Hello Friends
I apologize for the late response but my sincere thanks to all those
who responded for my query about RT shim values viz. Jerry Hischinger,
Bob Santini, Thomas Speck, Rainer Haessner and Jane Strouse. I learned
some valuable new knowledge through all of your replies. Much to my
uneasiness, I am making this mail rather long with the danger of
presuming that the information presented here may prove useful to many
of you, but I will quickly add my sincere apologies for taking too much
of your time if it does not turn out to be so.
My original query was:
" How many of you have measured the shim coil resistances on the Bruker
BOSS-2 shim stack and can you kindly tell me how far off these values
were from the (average) expected values ? "
I used this weblink of Bruker to get standard shim resistance
values:
http://www.bruker-biospin.com/nmr/techsupport/systems/shim_coil.html
I had reported that some of the measured resistances were a factor of
2 off, from the standard values above. I later got detailed mails from
Thomas Speck of Bruker who first confirmed to me that the values I have
reported are correct for my type of BOSS-2 shim system. i.e. with two
detachable cables A & B at the shim stack. (If any of you, just for
comparison, would like to look at the actual resistance values I have
got for each of the shim coils, please let me know and I will be happy
to forward that list to you. )
He had also passed other useful info. like shim current sources
21-26 on SCB-M are the ones which take care of RT shims (X2-Y2)Z^n,
where n = 0,1,2, nominally. ( I am tempted to summarize these and other
valuable info. I alluded to earlier, in a separate posting, but if you
forgive me, I beg you to bear with me while I stick to my original
story line, to complete the picture).
Several details were cut out of my last mail, to keep it concise.
But, the longer story behind my above question is the following:
* My magnet quenched last Septemeber and was brought back to field by
Oxford shortly after. After bringing the magnet to field, we put the
cryoshim currents that were originally recorded at the time of its
installation i.e. about 7 years ago. This magnet itself is a used one
and was apparently cranked out in 1985 from Oxford.
* The field drifted like hell for about a month and therefore, using
only the cryoline we adjusted the cryoshims twice (with the Oxford
engineer brought in for this).
* Eventually, the field drift slowed down but never stopped, but I could
shim and get a lineshape using the RT shims at this point.
Nevertheless, the cryoline simply kept on melting away, ever so slowly.
(I do not know, if the main i.e B0 coil drift alone can cause this or if
the cryoshim coils are also involved as well). With hindsight, I feel
that the RT shim system was holding up, correcting for this
ever-so-slooowww drift of the cryoline. The instrument was put to
regular use with no 'apparently noticeable' problems but something
indeed 'was' going on, as I learned later.
* Eventually, by beginning of February this year, the RT shims needed
to be moved to maintain lineshape in such a way that, the bsms
complained of too much shim system current/power being demanded.
Especially, it was the X2-Y2 values, which moved to about -54,000 (
Prior to quench, this shim still was of course high i.e. about
-48000) that triggered the BSMS error. With BSMS, when this error (i.d.
B069) is triggerd, it does not allow one to increase any shim current
and one can only decrease the shim currents then on (this is very much
in line with Jerry Hirschinger's comment that BSMS looks at the total
current driven by the systtem rather than the individual values, before
triggering an error condition of this type).
It is at this point that I started to ask if the RT shim system has
any problem to begin with and if my shim stack has any problem so that I
am not able to adjust my RT shims at will.
There were reasons for my doubting the RT shim system. Even though I
talked about the changed cryolineshape, it was not clear to me, if the
change was significant enough to suspect that something was amiss with
the cryoshims. When I showed the comparative spectra from Nov '04 and
Feb '05, Oxford did not think the change to be too much, but Bruker did
think that the cyroshims need adjustment. Also, it was almost five
months since the original re-install of the magnet and neither Oxford
nor Bruker thought that a supercon magnet can drift so slowly and be at
field this long.
I got a loaner set of shim boards from Bruker and I could see the
same 'overcurrent' error message with similar RT shim sets. This
prompted a look into the magnet, as a possible source of the problem.
By this time, my available resources were such that, I could not
simply get an engineer here for three days to complete the whole job of
cryoshimming and then adjusting the RT shims while I sat back. Instead
I started on this memorable and vexing journey, along with a colleague
of mine, of "doing it all ourselves". We used a shim power supply that
I borrowed from Oxford and after several days of trying to adjust the
cryoshims, found that two of the supercon shim switches were not opening
owing to lower than expected heater current from the shim power supply.
After this I looked around and found a better looking but old Bruker
power supply (B-CN70) which belongs to my facility and thankfully it is
working ok.
The journey is not over yet. After trying different sets of values
for the cryoshims ( I have Z, X, Y, XZ, YZ, X2-Y2 and XY) and trying to
obtain corresponding RT shim solutions, unsuccessfully, I brought the
cryoshim values back to the "original" values. I noticed that, changing
the current in one of the cryoshims, in many instances induces
additional magnetic flux in one or more other cryoshim coils, which
needed to be systematically 'dumped' into the power supply by visiting
each shim at a time and opening the heater switch and simply closing the
switch again. I also noticed that the Z cryoshim is so extremely
sensitive, that with my old power supply with analog vernier shell to
control the current, just touching the vernier pot completely takes the
cryoline to a whacky shape. The precision that I have with this turn
helical pot is 10 mA. When I 're-adjust' the Z-shim to bring the
cryolineshape to its original state and look at the pot reading, it
looks as if the change in current is smaller than the least count i.e.
within 10 mA. Either that is the case or there is some kind of a
hysteresis like behavior associated with the cryolineshape.
In any case, with the last set of cryoshims, I could manage a
reasonable RT shim set with the spinning sidebands essentially gone and
I was in the process of getting to the 3rd and 4th order transverse
shims. Even though this RT shim set has been having rather large X2-Y2
RT shim value (approx. -45000), (back to square one, apparently !) I was
feeling that I was making some progress, though a long way to go. But,
after I finished a He fill yesterday and came and saw the screen, the
3.5 Hz H2O line that I had, has jsut collapsed to a broad line, similar
to the cryoline itself. The conclusion as of now is, I have somehow
quenched one or more of my cryoshims and I have to get into the magnet
again to take care of the same. Originally, I wanted to converge to an
acceptable lineshape with more shimming, albeit large values for X2-Y2
and XY and then get back to the cryoshims and see if I can off-load some
of these currents into the cryoshims. In normal circumstances it may
not matter much, but, in my situation, it would have been valuable if I
knew, what my 'X' cryo-coil orientation is, given the fact that I know
in which direction my RT X-coil axis is pointing (thanks again to Thomas
Speck for pointing out how to figure this out). If I can align my RT
shim stack in line with cryoshims, presumably, it would simplify the
task of off-loading higher RT currents into cryo currents. I do not
know at present, in which direction my X-cryo coil axis is pointing.
Thank you for your audience and I will be more than to eager to hear
any reflections, lore, nostalgia, comments, critiques and advice on this
never ending do-it-yourself cryoshimming adevnture of mine.
Thank you.
very sincerely
rajan
--
*_______________________________
Rajan K Paranji, Ph.D.
*NMR Facility Manager
Department of Chemistry
Room 65, Bagley Hall
Univeristy of Washington
*Seattle, WA 98195
ph: 206 685 2581
fax: 206 685 8665
email: paranji_at_chem.washington.edu
___________________________________*
Received on Fri Apr 01 2005 - 18:15:29 MST