In addition you can do what I did, build an enclosure for the bottom of
hte magnet and flush it with N2 gas. I did a lot of work at 100k for
many hours. Just keep an eye on things.
"K.P.Sarathy" wrote:
> Dear Bruker Users: You can minimize this problem by passing
> N2 gas from top of the magnet and coming out of the shim
> stack base plate during Low VT work.
> Dr. K. P. Sarathy
>
> > --- Jane Strouse <strousej_at_chem.ucla.edu> wrote:
> > > Date: Thu, 11 Apr 2002 09:22:14 -0700
> > > To: ammrl_at_chemnmr.colorado.edu
> > > From: Jane Strouse <strousej_at_chem.ucla.edu>
> > > Subject: Another shimming weirdness
> > >
> > > Dear AMMRL'ers,
> > >
> > > I have had exactly the experience that Ben
> > describes below. We do a
> > > lot of VT work and I have fixed these connections more
> > times than I would
> > > like to remember. I agree 100% with Ben that Bruker
> > should consider making
> > > these connections a lot more robust.
> > >
> > > Jane Strouse
> > >
> > >
> > >
> > > >Date: Thu, 11 Apr 2002 09:05:16 -0400
> > > >From: Ben Bangerter <ben.bangerter_at_yale.edu>
> > > >X-Mailer: Mozilla 4.76 [en] (Win98; U)
> > > >X-Accept-Language: en
> > > >To: Karen Ann Smith <karenann_at_unm.edu>
> > > >CC: ammrl <ammrl_at_chemnmr.colorado.edu>
> > > >Subject: Re: Another shimming weirdness
> > > >
> > > >Karen Ann,
> > > >
> > > >Is it possible you have a bad connection to one of the
> > shims, perhaps at the
> > > >base of the shim coil asssembly? If this were the
> > case, when you have the
> > > >bad lineshape, one of the shims would have no effect.
> > I think you have
> > > >Brukers, right? From my experience with Bruker shim
> > stacks, the soldered
> > > >connections where the wires of the shim coils are
> > attached to the PC board
> > > >where the wires to the shim cable attach, at the base
> > of the shim stack, are
> > > >prone to fail. I have seen this a couple of times. I
> > believe the culprit
> > > >is electrolytic corrosion that occurs when there is
> > water condensed in this
> > > >area during low temperature operation, when the shim
> > assembly is cooled
> > > >below the dew point. A small amount of mechanical
> > disturbance can break or
> > > >open the connection, and when a connection is remade
> > it can "weld" slightly
> > > >due to the current density at the point of contact.
> > The first time I
> > > >experienced this, it was maddening until we removed
> > the shim stack,
> > > >disassembled the base, and examined the connections.
> > Lots of corrosion
> > > >products were evident, and an open connection.
> > > >
> > > >If this is the cause of the problem you are
> > experiencing, it is easily
> > > >fixed. You would need to remove the shim assembly to
> > the bench, disassemble
> > > >the base to examine the connections, clean off any
> > corrosion products, and
> > > >repair any break. I have long thought Bruker ought to
> > passivate these
> > > >connections because of the potential corrosion
> > problem, perhaps with a
> > > >coating of lacquer of some sort.
> > > >
> > > >Of course these comments may be way off the mark in
> > this case; if so,
> > > >"never mind!"
> > > >
> > > >Ben
> > > >
> > > >*****************
> > > >
> > > >Karen Ann Smith wrote:
> > > >
> > > > > AMMRLers,
> > > > >
> > > > > I have been seeing a pretty weird problem on my
> > 500. Sometimes when I
> > > > > swap probes, the shim goes to (really bad). As in
> > linewidths of (many)
> > > > > 10s of Hz. But only sometimes.
> > > > >
> > > > > The most recent example is that last week the 5 m
> > probe was in and
> > > > > shimmed pretty well. Swapped it for the 2.5 on
> > friday and all was
> > > > > well. Swapped the 5 back in on Monday and again,
> > all was well. I did a
> > > > > little shimming on Monday and saved the file.
> > Tues. I swapped the 2.5
> > > > > back in. The lineshape was terrible, the field had
> > to be changed to get
> > > > > the system to lock, and the o1 necessary to put
> > water on resonance was
> > > > > off by about 1100 Hz. Today I put the 5 back in,
> > read in Monday's shims-
> > > > > and the field was fine, but the shims... not so
> > good.
> > > > >
> > > > > Last month, this happened about every probe swap.
> > I looked up the bore
> > > > > and didn't see anything. I cleaned the bore
> > anyway, and the problem
> > > > > seemed to go away - until yesterday. When this
> > first started, I blamed
> > > > > the surrounding- esp. as it seemed that the nearby
> > (touching 5 gauss
> > > > > lines) 250 also seemed to be changing. Now, I
> > don't think so. I can't
> > > > > find any changes in the nearby labs- and the shims
> > are fine as long as a
> > > > > probe is left in the magnet. I am sure the probes
> > are aligned the same
> > > > > every time- I have marks on the probe and magnet.
> > > > >
> > > > > Last month, z4 was the big changer. Today, z4 was
> > not too bad, but
> > > > > lower x,y,z all needed some adjustment.
> > > > >
> > > > > Ideas, thoughts, suggestions??? Please note that
> > not-swapping probes is
> > > > > not an option.
> > > > >
> > > > > kas
> > > > >
> > > > > --
> > > > > Karen Ann Smith karenann_at_unm.edu
> > > > > Director, NMR Facility Adj. Asst. Prof.
> > > > > Dept. of Chemistry Clark Hall
> > > > > University of New Mexico Albuquerque, NM 87131
> > > > > 505.277.4031 url:
> > http://www.unm.edu/~karenann
> > > > > Out of this world screensaver:
> > http://setiathome.ssl.berkeley.edu/
> > > > > "There is nothing a good day of cycling won't
> > cure."
> > >
> > > Dr. Jane Strouse
> > > Department of Chemistry & Biochemistry
> > > UCLA
> > > Los Angeles, CA 90095-1569
> > > (310)-825-9841 - voice
> > > (310)-825-0393 - FAX
> > > strousej_at_chem.ucla.edu
> > >
> > >
> >
> >
> > __________________________________________________
> > Do You Yahoo!?
> > Yahoo! Tax Center - online filing with TurboTax
> > http://taxes.yahoo.com/
>
> Dr. K. P. Sarathy
> Department of Chemistry
> Texas A&M University
> College Station TX 77842-3012
> Tel: 979 458 0705
> Fax: 979 845 4719
--
Work phone (days): 802 656 0196
Web Site: http://www.uvm.edu/~jbreeyea
University of Vermont
Received on Sat Apr 13 2002 - 09:14:29 MST