogen will start to emerge in droplets from the outlet pipe within 10 minutes.
c. Reduce the gas pressure so that occasional droplets of nitrogen emerge. Excessive liquid mixed
with the exhaust gas will reduce the cooling efficiency.
f. Once the desired temperature has been reached the gas flow can be reduced further.
g. On the completion of the cooling run, remove the copper coil from the dewar and allow the system
to return to ambient temperature with a very gentle flow of nitrogen gas to prevent condensation.
D5. PERFORMANCE
The cooling performance will vary depending on many factors, including the nature of the sample, the
sample mounting arrangements, radiated heat from the chamber, vacuum pressure and experimental
conditions. The temperature will drop sharply once liquid has reached the evaporator, (when liquid
droplets begin to emerge from the exhaust.) The temperature
will continue to fall and stabilise after approximately 1 hour.
D6. BAKEOUT
Disconnect all plastic tubes from the liquid nitrogen feedthrough connections before commencing
bakeout.
