On Wed, 29 Oct 2003, Gregory L. Helms wrote:
> Dear Spinlanders,
>
> We have an Oxford narrow-bore round bottom magnet on our Inova 500 which
> was energized in May of 1990 and has been a pretty solid magnet since it
> was mapped and re-cryoshimmed in 1997. About two years ago we began to
> notice an increase in the helium can oscillations between 43-48 Hz which
> correlate to changes in atmospheric pressure (ie. they get worse when
> low pressure systems come through). Using the 1% CHCl3 lineshape sample
> on a low pressure day the band of vibration lines can be quite
> noticeable in the non-spin spectrum, sometimes being about 10 to 20% of
> the height of the C13 sattelite peak. The vibration lines are most
> noticeable in our triple-res C-H detected experiments, manifesting
> itself as "fuzz" around the residual water peak and obscuring important
> correlations. It also leads to more "dripping paint" in our NH detected
> triple-res experiments. The helium boiloff and magnet drift are lower
> than spec and have not changed at all and the non-spin lineshape can be
> reproduced without to much effort (0.5/5.2/10.5). Is there an internal
> structural problem developing inside the magnet ie. one of the
> positioning rods to the helium can becoming loose? I have tried moving
> the position of the one-way 0.5 PSI vent valve from right at the back of
> the upper stack to down at the output of the boiloff rotameter with no
> real reduction in size of vibrations. Does anyone have any experience
> with manostats for these magnets or have experienced similar increases
> in the helium can vibrations? I will be more than glad to post an
> anonymitized summary of your collective thoughts.
>
> Thanks,
>
> Greg
>
> --
> Gregory L. Helms, Ph.D.
> Director, Center for NMR Spectroscopy
> PO Box 644630
> Washington State University
> Pullman, WA 99164-4630
> (509) 335-3005 voice
> (509) 335-9688 FAX
> greg_helms_at_wsu.edu
> http://nmr.chem.wsu.edu
>
Greg,
We had exact same problem with our Oxford 600 on our Unity+.
The solution is reasonably simple. You require a manostat to
control the pressure in the magnet. Contact Varian, they will or
should be aware of the problem. It is unusual I believe in magnets
below 600 to see this problem, but it is the problem we observed and
was cured by the addition of a manostat,
Regards,
Tony Pemberton
*********************************************************************
Mr. A.J.Pemberton Tel: +121-414-3388
c/o Dept. Rheumatology, Fax: +121-414-6794
Medical School, E-mail: A.J.Pemberton_at_bham.ac.uk
The University of Birmingham,
Birmingham B15 2TT.
U.K.
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Received on Fri Oct 31 2003 - 11:21:26 MST