Bob's recent message suggests an important safety measure for
detecting magnet cryogenic abnormalities. While monitoring the boiloff
rates and liquid helium level sensor readings is of course important, both
of these measurements can be unreliable. In the case of flow rates, it is
well known that the helium boiloff strongly depends on air pressure in the
room, resulting in the familiar "NMR barometer" effect. For level sensors,
erroneous readings may result from improper installation, excitation
current, or icing.
The most reliable solution to monitoring a magnet's cryogenic
performance would seem to be to integrate the helium boiloff flow rate,
zeroing the integrator after each liquid helium fill. Once you have an
accurate count of the total liters of helium gas expelled by the magnet
since the last fill, you can calculate the total liters of liquid helium
evaporated so far using the relation:
1 liter LHe @ 4.2 K -> 700 liters He gas at STP.
It should be possible to count the liters of helium gas ejected
from the magnet using a device such as a commerical natural gas meter or a
mass flow sensor with a digital integrator. The mechanical solution, if
available, is preferable since the digital integrator could get reset to
zero if there were a power failure any time between liquid helium fills. I
have seen such a device in use at a Bruker NMR lab in Europe but have never
seen a helium-calibrated, low-flow gas meter locally. Does anyone know
where once can be purchased?
Regards,
Ken Fishbein
Facility Manager, NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8512
FAX (410) 558-8323 or 8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA