Thanks to all of you who responded to my question about keeping vs
replacing a 15-year old magnet.
The reason is purely economic, as magnets are big ticket items in a
spectrometer purchase. Of course, if funds are available you opt for a
new magnet.
Eighteen out of the nineteen spectroscopists that responded suggest
keeping and servicing the old magnet, most sharing their own positive
experience with console upgrades of 10-20 year-old systems.
That assuming a well behaving magnet in the first place, i.e. no major
change in cryogen boil-off rate, low field drift, no rust/corrosion etc
Once you decide to keep and service an old magnet replacing o-rings and
cleaning/remaking seals is a must. Other recommended replacements are
the RT shim stack and the spinner turbine.
Scott Chesnick provided a detailed magnet inspection and service guide,
please see his message at the end of this summary.
Resonance Resources (Peter Starewicz, 978-671-0811, ext 232) and
Magnetic Resonance Services (Ben Hamilton, MagResServ_at_aol.com ) were
mentioned by some as a knowledgeable source of (old) magnet service.
Lazaros Kakalis
Rutgers University at Newark
Department of Chemistry
73 Warren Street
Newark, NJ 07102-1811
Tel: (973) 353-5040 (office, lab)
Tel: (973) 353-5329 (secretary)
Fax: (973) 353-1264
Email: kakalis_at_andromeda.rutgers.edu
Scott Chesnick's message:
Hello-
I thought I would send in my opinion and two cents worth on your magnet question.
I think you should have no problems at all using the magnet forever.
Only if - you replace the "O" rings,
THIS ABSOLUTELY HAS TO BE DONE.
Nothing else really wears out.
(You may want to upgrade your RT shim stack and spinner turbine.)
When the magnet is disassembled use this time to visually inspect various
mating surfaces for signs of corrosion from water dripping from the liquid
nitrogen boil off ice etc. You should also replace any graphite quench bursting
plates if so equipped. The other item that may need attention is the pop
relief valve snorkel balls and seats. You may have to replace the
plastic hold downs if cracked. I would clean the ball seats and also
replace the balls if the material itself looks aged. I use a thin
coating of silicone vacuum grease on these ball and valves seats to
insure against any back pressure / cryopumping water ice contamination
back flow from the moist ambient room air.
The bore tube alignment spiders should be inspected at this time as well
if the magnet is disassembled this far. Depending upon the age of the
magnet I have found a few cracks and pulled threads on these.
The spiders often get damaged when something ferromagnetic
(i.e. typewriter, toolbox, etc.) that gets too near or "accidentally
stuck to the magnet dewar is removed.
This can happen when no one else is around so these events often go
unreported.( i.e. janitors, maintenance crews and errant students)
There is no visible outside damage to the dewar.
So do take the time to inspect the spiders and support structure parts
closely.
The indium soft metal thermal shorts can get knocked loose as well
during these episodes.
You may have to refinish the metal surfaces of any area that is rough
and would prevent a good vacuum / helium tight seal. I have used
a plastic varnish "GE Vacuum Sealant" to fine fill any suspect porous
metal areas near the compression metal to metal and "O" ring seals.
I am not sure if this is still made though since one bottle has lasted
about twenty years now. This is basically styrene plastic and some
solvent.
The Ham radio stuff called _"Q" Dope_ used to coat RF inductors will
work just as well since it is the same plastic. You may have to thin it
a bit you want a barley detectable thin coat for the "O" ring to seat
against.
You also should clean the gold connector pins or leads, for ramping as well
as inspect and replace as necessary any of the wires for the cryogen monitors.
I would add new aluminum finned tube heat sinks to the nitrogen boiloff
ports to prevent water dripping.
Your magnet should be ready for another 14 years or 100,000 spectra or
whichever comes first.
We have been using Ben Hamilton of Magnetic Resonance Services recently
for refurbishing our older magnets here when they have been moved at NIH.
he is very knowledgeable as well as cheaper then Oxford and MR Resources
for us. He can also custom make any oddball size "O"rings if you have
trouble finding the replacement kit from Oxford.
ooops
P.S. One more thing. You may want to increase the size of your helium activated
charcoal "getters" or add them now if the dewar is not so equipped originally. These
traps certainly prolong the life of the vacuum protecting the dewar space from
ravages of time and helium contamination.
Regards
Scott Chesnick
NIH Bethesda, Md.
Received on Mon Jun 04 2001 - 08:36:11 MST