Re: Raising a Dewar during Helium Transfers
rdw (rdw@interaccess.com)
Tue, 17 Aug 1999 12:03:27 -0500
----------
: From: Stephen Stanford Jones <jonesss@cofc.edu>
: To: AMMRL <ammrl@wwitch.unl.edu>; Charles Oaks
<"charles.oaks."@nmr.varian.com>; Chris Jones
<"chris.jones."@nmr.varian.com>; John Oatis <oatisj@musc.edu>; Don Barrett
<barrettd@cofc.edu>
: Cc: Rick Heldrich <heldrichr@cofc.edu>
: Subject: Raising a Dewar during Helium Transfers
: Date: Monday, August 16, 1999 1:11 PM
:
: Spinlanders,
: We have had difficulty performing an optimal helium filling in our
year-old
: Oxford 300/54 magnet. For this problem, I have an hypothesis of the
cause and
: an idea to rectify. I would appreciate thoughts and suggestions on the
: matter.
:
: 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
:
Steve,
I had situation similar to yours but with an R2D2 style Varian Magnet on a
TMC vibration platform. I never seen a fill tube for the newer style Oxford
300's, but I found that the flexible fill tube from our Oxford NB 500
worked very well. The only temporary modification I made to the 500 fill
tube was to place a small hose clamp around the fill tube so as to achieve
the same insertion depth into the magnet fill tube as the one supplied with
the magnet.
Hope this helps.
Regards,
Rick Wintermute
rdw@interaccess.com
Looking for a job!!!