Karen Ann,
We always run N2 gas through the shim stacks for VT here, on our Bruker and Varian systems. We've found this to be crucial to maintaining good shims during the VT runs. So looks like you've (hopefully) found the solution to your problems.
Flowing shim stack air was not something I was aware of prior to coming to Madison. Fortunately, my predecessors knew that this was important. They did do one thing wrong, however, that is relevant to this discussion. They set up our old Bruker AM's to flow air only through the shim stacks. All the other gases would get switched to N2 for VT runs, but apparently to keep the N2 pressure high enough, they decided to keep the RT shim stack gas on the house air line.
When we upgraded our AM-360 that had done the most VT (especially a lot of very cold work, regularly down to -150C) a couple years ago, the RT shim stack was quite difficult to get out of the magnet. We saw why quite graphically once we did get it out: it was severely rusted for more than two feet about the magnet center!!! (I don't believe we took a picture, but it's a sight I'll never forget!!)
In Madison's humid summers, we obviously had been condensing water regularly in the RT shims, and between the magnet and shim stack, during the cold VT runs. From that point forward, I've made certain that the gas through the shim stacks on all our magnets is switched over to dry N2 gas for all VT runs.
Charlie
At 11:05 AM 5/24/02 -0600, you wrote:
>A while back ( OK, a long while back) I posted a question about some
>lock weirdness we were observing. I am reasonably convinced I have
>found our answer, and so I am posting a summary.
>
>The problem was major lineshape issues that seemed to occur with probe
>changes.
>
>Some of the suggested problems were:
>
>thermocouple placement in the probes changing
>broken shim wire(s) - most likely at the plate at the bottom of the
>magnet. This can be tested by wiggling the cable or checking wire
>conductivity.
>Shim values not being loaded correctly.
>On some systems it is possible to check the actual shim current values.
>This would help diagnose both of these problems.
>Something in the bore.
>Overtightening the probe screws- this could either change the probe
>position enough to affect things, or alter the shims by compressing
>wires in the shim plate.
>Something in the can that is shifting.
>
>A discussion about this discussed VT and that that might contribute to
>shim wires breaking or being intermittent. A vendor suggestion was made
>to run air through the shims while doing VT.
>
>This turns out to be my particular problem. The shims (specifically z4)
>do not like to get cold. Even running at 6C was enough to make them
>unhappy. Once they warmed up, all was fine. I have done 2 probe
>changes at RT or above, and the shims were fine. 3 probe changes after
>6C runs, with rt air flowing through the shims were fine. My solution
>will be to run air through the shims whenever we do vt. (I did try
>wiggling the shim cable with no effect, but have not (yet) actually
>checked the wires.)
>
>thanks to everyone who responded,
>
>kas
>
>--
>Karen Ann Smith karenann_at_unm.edu
>Director, NMR Facility Adj. Asst. Prof.
>Dept. of Chemistry Clark Hall
>University of New Mexico Albuquerque, NM 87131
>505.277.4031 url: http://www.unm.edu/~karenann
>Out of this world screensaver: http://setiathome.ssl.berkeley.edu/
>"Something that has a one-in-a-million chance of happening...
> happens once every second at 1 MHz."
~~~~~~~~~~~~~~~~~~~~~~~
Charles G. Fry, Ph.D. Tel: (608)262-3182
Director, MR Facility Fax: (608)262-0381
Chem. Dept., Univ. Wisconsin
Madison, WI 53706 USA email: fry_at_chem.wisc.edu
Received on Tue May 28 2002 - 09:04:14 MST