Hi Steve,
This reply is likely to be too late to help you, but you described the
classic symptoms of a frozen lower seal set on the magnet cryostat in your
posting. The NMR probe has been leaking low temperature gas toward the
lower seals during VT operations, thus freezing the lower seals at the
bottom end of the RT bore. This particular failure mode has been seen
occasionally with every cryostat that is in the field.
It is likely that there is a fault in the NMR probe that was used to do the
low temperature work and/or the operator was not careful during periodic
cryogen fills to the heat exchanger container during his/her long VT
run. The probe fault may be due to improper assembly of the internal
dewar, or possibly, the glass internal dewar structure is broken. If that
fault is not corrected, you will see the same problem when you carry out
the next low temperature run.
The only thing that I can suggest concerning the magnet cryostat is as
follows -- provided you have not yet had a quench: continue to pump while
allowing the lower seals to warm to ambient temperature. If you get that
far, reset the screws holding the outer seal in place (with a torque wrench
only) to 40 inch-pounds maximum. FYI. there were variations is the seals
of the R2D2 magnets. If your lower seals are all Indium (applicable to
early R2D2 cryostats), then they may have deformed too far to be reset. If
they are polymer 0-rings (later R2D2 cryostats), then you have a good
chance to reset then properly. If you can reset, and you have not formed
an ice bridge in the cryostat, you will get additional use from the magnet.
Good luck,
-- Bob Santini
PS -- I am posting to AMMRL in order to remind everyone that low
temperature VT incurs large costs if it is not done very carefully.
At 03:48 PM 11/14/2003 -0600, you wrote:
>Hi,
>
>I'm not sure just what is going on. Today a user on our R2D2 magnet
>was running a long low temp experiment, and after a few hours he
>noticed a plume at the LN2 stack. Both LN2 and LHe were boiling off
>at a greatly increased rate, though the LHe was quite a bit lower.
>This didn't seem to be a quench, given the relatively lower LHe
>boiloff, but the outer can was cold, and it was clear that something
>is seriously wrong.
>
>The only thing I could think of to do was to hook up a pump to the
>dewar vacuum line and try to pump it down, though I would expect
>that a softening of the vacuum would affect the LHe more than the
>LN2. So far it has had no effect, though it has been running under
>an hour at this point. We still have a magnet, but at these boiloff
>rates it won't last very long.
>
>Any ideas of what may have happened, and if there is any course of
>action I should follow (aside from just letting it quench)?
>
>(If nothing else this demonstrates the well-known rule that bad
>things usually happen on Friday.)
>
>Steve
>--
>Steve Philson philson_at_nmr.chem.umn.edu
>Director NMR Lab 612-626-0297
>Chemistry Dept. University of Minnesota
Dr. Robert E. Santini
Director of Instrumentation
Department of Chemistry
Purdue University
560 Oval Drive
W. Lafayette, IN 47907-2038
E-mail: rsantini_at_purdue.edu
Voice: (765) 494-5227
Fax: (765) 494-9388
Received on Tue Nov 18 2003 - 12:43:26 MST