Hi Wendy,
We have had partial and full nitrogen port blockages several times even with
heat exchangers installed on the turrets, probably due to some combination of
bad seals and suboptimal filling practices.
While I have not heard any complaints about bad lineshapes from that (though
that might not mean much), I have used a piece of large-gauge copper building
wire (the kind that contains a small number of relatively large individual
strands) to check for a blockage and, if one is there, remove it. I have o
cut a piece of that about 30-40 cm long, stripped it and put a dogleg in
one end to avoid dropping the whole thing into the nitrogen tank, and also
keep the strands together a bit better.
I use this to rasp the ice away (rotate at first, then at some point scrape
the walls of the tube). The copper should be sufficiently soft to not hurt
anything, at least that is my hope.
Cheers
Matthias
On 16 Aug 2024 01:13, 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
On Aug 15, 2024, at 5:32 PM, Karel Klika wrote:
Hi Wendy,
Your problem reminds me of a problem we had many, many years ago. We had an
NMR for which the shims would suddenly go bad, as in 20% spinning sidebands.
This would happen intermittently and seemingly out of the blue for no apparent
reason. Readjusting the shims (even by the instrument manufacturer engineers
since the instrument was still under warranty) would resolve the problem for
a period of time but sooner or later the jump to bad shims would happen. Looked
at every possible cause you could imagine... But one day when I was tuning and
matching the probe by physically getting down on the floor and adjusting the
tuning knobs I noticed the shim stack could move. Turns out the problem was
simply a loose holding screw, tightened it, adjusted the shims, and that was
the problem solved. So if you are lucky, whilst you might be getting driven
crazy by the problem as I am sure you are, it could be something very simple.
Regards,
K. Klika
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