We recently had a quench start on a Varian 200 MHz NB "R2D2" magnet due
to low LHe levels. Symptoms noted were high He gas flows (increased
from ~ 4 cc/hr to ~ 20 cc/hr) and extreme lock problems due to the
decreasing field (2H lock frequency loss rate of ~ 300 Hz/hr). We were
able to arrange for same-day delivery of LHe. After filling the magnet,
the field stabilized. Net result was 230 ppm loss of field. After
adjusting NMR frequencies, the system was able to meet all specs
(including lineshape!). This happened in November 1996. The magnet has
been fine since then. No pumping required.
Bottom line: Be lucky!
-- David R. Kinney |All opinions are purely mine. ARCO Chemical Company | 3801 West Chester Pike | Newtown Square, PA 19073 USA | Voice: (610)359-3455 | Fax: (610)359-5573 | EMail: cvxdrk@arco.com |