Hello all,
What follows is a summary of the wisdom gleaned from the many expert
responses on this issue of conserving He during properly executed fills.
I have little experience with newer Agilent/Varian/Magnex magnets, so
while this is mostly generic, some information is Oxford, Magnex and
Bruker specific. I put what I thought was most important first. Credit
to Paul Cope, Jon Decatur, Boris Itin and many others.
Matthew
1. Precooling the transfer line with He gas from the head of the dewar
(pressurized by a He cylinder if the tank has no heater) saves liquid
He, i.e. don't shove the transfer line into the liquid while warm. A
liquid He "flame" must appear at the tip of the transfer line before
inserting into the magnet to assure that you don't introduce He gas into
the magnet. I wonder how many ammrlers out there are pressurizing tanks
the old school way with a bladder. I know some are! I use the adapter
supplied by Varian with the old Oxfords to pressurize a He tank with a
cylinder (one cylinder lasts me about six fills). I would be interested
if any one knows where to purchase those modified Parker Fast & Tite
fittings, just to have a spare on hand. Though, in the future I may
switch to 250L tanks (usu. w/heaters, no?) for the discount (10%). If I
can figure out a regime to fill less often. That old Oxford just doesn't
have the hold time!
(A good follow-up discussion for this topic would be the different
apparatus/techniques for tank pressurization. Several queried about this.)
2. It does conserve He to stop filling before it reaches the manifold.
He level sensors can be inaccurate during filling (He splashes around
making the level reading appear high), the level inside the magnet may
not be as high as indicated, even at 100%. This should be sensor/magnet
specific and experience should be your guide. On my older magnets, once
filling starts the meters indicate between 83-86% if the level is below
that, no matter how far. Once the level gets above ~86% the
meters/sensors are accurate. Keep them calibrated!
3. Try to use all the He that you order, even if it blows out the
manifold. Or better, do not order more than you need. There are no
credits for returned merchandise.
4. Decrease the pressure of the fill. Filling at 0.5 - 1 psi reduces
vaporization caused by agitation (liquid He has a very low surface
tension). And it's safer in terms of inducing a quench. This was
strongly recommended by one magnet engineer.
5. It would be best to use one tank of He per magnet, not wasting any in
moving from magnet to magnet. This is not usually possible or practical,
though. And, our vendor gives a 10% discount for buying a 250 vs 100 L
tank. If you do fill more than one magnet at a time, you can keep the
transfer line cold by opening the transfer tank vent valve partially to
keep He from flowing as you prepare the next magnet for filling (don't
forget to seal the fill port and one way valve of the magnet you just
finished!). You can warm the magnet end of the transfer line to room
temperature with a heat gun and wipe dry with a rag. Then start the flow
until you see the "flame" before inserting into the next magnet.
6. You may be able to adapt one transfer line for use with more than one
magnet so you don't have to cool multiple transfer lines. For instance,
the standard Bruker and Oxford transfer lines can be fitted with a 3/8"
to 1/2" adapter for use with Magnex magnets. If using one line, the
phase separator should be used for Oxfords, but can removed to reduce He
loss when filling Bruker magnets. And, on Bruker magnets make sure you
use the O-Rings provided with the transfer lines. These o-rings can be
cut, as needed, if the level sensor is close to the port (Oxford
magnets), but using them saves He and reduces the risk of ice build-up
in the magnet. I use the threaded cap with brass washer and O-ring for
the Oxford and Magnex, then slip the O-ring on for the Bruker magnets.
Received on Wed Oct 02 2013 - 11:14:23 MST