The Problem:
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.
Characteristics of the filling are as follows...
With the magnet on an antivibration table, the storage Dewar must be raised 14
cm.
Even with the raising, the longest stinger, 48 cm, is used.
The transfer-tube is evacuated to 0.25 Torr.
The transfer-tube is not cold to the touch.
The fittings between the transfer tube and the magnet and between the tube and
the storage Dewar are properly snug.
Liquid levels in the storage Dewar start around 26 cm and end around 10 cm.
Gaseous helium flow and pressure are within published parameters during the
entire filling: 6 l/min and 1.3 psig.
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).
Our only truly successful fillings to absolute fullness have been from 60
liter storage Dewars where the liquid-levels began around 34 cm and ended
around 23 cm. Only a 2.2 times excess helium was used.
The Hypothesis:
As the filling progresses and as the level of liquid helium in the storage
Dewar drops, the uninsulated stinger becomes progressively exposed to the warm
UHP helium gas which is used to pressurize. As the stinger is exposed, early
evaporation within the transfer-tube becomes unacceptably high, resulting in a
large plume and little transferring of liquid.
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.
I see in the archives that Ken Fishbein posted a similar question in January
1996, but I do not see any replies.
Thanks in advance,
-- Stephen Stanford Jones, College of Charleston, Chemistry Department Facility for Chromatography and Nuclear Magnetic Resonance jonesss@cofc.edu, (843)953-4965, facsimile -1404 virtual location: http://www.cofc.edu/~jonesss/ssjHome.html physical location: SCIC 325 and 329