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
Received on Wed Oct 29 2003 - 17:52:21 MST