5
Keep the plastic cap and the pressure relief valve in place on the dewar top fitting, to
prevent the ports becoming blocked by ice.
Alignment of Nozzle
The nozzle should be placed as close to the sample as possible to prevent icing - 5 mm is
recommended.
The jet should be centred on the sample. One way to do this is to use the Oxford
Instruments alignment tip. Slide the device into the nozzle, and tighten the knob to
clamp it in place. Place a sample mount with no sample on the goniometer. Adjust the
nozzle so that the sharp tip is almost touching the place where the sample would be.
Remove the sample mount. Loosen the knob and remove the alignment tip. Then place
the sample on the goniometer. If required, move the coldhead forward using the
micrometer.
Using the dipstick to measure the level of liquid nitrogen
Slowly lower the dipstick into the dewar through the fill port until it hits the bottom, and
wait a couple of seconds. Then pull it out, and wait a few seconds for frost to form on the
surface. The length of the frosted section is the depth of liquid in the dewar. Do not
touch the cold part of the dipstick with your bare hands.
Warming up the system
The system should be switched off whenever it is not in use. To prevent the remote
possibility of ice or water building up in the coldhead (which is designed to prevent this
happening), either
(a) block the inner nozzle with a rubber stopper. The cold head can then be left to warm
up naturally. Or
(b) set the temperature to 300 K and allow the displayed temperature to reach room
temperature before switching off. To speed this up, reduce the sample flow to about 3
l/min.
