You can measure the LN2 level any time, trivially, with a plastic/
fiberglass "dipstick" rod such as Varian routinely provides with
their systems. You can pretty easily measure the LHe level with
a flutter tube. Once you know how long you can go between fills,
if you keep an eye on your boiloff, and measure the levels very
occasionally, you should be set.
The cryogen level gauges are not completely reliable, especially
the LN2 sensors, which depend on the difference in dielectric
constant between liquid and gaseous nitrogen to change the capacitance
between a long pair of concentric tubes immersed in the LN2 reservoir.
I have never gotten into trouble ignoring or not having cryogen level
sensors; I _have_ on a couple of occasions gotten into trouble (let
a LN2 can go dry) by _believing_ over-optimistic LN2 level gauge readings.
(But not in a long time! :-)
Regards,
John Wright
Dr. John M. Wright, Dept. of Chemistry & Biochemistry, M/S 0314, UCSD,
La Jolla, CA 92093-0314; email: jwright@ucsd.edu; phone: (619) 534-3049
> Subject: Cryogen sensors: summary
> Date: Mon, 3 Aug 1998 13:43:45 -0600
> From: Bill Thurmes <thurmes@displaytech.com>
> To: "AMMRL" <ammrl@wwitch.unl.edu>
>
> To those who responded to my question about the usefulness of cryogen
> sensors, thank you!!
>
> I had heard from a couple sources that some people liked the sensors,
> others disliked them. Most people who responded really liked them,
> especially the helium sensors (they were more ambivalent about the
> nitrogen sensors). Out of fifteen respondents, only one said "don't
> bother", and another said they probably weren't necessary. Several people
> said they were essential. A couple people pointed out that the sensors
> should have an On/Off switch, so they don't deplete the helium so fast.
> Some other points mentioned:
>
> (They provide) an additional indicator during filling besides the change
> in the plume.
>
> In the couple of the last 7 years with altogether 5 spectrometers we had
> 3 dangerous situations. In all of these cases we had thermoacoustic
> vibrations within the helium tank. This caused the helium evaporation
> rate to increase about three times. The problem itself each time was easy
> to solve, but we needed the sensors to detect the problem.
>
> Thanks again for your responses.
> --Bill Thurmes
>
> Bill Thurmes 2602 Clover Basin Dr. Phone:(303)772-2191
> Sr. Research Chemist Longmont, CO 80503 Fax: (303)772-2193
> Displaytech, Inc. thurmes@displaytech.com www.displaytech.com
> When aiming for the common denominator, be prepared for the occasional
> division by zero
>