Re: odd magnet quench

Daniel W. Bearden (bearden@manta.cha.nmfs.gov)
Tue, 30 Jul 1996 13:18:40 -0400

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
National Marine Fisheries Service	FAX:	(803) 762-8700
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