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A Description of the liquid nitrogen filling process

It is assumed that the cryostat has a good vacuum and that it starts at room temperature. The pump should be fitted in the nitrogen storage dewar and left until fully pressurised (see indicator light). Then the fill pipe should be fully inserted into the cryostat and the pump switched on. It is important that a full length tube is used - otherwise very little liquid nitrogen can be transferred into the cryostat during the filling process.

Initially there will be a strong boil-off with perhaps some liquid blowing out as splashes. As the fill-pipe freezes and the cryostat cools liquid should be seen flowing along the pipe. After a few minutes it is quite common for a brief increased boil-off plume and a "whoosh" for 10 seconds or so; filling must be continued for 5 - 10 minutes typically. The cryostat is full when there is a steady stream of liquid overflowing - the strength of this is best recognised by experience. Do not move the plastic pipe until it has thawed out.

After this initial cool-down, the boil-off will be quite strong and will stabilise over several hours; the detector takes 2-3 hours to cool down. During subsequent operation it is usual for ice to form on the neck; if this ice ``collar" is seen to melt this could indicate that nitrogen has run out. It is possible to detect the normal boil off (to ensure nitrogen is present) by momentarily blocking the exhaust port with a finger and on release one should see a brief puff of condensation or feel gas.