We have a Gemini 2000 NMR spectrometer (Varian R2D2 magnet) which has
the magnet's helium power port severely clogged with ice. We pulled the
cap off the port (to replace the O-ring in the cap) and found that it
was very difficult to pull the hollow safety tube out and get it back in
due to ice down in the power port neck. We knew it was ice because we
had to chip away at it in order to get the rod back in - it looks like
there could be ice in several inches of the power port neck tube. Also
when we had initially vented the port through the top, no pressure was
released indicating that the portion of the neck on the outside of the
safety tube was clogged with ice. Tying off the Tygon tubing of the
helium fill port has still resulted in normal flow rates, though,
indicating that the safety tube itself is not clogged. Helium boiloff
rates are still normal. But it looks like we need to attend to this as
soon as we can.
We were recommended by Varian phone support to top off the magnet with
liquid helium and use the L-shaped cleanout tube to melt the ice in the
power port by using helium gas at a flow rate that's barely detectable
when we hold the tube against our ear (manual indicates similar flow
rates to those used for helium fills: about 4 to 8 LPM). We are to
lower the cleanout tube into the port by lowering it against the side of
the neck tube and once we reach the ice plug go "back and forth and
around and around" until the ice plug is melted. Has anyone ever done
this before with a Varian R2D2 magnet? (Yes, we know the risks of
quenching - the adrenaline will be pumping hard that day.)
Questions:
1) About how long might we expect to take to melt this ice plug which
may be several inches long in the power port neck tube? Should we stop
and check the helium level after a couple of minutes, or can we afford
to go longer?
2) As we melt the ice plug, it looks like it might be hard to avoid
blowing some of the fairly large volume of air vaporizing from the ice
plug back into the magnet. We plan to leave the helium fill port cap on
during the procedure and tie off the Tygon tubing of the fill port,
which may help force most of the expanding air from the large ice plug
back out of the power port neck. It's possible, though, that we could
at least partially cause the ice plug to "trade places." Perhaps we
should stop periodically to make sure that the helium fill port is not
forming an ice plug from all the air that is being vaporized from the
power port neck tube?
3) Regarding how far down to go in the neck tube, I'm told to insert the
clean-out tube until I've reached the electrical connector for the power
stick, which may be about 17 inches from the top of the neck tube (also,
the length of the safety tube on the port cap is a rough guide for how
far down to go in the neck tube). From the magnet diagram in one of the
manuals, the power connector lies on a "shelf" on top of the solenoid
and blocks further access into the magnet dewar through the power port.
Obviously if we get this far we want to stop immediately. I'm planning
to estimate how far down to go in the neck tube by marking off the
length of the port cap's safety tube with tape on the L-shaped cleanout
tube.
Any further pointers from someone familiar with doing this type of
procedure on the Varian R2D2 will be helpful.
Thanks -
Ruth
Ruth E. Nalliah, Ph.D.
Associate Professor of Chemistry
Huntington University
Huntington, Indiana
(260)359-4203
rnalliah_at_huntington.edu
Received on Mon Dec 12 2005 - 12:31:58 MST