Hi,
although retired since about 5 years, there are nice challenges from time to time.
The recent one within my former university gave me some questions.
==========
Equipment
==========
unshielded 360 MHz magnet, discharged to 300 MHz about 10 years ago
helium level (dipstick measured): about 80%
AV III console
Windows workstation running TopSpin 3.2
==========
Magnet behaviour
==========
Since about one year, the helium level meter always shows the same value.
To make the story short, with very high probability a lot of air condensed
within the magnet,
maybe due to a broken sealing.
Now the magnet began to drift with about 100 Hz/hour.
Very bad, presumably an ice bridge between one of the joints and a warmer
part of the cryostate,
but, of course, I am not sure.
But now the unexpected things began ...
==========
Drift direction
==========
The magnetic field increases! It was possible to get the spectrometer back to operation
by increasing the base frequency by 100 MHz (from 300.13 to 300.23) using cf.
==========
TopShim
==========
After changing the base frequency the data sets have to be corrected using expinstall.
This works perfect for nearly all data sets stored within
.../exp/stan/nmr/par
with the exception of topshim1d2H.
This data set becomes *deleted*. Tried to manually install a copy from par.AVANCE, but running
expinstall again to configure the parameters deletes this data set again. Only this one, nothing else.
The data set for topshim1d1h is fine and produces the expected results. Didn't test the 3D data set.
==========
user defined data sets
==========
For ICON-NMR these are stored within
.../exp/stan/nmr/par/user
expinstall doesn't touch these data sets. I tried a dirty trick and moved one data set from
.../par/user
to
.../par
but even in this case the user defined data set remains unchanged doing expinstall.
I fear, the magnet is lost. Nevertheless I am very interested to understand this unexpected
drift behaviour.
Greetings
Rainer
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Received on Thu Apr 11 2024 - 10:16:47 MST