Dear Spinlanders,
Thanks to all who very promptly emailed in suggestions for the 40-48 Hz
vibration lines being seen in lineshape and H2O supression spectra. I
failed to mention in the original email that we can reduce the magnet
backpressure and watch the vibrations diminish and grow back so it is
very much correlated to the pressure of the helium can. I had also
replaced the 1-way valve (Circle Seal 232-1PP-.15) with no effect. We
are also fairly certain that the vibration lines are not due to airflow
problems in the probes or from floor-conducted vibrations.
Many people emailed with suggestions and I will relay their collective
advice.
Most people suggested a modified Bunsen valve consisting of 20 to 40
feet of latex or tygon tubing connected directly to the exhaust port on
the magnet without the 1-way valve in place. The end of the tube is
crimped off and a small slit (1 to several inches) is cut in the tubing
lengthwise to allow for positive pressure helium gas to escape while
still keeping the system from cryopumping at negative pressures. The
length of the tubing helps in damping out the helium can thermo-acoustic
oscillations. I will be trying this approach tommorrow before our
weekly nitrogen fill.
Several people also suggested Acoustic dampeners which are tubes with a
small appeture at the inlet which allows for expansion of the gas into a
larger volume upon entering the tube. I am assuming that the tube has a
1-way valve at the end to prevent cryopumping.
One person reccomended removing the radiation baffles from the port
where the energizing lead goes in. Apparently this is a way to solve
the problem on some Bruker magnets. I need to find something else to
take the place of the baffles to try this approach. I am also not sure
whether this would lead to an overall higher helium boiloff.
Another approach favored by several people is to get a manostat for the
magnet. While I am fairly certain this will help solve / reduce the
problem the price of a manostat from Varian is > $9000. Hopefully the
rubber/tygon tubing solution will be as effective.
Many people seem to have fought against these sorts of vibration lines
and have found various sources besides helium can oscillations. We did
indeed experience vibration problems in some of our probes from too high
a VT airflow and or "probe body" cooling air flow. The vibrations that
prompted my email are definately related to cryogen oscillation. Other
sources mentioned were floor vibrations and HVAC air flow oscillations.
The latter could be part of the problem with the lines getting worse in
the last two years as we have had two separate "quench protection"
systems installed in the lab which have had an effect on the overall
airflow / internal pressure in the lab. Pending my findings with the
tygon-tubing suggestion I will be going to our Physical Plant engineers
with some new complaints.
Many thanks to Karl, Klass, Tony, Josh, Jerry, George, Bob, K.P.,
Michael, Joyce, Jerry, Charles, Rainier, Edward and Garreth for the
advice and encouragement.
Thanks again,
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 - 11:21:26 MST