We had a sudden onset recently involving a 3 minute cycle of the lock
level on our 600 equipped with a cryoprobe. With a lineshape sample in the
probe we could achieve stellar lineshape. Then the lock level would
gradually dive for about a minute to a much lower level and then gradually
return to perfect shims. After examining the performance it was noted that
the basic lineshape was not perturbed but the resolution would suffer by a
factor of 2 or 3 in the lower state. Because it was such a slow and
deliberate transition we could easily capture high resolution single scan
spectra in each state. We conferred with the manufacturer and we proceeded
to "round up the usual suspects". We replaced cooling fans in the shim power
supply and we started accurate measurements of the current in the Z1 and Z2
shim coils. To our surprise the shim currents were very stable within +/-
0.01ma. Finally we began to question other factors and a very slight hissing
sound could be heard from two control valves of the TMC vibration dampers in
the 3 magnet legs. So, out of desperation we killed the air supply to the
magnet legs and allowed the magnet to sit on the ground--albeit slightly off
level. To our amazement the lock was completely stable.
TMC supplied us with a replacement set of valves ( 3 x $180 ). We
installed them ourselves and the mystery was solved. Apparently the leak was
so subtle that the magnet was very gradually tilting just enough to perturb
the resolution but not far enough to notice visually. Then it would recover
according to the properties of the vibration damper and then succumb once
again in an endless slow cycle.
Once the old valves were removed we could observe a soft seal that had
clearly begun to leak. This magnet is vintage '98. We have a much older
Oxford 500 magnet with the same style control valves. It has never exhibited
this problem. We thought that some other lab may encounter this event
someday so why not share the experience.
Regards,
Jerry Dallas & Jeff Walton
UC Davis NMR Facility
Received on Tue Jun 21 2011 - 17:28:07 MST