LN2 running out of cryostats

[Paul Jorden]

Hi everyone-

I guess most of us use Liquid Nitrogen Cryostats for our CCD cameras.
At our La Palma observatory there are irregular, but inconvenient, instances of 
the dewar warming up because the LN2 has run out in the night. The cryostats 
generally have 16-20 hours hold-time when upright. However, some telescope 
attitudes are particularly unfavourable. The worst case is a cryostat configured 
for inverted use, with an inserted (offset) siphon tube. If the cryostat is 
rotated, and then the telescope tipped right over much of the nitrogen can be 
lost. Our policy of sharing detectors between different instruments, means that 
they are not always optimally designed for all uses.

We can monitor temperature to determine when LN2 has expired, and we have also 
experimented with a nitrogen gas flow-meter, but have not established a working 
technique to completely solve the problem yet.

Can anyone say if they never get this sort of problem?
Does anyone have any good practical solutions?
Does anyone implement a reliable LN2-loss indicator?
Any other comments?

Regards, Paul Jorden

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