>A typical filling from a 30 liter storage Dewar requires three times excess
>helium than is actually added. e.g. At the last filling, 19.5 liters were
>used to add 5.5 liters, a 3.5 times excess. I naturally expect an excess of
>helium to pre-cool the transfer-tube and to account for losses along the way,
>but I do not expect such an excess.
I've found that it usually takes about 3-5 L of LHe to precool the
transfer tube, so your useage does seem excessive. However, measuring the
helium in either your NMR or the dewar is only as accurate as the charts
you have to correlate helium depth with helium volume - I've found them
to be none to accurate, especially the ones on the dewar.
>The exhaust plume begins small as expected.
>(This is the disconcerting part.) The plume does NOT stay small during the
>filling and suddenly increase markedly when the magnet is full. The plume
>DOES, however, increase gradually such that we cannot discern an obvious
>ending. We must rely upon conservative timing to decide when to stop
>(circa 1
>min per liter used).
The plume usually gets bigger as you add helium - I always thought it was
because the dewar sides above the liquid level were a bit warmer than the
parts immersed in the liquid. It's harder to see the endpoint with the
newer Oxford magnets than it was on my 15-yr-old R2D2 (I'd still never go
back!).
>The Idea:
>
>I would like to raise the 30 liter storage Dewar at least to 36 cm so to use
>the smallest stinger of 26 cm OR BETTER to raise the Dewar to 62 cm off the
>floor to use no stinger at all.
>
>The Plea:
>
>Do any of you raise a Dewar so high? Can we safely do so without violating
>too many OHSA regulations? Would the raising even help me to fill better?
> Am
>I overlooking a simpler solution? The magnet still has one year under
>warrantee; I would not want to do anything to nullify it.
With my old R2D2, the only way I could get it filled was by building a
little platform to put the dewar on. I never checked into OSHA rules
about this, but I really don't think they'd care. The platform I built
was about 9" high, had four steerable wheels, and had little walls above
the platform on three sides to keep the dewar from rolling off.
Getting the dewar onto the platform is the only part that OSHA might care
about. I did it by lifting, with the help of someone else. A 60L dewar
wasn't too hard to lift up onto the platform (on that fourth, unwalled
side), although a bit ungainly - I would suggest getting someone to help
you. If you're filling an Oxford magnet, you have someone there anyways,
right? Having the stinger as short as possible should help the whole
process.
Good luck, --Bill
William Thurmes, Ph.D. 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
"The future is purchased by the present" -Samuel Johnson