Of the 13 who responded, no one said that there would be an immediate
danger of a quench if the LN2 dewar goes dry, although at some point the
He evaporation rate would rise (as I suspected it would). The rise may
either be slow or dramatic.
Having the LN2 dewar go dry actually happened to 9 respondents. One whose
LN2 dewar warmed up to room temp before LN2 could be replaced noted that
the He evap. rate never returned to the levels they were before. This, he
explained, was due to the charcoal "getters" in the vacuum warming up and
losing some gas-absorbing efficiency. Another one noted that having the
LN2 can warm up would subject both cans to thermal stresses.
The solution would then be to fill with LN2, then fill up LHe. However,
someone recommended filling LHe first, then LN2. One respondent purged
the N2 can with nitrogen gas first before filling LN2.
As for water getting into the dewar, two replies mentioned the
possibility, including the formation of ice blockages, but the presence of
a check valve puts this in check. (Of course! How could I possibly forget
the check valve? We have this installed on ours.)
I think the consensus is that even though you might get away with it, it
is not a good idea to deplete the dewar of LN2. I haven't done this yet
in our two years of operation, although lately I have come close to the
manufacturer's minimum level of 10%. Our magnet has He and N2 sensors,
and our software raises an alarm whenever it senses that the cryogen
levels have become dangerously low.
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Edward T. Chainani Nuclear Magnetic Resonance Laboratory Manager
National Chemistry Instrumentation Center (NCIC)
Ateneo de Manila University phone: (632) 426-6001 loc. 2309
Loyola Heights, Quezon City e-mail: etch@ginto.chem.admu.edu.ph
Philippines FAX: (632) 426-5985
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