I read your ammrl message concerning N2 boiloff. You don't mention how long
your magnet has been in service. We have an Oxford 400WB and had (traded in)
an Oxford 250 magnet both of which developed ice bridges after being in
service for 10-12 years. I think that air very slowly leaks through the
o-rings and freezes on the He can eventually causing a touch. The symptoms
for the magnets were a little different, but were solved by having the
magnets rebuilt. This means discharging, warming up, replacing the o-rings,
repumping, recooling, recharging, recryoshimming, and reshimming. You are
down for a couple of weeks and it costs at least $12,000. Try to get a
closed quote where the price does not go up if extra days are required
beyond the estimated time. I would talk to the Bruker magnet people and
Bruker service people about this describing in detail your symptoms. Our
400WB symptoms most closely mimic yours: both the He and N2 boiloffs
doubled, but this would last a week and then return to normal. After a few
weeks, the same thing happened again. During high boiloffs, the bore
temperature fell perhaps 10 deg. C. Our 250 was a little different. The He
boiloff skyrocketed on N2 fills, but would return to normal after a few
hours. The bore temperature fell during this time and frosted inside. As I
said, both magnets returned to normal operation after rebuilding. Good luck;
go find a lot of money.
Jim
James E. Van Epp, Jr.,Ph.D.
Department of Chemistry
Brown University
324 Brook St.
Providence, R.I. 02912
401-863-3069
401-863-2594 FAX
e-mail: James_van_Epp_Jr@brown.edu