Thanks to all those who replied to my query. I was arranging for
emergency liquid helium to be delivered, but the magnet decided to go
overnight. I guess I'll have to call JEOL.
The funny thing is that the vacuum in the magnet dewar seems a-OK: there
are no cold spots, and the nitrogen boiloff is normal. It was about time
for a helium refill, but the tank was nowhere near empty.
Edward
On Thu, 14 Feb 2002, Edward T. Chainani wrote:
> We have a Magnex supercon (400/54 Year-hold circa 1996) used with a JEOL
> Lambda 400 NMR spectrometer.
>
> We have a problem that seems like field drift: lock was lost and the usual
> value of Z0 required to lock onto the sample has changed drastically.
>
> As of yesterday, boil-off of liquid helium was only slightly above normal
> (220 mL/min). This morning it was 280 mL/min and has slowly climbed to 350
> mL/min at noontime.
>
> I turned off then disconnected the RT shims, and tried to obtain the
> spectra of 10% H2O in D2O. I notice that the drift is very bad: using
> single-pulse acquisition and peak picking of the water peak, the peak
> position has changed -1486 Hz in the course of 6 minutes.
>
> JEOL Co. (Japan) has surmised that it could be a slow quench of Z0 or the
> main magnet field, although nothing unusual has happened to the magnet.
>
> Any advice is welcome!
>
>
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Edward T. Chainani Nuclear Magnetic Resonance Laboratory Manager
National Chemistry Instrumentation Center (NCIC)
Ateneo de Manila University phone: (632) 426-6001 loc. 5629
Loyola Heights, Quezon City e-mail: etch_at_ginto.chem.admu.edu.ph
Philippines FAX: (632) 426-1323
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Received on Fri Feb 15 2002 - 13:28:55 MST