Dear Spinners
The veterans of this group can't help but feel nostalgic when I ask
the following. We all know that, extreme temperatures of the RT shim
stack is a bad thing. I am trying to understand two things : 'how bad'
and 'why is it bad'. Referring to my 700 MHz Bruker Avance III system,
and a new generation Z-gradient probe (which is rated to reach -150C),
when I reach somewhere around -70 C for the sample temperature, the shim
stack reaches 0 C in about 10 minutes or so and the system gives a
warning and eventually shuts off the BSMS shim current to the stack.
Now, why would they do that ? In other words, what is the mechanism of
failure that includes current in a coil and the 0 C temperature of the
shim circuit. If the shim currents are switched off, does it mean that
the shim coils are safe even if the temperature goes below 0 C ?
I did a local poll in my electronics shop and one of my colleagues
hypothesized that ( any condensed water + copper/aluminum conductor +
electricity) = electrolysis and that's indeed a bad thing. If that's
the mechanism of failure, then when stack goes above 0 C when I warm the
system back up, the condensed water will still be present and does it
not now pose the same danger at these higher temperatures, until the
water is evaporated ?
So, are there any other mechanisms of failure out there that you are
aware of ? One possibility is sheer mechanical stress when condensed
water becomes ice, possibly damaging the shim circuits. Even a failure
of one shim is bad enough. But if mechanical stress is the culprit,
that can cause the damage whether the shim currents are being driven or
not. Why take the step of shutting down the shim current boards ?
My description thus far assumes that, what we are trying to protect
is the RT shim stack. Of course, if there is a short circuit in the
load, we do have to worry about the shim power supply. But my
understanding is that these are constant current sources (I might be
wrong about this), and if it is so, they can handle a shorted load
without issue. Even otherwise, I suspect that there will be some kind of
short circuit protection in these systems. So, why shut off shim power
supply when the stack reaches 0 C ?
I have benefited so much from the shared knowledge from this
fantastic group, that I can't find a fitting way to express my gratitude.
Still, my humble thank you for your insights on one of the many nagging
doubts I have had in this fascinating Field (pun intended :-) ) of ours !
There you go, I have put the bug in your brains to mess up the weekend :-D !
Happy Spinning.
Regards
Rajan
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____________________________________
____________________________________
Rajan K Paranji, Ph.D.
NMR Facility Manager
*Department of Chemistry*
*Room 65, Bagley Hall*
University of Washington
Seattle, WA 98195
phone : 206 685 2581
fax: 206 685 8665
email: rajanp_at_uw.edu
____________________________________
Received on Fri Nov 04 2016 - 11:32:14 MST