We have had a number of these magnets over the years, and have pumped
magnets at field on a number of occasions with no nasty accidents and
with considerable improvements in boiloff. The seals on these magnets
seem very susceptible to He diffusion, so a good pump every few years
can save on helium bills. You need a good (e.g. turbomolecular) pump,
an adapter (Cryolab p/n VO3-088-5L7-SP, cost a few hundred dollars ten
years ago), a good vacuum gauge, a short length of flexible stainless
vacuum tubing, and nerves of steel. Our power supply hardly ever fails
(DV) so we have not bothered with precautions like failsafe vacuum valves;
like Charlie Mayne, we carry on normal spectrometer operation during pumping.
We have always found the local Varian organisation extremely helpful about
such operations; they carry them out routinely, and if they've had any
disasters I haven't heard about them. The have a very nice portable Varian
turbo pump, which is compact, efficient,and easy for a high vacuum novice
to use.
Talking of helium, R2D2 magnets and disasters, I should perhaps own up to
an embarrassing mistake here in the past, as a warning to others. Our lab
ceilings needed to be replaced, so we were naturally anxious abut the magnets.
We carefully swathed our XL300 loosely in polythene sheet, and built a large
wooden box around it. When the ceiling repairers drilled in to a water pipe
and sprayed the lab with dirty water, we congratulated ourselves on our
foresight. Unfortunately when we unwrapped the magnet some days later we
felt rather less smug: there was no nitrogen, no helium, and no field.
There had been no sign of trouble, so we were forced to the conclusion that
wrapping the magnet had led to a buildup of helium gas around the seals,
which had softened the vacuum so much that the He hold time had dropped to
only a few days. Moral: keep good ventilation around the magnet.
On another topic, we are thinking of buying a Postscript laserprinter for
our small Apple network, and are wondering whether to go for the Apple
LaserWriter Pro 630, which has an Ethernet interface. Does anyone have
any experience of using such a printer with Sun workstations &/or Varian's
VNMR software?
Gareth Morris
University of Manchester
Acknowledge-To: <MBDPSGM@UK.AC.MCC.CMS>