Hi Tianzhi,
It sounds more nerve-wracking on paper than it turns out in practice. The
"warmed" ice is unlikely to collect further down and refreeze into an even
worse blockage. And even if it did, you would continue on just the same.
The procedure is generally safe to perform after hearing some general
suggestions. Not necessary to see it demo'd before attempting yourself.
That said, I wouldn't ask graduate/undergraduate TA's to do it without
supervision.
Identifying blocks:
Take a thin hollow stainless steel tube, thumping rod is ideal dimension,
and gently lower it into the exhaust port. Slide the tip slowly along the
interior walls. Try to keep moving constantly. The rod can freeze in place
and become stuck if stationary for too long, particularly if the laboratory
humidity is high. If it does get stuck then you may need to pass warm nitrogen
gas through it so that it warms enough to dislodge. For that reason avoid
wooden dowels and non-hollow rods. The interior walls should feel smooth.
If you can notice texture, uneven portions, or "crunchy-ness" then ice blocks
are likely. If allowed to grow they could seal off the exhaust port entirely
and dewar pressure will begin building. A fully blocked port *should not* not
destroy a magnet immediately, but I refrain from suggesting how long it might
survive for fear of downplaying the urgency. If ever found it becomes an
immediate high level priority to open it back up. Blocks can form due to
worn out O-rings on your condensers, leaving exhaust port open for extended
periods following nitrogen fills, and excessively long helium fills chilling
the stacks on top of magnet.
Clearing blocks:
You want a source of warm dry nitrogen gas. A cylinder is fine. If you have
house nitrogen which passes through filters and dryers upstream then that
is preferable. Connect your nitrogen to a thin hollow stainless steel tube,
thumping rods lines are ideal. Use long flexible rubber hosing to make handling
the rod convenient, you will appreciate the comfort. Test your gas pressure
before approaching the magnet. Key detail is to maintain LOW PRESSURE coming
out of the tube, start with just enough to feel against your bare skin. Goal
is to avoid agitating the liquid N2 in your magnet dewar; you'll always have
the option to increase pressure afterwards if dissatisfied with the performance.
All movements from here on out are done SLOWLY and GENTLY. Place the tip of
the thumping rod into the exhaust N2 exhaust port and search for blocks. Begin o
chiseling away at blocks with the tip of the thumping rod. The warm nitrogen
gas will soften the surface of the block, and the thumping rod tip will scrape
it away. Continue until all blocks are remove d.
-If the port is fully closed off then at some point you will have to breach the
barrier to the other side. The pressure buildup with rapidly relieve itself.
Depending on how long the block was there this can be surprising, loud, and
may blow some liquid N2 and ice pellets out the port. Larger magnets may
require precarious positions or balancing on stairs and ladders. Don't lose
your footing and take a dangerous fall to the lab floor!
-Take your time. It may be 30-60 minutes of careful tedium.
-Many magnets are equipped with pneumatic supports to isolate from floor
vibrations. Remember to lower the legs before beginning.
-The liberated ice will fall down into the N2 dewar section. I'm unaware of
how to avoid this after they've formed. Best long term strategy is prevention.
Kind regards,
Luke
Luke Fulton, PhD
CHEM BLDG R003
NMR Core Facility Director
Unit 3060
COR2E & Department of Chemistry
55 N Eagleville Road
University of Connecticut
Storrs, CT 06279
________________________________
> From: main_at_ammrl.groups.io on behalf of Wang, Tianzhi via groups.io
> Sent: Friday, April 3, 2026 2:48 PM
> To: main <main_at_ammrl.groups.io>
> Subject: [AMMRL] Liquid nitrogen filling problem of 800 Oxford pumped magnet
Dear All
I am experiencing a problem when filling nitrogen into our Oxford 800 pumped
magnet, which has been in operation for over 21 years.
Recently, the boil-off pressure has been significantly higher than in previous
years, while the filling rate has decreased. This suggests that there may be
an ice blockage somewhere in the vent port line.
I am considering whether it is safe to use dry nitrogen gas to gently purge
the vent line in order to sublimate the ice or push out any loosened blockage.
However, I am concerned that this approach could potentially worsen the
situation if melted or dislodged ice is pushed back into the nitrogen vessel
and blocks the vent port further.
I would greatly appreciate any advice or shared experience on how to safely
diagnose and remove a possible ice blockage in the vent line.
Thank you very much for your assistance.
Best,
Tianzhi
-------------------------------------------------------------------
Tianzhi Wang, Ph. D.
NMR Manager
Sealy center for Structural Biology & Molecular Biophysics
University of Texas Medical Branch,
301 University Bvld, NMR dockside building, Rom 1.104
Galveston, TX 77555-1157
Email: tiawang_at_utmb.edu
(409)7476821 (O)
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Received on Fri Apr 03 2026 - 13:13:51 MST