I am wondering what the conventional wisdom is with
regard to maintaining the optimal heater current on
Varian cold probes. We have two Varian cold probes. The
unit on our 500 MHz system only has the TASK system while
the unit on our 800 MHz system has an ion pump. The
cold probe on our 500 is about a year and half older
than that on the 800 MHz system. Since we have excellent
cooling with a dual Liebert chiller that provides 15C
chilled water in an insulated line and the units are on
emergency power with generator backup, we rarely have the
cold probes go down accidentally.
I have found that the cold probe on the 800 with the
ion pump seems to lose heater current over the timespan
of a year (dropping from 4.5 to 3.5 in heater current).
So far, I have found that pumping the cold probe down
on the TASK sysem while warming it up appears to recover
this lost heater current. I have suspected that the
cold finger from the He coolant line may become encased
in 'plasma' (aka ice) as can the coils of the probe. It
would seem to me that this could insulate the cold finger
of the coolant line and reduce the effectiveness of the
He compressor system. I would also note that we never
replace the He gas in our cold probe when we suffer
a warmup but reuse it (with the idea that the gas in
the line has been cleaned up by the builtin molecular
sieve more than the ultra pure He gas tank supply).
I would be interested in what the conventional
wisdom is on these issues. Doesn't it make sense to
warmup the cold probe once a year to be able to blow
off any plasma that has builtup on the coolant system's
cold finger?
Jack
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Jack W. Howarth, Ph.D. 231 Albert Sabin Way
NMR Facility Director Cincinnati, Ohio 45267-0524
Dept. of Molecular Genetics phone: (513) 558-4420
Univ. of Cincinnati College of Medicine fax: (513) 558-8474
Received on Mon Aug 18 2008 - 06:57:27 MST