MAS air failure

marat@cc.UManitoba.CA
Mon, 14 Nov 1994 17:58:59 -0600 (CST)

This past weekend, we had a power failure which caused our AMX500
lab to loose air pressure during a weekend long MAS experiment.
When we came in, we found both the rotor and stator of the probe
to be totally destroyed, and a very valuable sample to be deposited
as dust into the shim system of the spectrometer. This was a
5 mm high-speed MAS probe running only at ca 6 kHz - according to the
rather pathetic excuse that the manufacturer calls a spin rate detector.
We are not sure if the accident happened when the air failed, or when
air pressure was restored, or (possibly) was totally unrelated to the
loss or air pressure.

I should point out that the compressed
air system has a buffer tank and probably took on the order of several
minutes to drop to zero pressure. We are using regulators on both
the bearing and drive air supplies and, typically, the bearing
runs at much higher pressure than the drive. For 6 kHz we would typically
use 2 bar of bearing pressure and less than 0.5 bar of drive pressure.
As the building air pressure dropped, the bearing would therefore
lose pressure first and the drive pressure would remain constant
until the supply pressure dropped below the drive set point.

Does anyone out there have any experience with situations like
this? Is it normal for an MAS probe to suffer such severe
damage (approx $7,000 CDN) from a loss of pressure, especially
at relatively low speeds?

If this IS a commmon problem, does anyone have any ideas for
a protection system?

Even if your system has had loss of pressure WITHOUT damage, I would
appreciate hearing about it.

Thanks in advance!

-Kirk

-- 
Kirk Marat, NMR Facility Manager                  | We Keep the "N" in
Dept. of Chemistry / Prairie Regional NMR Centre  | NMR
Winnipeg, Manitoba, R3T 2N2, CANADA               |
ph: (204) 474-6259, FAX: (204) 275-0905           |