Cold-Pumping

Ron Nieman (nieman@asu.edu)
Thu, 01 May 1997 10:22:26 -0700

I'd like to add one comment. Cold-pumping can be very effective at
decreasing boiloff rates in some circumstances. The helium cryostat is a
very effective cold-pump for O2, N2, CO2. It is not effective at all for
helium. You can get a significant increase in boiloff rates over time by
diffusion of helium gas through somewhat leaky o-rings or seals into the
cryostat. The smaller magnets, e.g., the older Varian R2D2s and others as
well, have helium fill ports that are very close to the upper magnet seals.
Boiloff can increase significantly after fills on these magnets, or even
large magnets in poorly ventilated rooms.

In our hands, monitoring he flow over time is an extremely effective and
sensitive detector for increased boiloff rates (and discovering that it is
raining outside!) and correlates well with measurements made by dipping or
direct monitoring.
---------------------------------------------------------------------
Ron Nieman, Director
Nuclear Magnetic Resonance Facility
Box 871604
Department of Chemistry and Biochemistry
Arizona State University
Tempe, Arizona 85287-1604

(602) 965-3613 http://www.asu.edu/clas/chemistry/
Fax (602) 965-2747 http://asnmr4.la.asu.edu/nmr/
email: nieman@asu.edu http://www.public.asu.edu/~nieman/