Shaoxiong Wu, Ph.D.
Director, NMR Center
Emory University
On Tue, 30 Jul 1996, Daniel W. Bearden wrote:
> On Jul 29, 4:25pm, T. Pratum wrote:
> > Subject: odd magnet quench
> > Hello everyone, we just had an odd spontaneous magnet quench here that I
> > find puzzling. It occurred on an Oxford 300 with a "family style" dewar.
> > The magnet quenched at about 2 PM in the afternoon, however I was working
> > on the instrument between 12 and 1 PM and noticed nothing unusual- in
> > particular there was no frost, ice, or condensation anywhere, and the
> > probe was not cold at all (all of these things might indicate a vacuum
> > leak). I had set the machine up for a long term experiment, and it was
> > during this time (when noone else was using the instrument) that it
> > quenched. The helium level a few days ago was about 60%, and we usually fill
> > it around 40%. I notice that even after the quench there is a fair amount
> > of helium in the magnet (helium level says 18%, but from a thistle tube I
> > can tell the helium level is below the level of the solenoid- I am
> > guessing about 10-15 liters left). There is still no sign of any vacuum
> > leak- the helium boiloff rate is low.
> > From this point it is very clear what needs to be done, but I am wondering
> > if anyone has any idea why such a quench might occur?
> >
> >
> > Tom Pratum
> > Dept of Chemistry
> > Box 351700
> > Univ of Washington
> > pratum@u.washington.edu
> >
> >-- End of excerpt from T. Pratum
>
> This sounds like the "slow-quench" I had a few years ago on a 300 MHz
> magnet. I am pretty sure it was an Oxford magnet. The instrument had been
> installed for over a year with no signs of trouble. One day a vendor
> application scientist was in the lab helping me set up something, and the
> signal dissappeared suddenly. We spent some time looking for the 'bad
> connection' when I noticed the helium flow meter was pegged at maximum. There
> was a slight frost near the check valve, but the bore was OK. We were able to
> 'follow the field down' for a while by looking for some resonance, but soon it
> was out of range of the electronics or something like that (this was more than
> 5 years ago). The helium loss was significantly larger than our 'normal' usage,
> but it did not drop below our re-fill point. Fortunately the instrument was
> still under warranty, so we re-energized the magnet after a thouough vacuum
> overhaul (there was no sign of a bad vacuum or 'touch', but it was felt that
> starting over completely was the best course). In about a month, the same thing
> happened again, and the magnet had to be replaced. I spent come effort in
> trying to get an explanation from the magnet vendor, but it seems to have been
> one of those 'things'.
> Does this match your experiences? Does anyone know about these 'slow
> quenches' as I have called them? I hope I never have another.
>
> Dan
>
> --
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> Daniel W. Bearden Phone: (803) 762-8527
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