Hi Tara,
Leaking TMC valves are normally audible due to the pressures involved. Disassembly and cleaning will normally take care of them. Sometimes the large bonnet ring just needs tightened. Using relatively dirty, oily house air will not harm the legs, and will reduce load on the clean NMR air source. If you find yourself cleaning them often, I recommend putting a particulate filter in the air line.
You are correct that there should be no gas consumption in a static condition, but users leaning on the magnet, cryogen fills, etc. can disturb the stasis. Since N2 gas cylinders are so cheap, you might just hook one up and determine the longevity empirically, but it will vary according to how much the stasis is interrupted. Running out of N2 gas during such an experiment would cause no damage except to spectral quality.
There is no need to ever lower the legs for normal operating and maintenance. AFAIK, Bruker no longer does it. I stopped lowering my magnets for fills many years ago. Lowering and raising the magnet not only uses up more gas, it can require complete realignment of the TMC gymbals each time, or slowly moves the magnet away from the center of the leg bases, and can increase the need to re-shim.
The only major difference I have noticed between lowering them and not lowering them is during the LN2 fills. LN2 fills develop a travelling wave of LN2 going round and round in the circular LN2 chamber, which rocks the magnet in a small circle. This condition is eliminated by lowering the legs, but it is not harmful. Allowing the magnet to rock naturally with the travelling wave reduces cryostat stresses by allowing the legs to absorb some of the wave forces instead of the cryostat internal structure.
Cheers, -Hirsch
Jerry Hirschinger, NMR Instrumentation Specialist
Purdue Interdepartmental NMR Facility
560 Oval Dr. West Lafayette, IN 47907-2084
Office: Wetherill 365A
Phone / Fax: (765) 494-5288 / 494-0239
Cellular: (765) 427-3034
-----Original Message-----
From: Tara Sprules [mailto:tara.sprules_at_mcgill.ca]
Sent: Wednesday, January 14, 2015 2:17 PM
To: ammrl_at_ammrl.org
Subject: AMMRL: testing gas consumption/leaks on TMC anti-vibration legs
Hello,
I was wondering if anyone has a method to determine how much compressed gas their TMC antivibration legs consume? I was envisaging putting some kind of splice into the airlines leading into each leg them with a flowmeter and valve to cut flow to the flowmeter when raising and lowering the magnet...
As I understand their operation, as long as the compressed gas going into them is ca 20psi higher than the value required to keep the magnet floating, there is very little actual consumption of the gas. Assuming no leaks.
I need to both check for leaks, and also determine how long a standard compressed nitrogen cylinder would keep the magnet up.
Thanks for any suggestions/experience,
Tara
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Dr. Tara Sprules
Quebec/Eastern Canada High Field NMR Facility
www.nmrlab.mcgill.ca
phone: (514) 398-1721
fax: (514) 398-8254
3420 University St., Rm 023
McGill University
Montreal, QC, H3A 2A7
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Received on Thu Jan 15 2015 - 09:00:07 MST