Hi all,
Recently our 9.4 Tesla (400 MHz) narrow-bore Magnex supercon was
energized by JEOL engineers. In the days following the energization and
top-up of liquid helium, the helium boiloff was zero (in normal operation
it is around 180 mL / min). I was assured that this was normal, but it
has been 14 days since, and the boiloff is still zero.
The engineer (when he was around) has checked that there is no ice
blockage in the helium stacks. And by replacing the one-way check valve
(25 psi) with a straight through connector, I at least get a helium
boiloff of 80 mL / min. When I put back the one-way check valve, I get
zero again. (Obviously, the helium pressure is to low to open the check
valve).
Thinking the low pressure might be due to a leak, I have checked for any
possible leakage of helium in the connections in the helium stack, exhaust
port, etc. and found none. (Anyway, I figure we would have seen the leak
during helium refill).
I was told to see if the Z0 field drift was normal or increasing, but I
found that it is decreasing and is now very much within specifications.
Liquid nitrogen boiloff is normal (around 2.1 L / min). And we can get
good spectra from the machine.
I am in touch with the JEOL engineers and have gotten good advice from
them, but I thought I could ask the group to see if any had a similar
experience. I know ours is supposed to be a long-hold dewar, but can it
be that efficient??
Best regards,
--
Edward T. Chainani
Chemistry Department &
National Chemistry Instrumentation Center (NCIC)
Ateneo de Manila University
Loyola Heights, Quezon City
Philippines
phone: (632) 426-6001 loc. 5628
FAX: (632) 426-1323
e-mail: etch_at_chem.admu.edu.ph
Received on Thu Oct 09 2003 - 16:44:34 MST