Many thanks to the two dozen or so who replied to my inquiry on how
to increase the boiloff from an N2 dewar for VT mas. One thing that I didn't
mention, is that this was for a ChemMagnetics pencil rotor system, which uses
considerably more gas than, say, Doty or Bruker systems, due to the separation of
the VT gas from the drive and bearing gas. This isn't really a flaw in the
ChemMagnetics VT system as actual VT (and MAS) performance is superb.
One respondent commented on the silliness of my statement:
"we can't get enough flow for the bearing, drive, purge, and
VT requirements (pressure is fine)." What I meant was that the static
pressure was fine, but that when the evaporation can't keep up with flow
requirements then, of course, the pressure will drop.
Summary of solutions:
- Use several Dewars in parallel. This is probably the easiest and most practical
solution. We would want to consolidate the VT MAS experiments in order to
use up that much liquid N2, which costs us the equivalent of 1.50 US$ per liter.
- Some sort of heating mat on the outside of the Dewar where the pressure building coils
are located. Since there will still be some condensation, electrical safety might be
a bit of an issue.
- Use an N2 gas cylinder to pressurize the Dewar with warm N2 gas through the liquid port.
- Pass liquid N2 from the liquid port through an evaporator and ballast tank of some
type.
- Try a different Dewar, as the pressure building system might be working as well
as it should, or might need adjustment. Hmmm.... Yes that is worth a try, as our
experience was limited to one Dewar.
Thanks again. I'll post a follow-up if we find an easy workable solution.
-Kirk
Kirk Marat, Ph. D., NMR Facility Manager
Dept. of Chemistry
University of Manitoba
Winnipeg, MB, R3T 2N2, CANADA
ph. (204) 474-6259 FAX: (204) 474-7608
kirk_marat_at_umanitoba.ca
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Received on Thu Oct 28 2004 - 11:10:00 MST