Hi Monika,
We have a bagless QT system with 20 L/day capcity, 3 connected magnets, and
combined daily boil off of ~2L. The manufacturer told us that's a low flow
volume which I assume is equally relevant for your system. I've wanded
everything with a He sniffer multiple times, and am stuck with the same
question: measured liquefaction is low, what gives?
I've talked with QT about this problem and we're still working through it.
The liquefier has several internal checks and operational modes it will switch
between, all under the umbrella of "liquefy" mode. It's possible that some
of these modes are more vulnerable to leaks in our plumbing, and the duty
cycling across them depends on circumstances such as purity and flow volume.
The all important question: how do we know we're actually losing helium?
-Usually by comparing two independent measurements. First, our He level meters
within the magnet(s). Second, a reading from the liquefier production dewar.
If calculated boil off from monitoring He levels in magnet exceeds liquid
collected in projection dewar, then we presume inefficiencies/leaks.
-The calibration of the He level meters can be wrong, or at least differ slightly
from the charts in our magnet books. Those deviations are likely small and
wouldn't jeopardize our fill scheduling. Still, they would contribute to net
error when we're attempting to account for every atom possible.
-The production dewar levels are more suspect. Most (all?) dewars report liquid
contents using a Differential Pressure Indicator (DPI). The connected tubing
extends down into the dewar and can clog with frozen impurity. When that happens
measurements can become strange such as non-linearity, highly variable, and
non-responsive with regards to actual liquid contents. Sometimes venting gas
through the DPI's tube fittings can clear the clog. But if not, then the nuclear
option is to empty the production dewar and draw vacuum to remove all impurities.
By no means a casual decision.
-If the situation calls for it then you could thump your production dewar to
check contents. It would introduce impurities, and is ill-advised, perhaps
even dangerous to the cold head. But saving a magnet at critical levels might
shift the calculus...
I submit that the only metric at that truly matterss is the amount of liquid
actually obtained and filled back into magnets. I at least transfer/fill too
infrequently for that to be practical. Not enough data points, and some what
annoyingly our mobile dewar warms back to room temp between transfers and
requires full cooldown each time.
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
email: fko24003_at_uconn.edu
(alias): luke.fulton_at_uconn.edu
mobile: (603) 953-5275
Office: (860) 486-4069
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Received on Fri Apr 03 2026 - 11:08:20 MST