whoops. My magnet is 30 years old!
> On Aug 15, 2024, at 7:13 PM, John Decatur <jdd13_at_columbia.edu> wrote:
>
> Hi Wendy,
>
> Many years ago my 500 spectrospin magnet (now 40 years old coincidently)
> would suddenly go to a terrible line shape (and this was before Topshim so
> it was very painful to shim it back.) Turns out the source was a nitrogen
> port that was getting partially or fully blocked and this caused the
> magnet/dewar to shift and the homogeneity to change. I didn’t have the
> metal heat exchanger fins on the exhaust ports but only tubing over the port.
> The seal between tubing and port was not perfect so when we did helium fills,
> air would sometimes get sucked back into the LN2 can, blocking the port.
> Replacing the tubing with metal fins solved the problem and the magnet has
> been stable ever since.
>
> Another possibility (remote likely) could be changes in the RT shims during
> VT experiments. Do you do VT often? I find my shims get hot or cold during
> VT which causes homogeneity shifts and then they drift back to RT slowly
> and that is with a large amount of air purge down the bore.
>
> And of course, make sure that RT shim stack is tight as others have suggested!
> To me it doesn’t sound like a potential quench but something is moving
> or perhaps as Charlie has suggested, an unstable power supply or bad contacts.
>=
> john
>
John Decatur Ph.D.
Director of Chemical Instrumentation
3000 Broadway
Department of Chemistry
Columbia University
New York, NY 10025
212-854-2155
jdd13_at_columbia.edu
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Received on Thu Aug 15 2024 - 16:15:10 MST