Original Message:
We have a 200 MHz Varian Gemini 2000 which is used for our undergraduate
students. About 3 weeks back I did a He fill. The day after the fill
there was some construction going on in the Department (some serious
jack hammering) and I noticed that the flow rate was at 20 instead of
3.5-4 (which has been the case for the last couple of years). I
immediately released the pressure by opening the fill port and there was
quite a large plume of Helium that came out. After closing the valve I
kept on monitoring the drift of the field. Initially it was high (around
8-10 Hz every hour) then it stabilized to about 4 Hz every hour.
Yesterday I noticed that the flow rate was again up to 8. The line shape
looked quite unacceptable and the shims had obviously changed quite a
bit. I called up Varian and they suggested that there was a possibility
of icing inside. They also suggested using a stainless steel T to blow
He gas into the magnet. I have not done this before and was wondering if
some of the NMR gurus could shed some light. What in your opinion would
be the short term fix and what can I do as a long term fix.
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We have the same equipment and for your comments I believe that you had
some ice on top of the helium recipient. What I suggest is to refill the
Helium and then blow He gas to remove the ice. Also it is possible that
vibration produce enough energy to star the boiling and this made all the
drift in the magnet. After you fill on again an let stabilize the magnet
check out if the boil off is in the standard levels.
What the machine will require is a new set of shims, but this most be done
until the drifting/boil off is stable.
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Higher boil off is natural when someone is jack hammering nearby.
You should not have opened the fill port, which might have allowed some
moisture to enter in the magnet. High boil off due to vibration is quite
common. Monitoring the boil off over a period would be prudent before
taking any drastic step. I would rather watch the magnet over a period
before doing some thing. Make that there is a plenty of liquid He in the
magnet. Do not let He level to dip below 50% (Well above the solenoid).
Good Luck.
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I would be very careful to blow gaseous Helium into the magnet. In these
type of magnets, the coil does not have a shield above it. So if you blow
gaseous Helium onto the coil, the likelihood of quenching is extremely high.
(It happened to us twice of the years just with the initial Helium input
when filling the magnet). On the other hand, if you really have an ice
block, that is what you may have to do. I hope you get other recommendations
to try first.
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We had an ice block on a Gemini-200 on the side not used for
helium fills. To diagnose this watch the helium boil-off meter and pinch
off the line from the stack one side at a time before the Y that joins the
two lines. A blockage will be detected as a change of flow rate when the
open side is pinched off and nearly no change when the blocked side is
pinched off. If a blockage is found, you may want to check the conditions
of the "o" rings in the helium stack caps and replace them to avoid a
repeat of the ice blocks.
We had Varian do this on our GEM-300 to remove an ice block. We
did it ourselves on the GEM-200. We used the flutter tube first, then
"L"-shaped stainless steel tube that came with the magnet to gently blow
helium gas into the ice block (probably nitrogen ice) to clear the
blockage. This must be done carefully to avoid blowing off too much helium
liquid from the magnet. It is safest to have the magnet topped up with LHe
first if possible. If in doubt, call the telephone tech support for
further instructions.
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If this is an r2d2 magnet then one possibility is that the harmonic
dampened tube is blocked. These are the long tubes that pass into the
magnet in the fill and charge port. During helium fills in the old r2d2
magnets the dampened tube sometimes gets blocked causing an imbalance in
the helium can. You can fix this by taking out the tube on the charge side
and taking it away from the magnet. If the tube is blocked in a few seconds
a plug of frozen air will come out. Then wipe off the tube and put it back
in things may then return to normal.
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Dipanjan Nag, Ph.D.
Director of Instrumentation
University of Nebraska-Lincoln
Department of Chemistry
Lincoln, NE 68588-0304
Phone:402-472-6255
Fax: 402-472-6964
Received on Mon Apr 08 2002 - 13:01:44 MST