Date: Mon, 25 Aug 2025 12:58:37 -0700
Hi Hasan
I have always used 99.999% helium to pressurize the dewar for helium fills
so that is what is on hand to clear magnets. I can't say I remember a flow
rate (sorry) but you want a good flow you can feel on your hand. The gas is
meant to melt ice and blow the "air" back out of the magnet. So it
needs to be present but not too forceful. Putting a rod of any kind into the
magnet is going to cause boil off so your initial inserts are going to be
what gets the ice out. Once the boil-off starts things freeze again. So heat
the rod up, this is to make sure it has no moisture and to help warm gas
and ice if you hit any. Pull the rod out, heat it again and wait for the
magnet exhaust to calm down, and repeat. So the rod is not staying in the
magnet this is put it in, pull it out, and warm it up, repeat. If you do hit
ice push down some on it then bring the rod out. Yes you may need to do both
towers. I have even had to take the tower top hat off before when doing this
when it was bad enough (don't wish that on anyone).
I have included pictures of one of the rods we use. It is a copper pipe (1/2 inch)
with a cap on it. The cap has holes on the sides and bottom, in hopes of clearing
ice that maybe on the side walls not just below it. Should be a nice "cushion" of
helium gas around it. This works well on the charging side of oxford magnets,
you can also use the plug puller on this side as well.
I have always done fills at 50% or higher on the meters. This comes from the
days of never knowing exactly when you would get helium delivered. Now with
helium recovery not an issue but old habits die hard. The level meter should
not include the towers? (I think that is correct) So you can try this pretty
much anytime but I like to have the helium ready so I can top the system off.
But if you don't have helium recovery you may want to wait until you have places
to put liquid if you have to order it. This is usually what determined when we
did this kind of thing before we had helium recovery. As long as your boil-off
seems to be normal you probably still have time
I have had ice blockages or something in a transfer line that cause similar
issues but yes next fills went smoothly. So there may still be a chance that
is what you had and things are all fine.
Greg
Gregory P. Wylie, Ph.D
NMR Facility Manager
Texas A&M University
Department of Chemistry #3255
580 Ross St.
College Station, TX 77843-3255
gpwylie_at_tamu.edu
979.458.0705 (voice)
979.845.4719 (fax)
706.206.0007 (cell)
http://nmr.tamu.edu
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