Cold Pumping
Walt Niemczura (walt@hydrogen.nmr.hawaii.edu)
Mon, 13 Dec 93 12:44:16 HST
Balzers sells a nice turbomolecular pumping system that one can
also buy a Helium leak detector to incorporate into the pump. They also
put together a He leak test station if you wish. We bought ours with CIP
monies and justified it due to our isolation and the cost and length of
time it takes to get equipment and an engineer here. We obtained ours
in 1991 and the turbo pump (with rough pump) was about $5K, the leak
detector another $8K and all the hardware to put it together for our
Oxford magnets another $1.5K. I believe the intact and preassembled
system runs around $18K. It's a nice unit that runs MS on gases up to
Kr so that you get the whole picture if you wish. Even interfaces to a
PC if you want to get fancy output (read-out only, no control over
system).
I hate to sound ultraconservative of like a wimp but I would
not attempt to try to cold pump an energized magnet. While a turbo
pump probably could get a bite on the He at those temperatures the
possibility of a power failure or other unforeseen event would be too
much for my weak heart to stand. I moved a magnet at field once (even
rode down an elevator with it) but that was in my young and cavalier
days. Recently we move a cold but de-energized magnet and I was worried
about that. I would be very cautious about pumping a live cryostat.
Solenoid actuated vacuum valves that close with a power loss are a must.
Considering the cost of a new magnet (> $50K) I would power the thing
down before attaching a pump.
Maybe I'm missing something here. I've have three Oxford's up
in our lab, the old narrow bore 300 has been in operation since Oct.,
1981. The boil-off on that cryostat has crept up from 1.1% a day
to about 1.25% averaged over the past six months. Are some magnets
prone to He diffusion? I haven't observed this for any of our's. I vent
the boil-off gas out of the room down the hall and out the building with
surgical rubber tubing. I thought that if one was careful to keep the
He level in the air near ambient and not use He gas to transfer nitrogen
that this diffusion problem was minimized. I guess that internal
diffusion can occur.
I suppose that if one was experienced at cold pumping a cryostat
that's OK but I would not attempt it myself and would hesitate to
recommend others to try this procedure.
Walt Niemczura
Dept. of Chem.
U of Hawaii
Honolulu, HI 96822.