Howdy All,
In my experience, helium can oscillations can be tough to beat. First
make sure it is helium can oscillations and not new floor vibrations
from an air-conditioner bearing going bad and etc. Also make note of
whether the liquid helium level in the magnet makes a difference.
Sometimes a full or empty magnet will be different from a magnet half
full of liquid helium.
I have seen Helium can oscillations come and go on their own for
extended periods of time. I always practice that "problems that go away
by themselves come back by themselves". Something needs to be done to
dampen the tendency to oscillate. Things to try:
1 Raise the pressure in the helium with a 1-2 pound popper.
2. Put a tee where the helium line exits (as close to the exit as
possible) and place a big plastic bottle on one end of the tee. This
changes the helium volume and can dampen or change the frequency of the
oscillation. Sometimes adding the bottle to a different helium port exit
that is normally sealed also works. Thin bottles are better and even a
balloon will work. These deteriorate with time and their condition needs
to be monitored.
3. Add a long tube to the helium exit. Seal the end of the tube and cut
a slit in the tube end. This makes a low pressure value for the helium
exit.
4. Put a ball flow meter on the helium gas exit. Use one with small
enough flow rate that you can see the magnet boil-off. With today's low
boil-off magnets this may not be practical. (An oil bubbler with backup
prevention can also be used).
5. Tilt the magnet slightly. Try different directions.
At one time or another I have successfully used each of the above
tricks. I have also have had magnets which I could not stop the
oscillations.
woody_at_acornnmr.com
> -----Original Message-----
> From: Gregory L. Helms [mailto:greg_helms_at_wsu.edu]
> Sent: Wednesday, October 29, 2003 2:51 PM
> To: AMMRL
> Subject: helium can vibrations getting worse
>
> 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 Fri Oct 31 2003 - 11:21:29 MST