The issue stems from two primary sources: either heat ingress into the
system and/or a deteriorating vacuum. I recommend replacing the O-rings at
the helium entry port as a precautionary measure. Even minor imperfections
or signs of wear in these components can significantly escalate helium
evaporation rates. Additionally, given the magnet's age, it's prudent to
acknowledge that seals can degrade over time.
Regards
Vas
On Wed, Dec 6, 2023 at 3:19 PM Michael Groves <m.groves_at_utah.edu> w=
rote:
> Hi Ulli,
>
>
>
> I have a few thoughts:
>
>
>
> Do you have MICS running on that spectrometer? It is possible to save a
> long-term log from that and send it in for analysis. I know the Bruker
> guys have a utility that will estimate the boil-off of both cryogens (if
> you have an N2 level sensor as well) and could potentially see a change in
> the N2 boil-off as well. Having said that--If your magnet was installed in
> 2004, I don’t think N2 sensors were standard back then but you conceivably
> could have added one with a console upgrade. If you don’t have an N2
> sensor then the MICS log won’t help. It would confirm that the He boiloff
> has increased but that’s it.
>
> Having said that—if you don’t have an N2 sensor, how are you confirming
> that N2 consumption has not increased?
>
> Is the SLCB calibration correct? If you plug in either the black
> calibration plug or the other one and run a helium level measurement, do
> you get the sensible 0% and 100% readings when you measure the helium
> level? Do you have a flow gauge on the Helium exhaust? If so, is that
> showing an increase corresponding to the change in your data? I would
> think that a tripling in the boiloff would be easy to see on a flow gauge.
>
> Icing on the helium sensor I think is unlikely. That usually manifests
> itself as a period of time that appears to be zero boiloff followed by a
> jump in the helium level. Changing the slope of the boiloff curve is not
> something I would expect if it were ice on the sensor.
>
> Given the plot of helium consumption, my first guess would be that the
> vacuum is deteriorating, and since your magnet is around 20 years old,
> that’s not impossible. It is possible to pump on the magnet while it’s
> energized so it might be worth contacting the manufacturer and seeing if
> they would recommend that. If the o-rings need replacing, though, that
> would only be a temporary solution without rebuilding the magnet. However,
> if that did improve things temporarily, that would be a good indicator of
> what the problem is.
>
> Cheers,
>
> Mike
>
>
> *From:* main_at_ammrl.groups.io <main_at_ammrl.groups.io> * On Behalf Of *Ulrik Werner-Zwanziger
> *Sent:* Wednesday, December 6, 2023 12:46 PM
> *To:* main_at_ammrl.groups.io
> *Cc:* Josef Zwanziger <Josef.Zwanziger_at_dal.ca>; Ulrike Werner-Zwanziger <
> ulli.zwanziger_at_dal.ca>
> *Subject:* [AMMRL] magnet helium evaporation increasing steadily
>
>
>
> Dear Colleagues,
>
>
>
> May we please draw on your expertise for help with our (700 MHz) Bruker
> magnet problem? Here is the issue: Our daily evaporation rates have
> continually increased starting about March 2023. I show a graph to that
> effect at the end of this e-mail. What I plot there is the average slope
> between magnet fills. From when the magnet was set up in 2004 until about
> March 2023 the slope of the helium content has been on the order of -0.33
> percent per day. We are now at about -0.8 % per day tending towards 1% per
> day..
>
> We are stumped on what is going on and how we can get back to a lower
> evaporations. Here are some tests, observations and data we have:
>
> 1) Vacuum not getting worse: The nitrogen consumption is unchanged. Unless
> the helium coil is in its own vacuum chamber, we think it is unlikely
> that the vacuum is deteriorating. Do you agree?
>
> 2) Cryoshims are not getting bad: Without any RT shims, the liquid
> lineshape of a static liquid sample is about 93 Hz wide, FID lasts 0.5s,
> which is plenty good for our solid state NMR. Using our "latest" shim file
> (you will laugh, from March 2022) the lineshape easily resolves 6 Hz. The
> shims are good for ssNMR. We checked every single time and had no reason to
> change the RT shims. Also, if a cryoshim would have gotten bad, I would
> have expected a short burst and then no change again.
>
> 3) Shim coil temperature? Without any RT shims, the coil temperature reads
> 25 degree C. With the "latest" shim file the temperature reads 26 degree C.
>
> 4) EAPD : We took it out of the system for a while (one week or so). The
> helium evaporation does not change.
>
> 5) Helium recovery: The helium liquifier system was installed in Feb.
> 2022, more than a year before the evaporation rate increased. In our rather
> noisy data analysis and limited data, our recovery fraction has not
> changed, meaning we are recovering a lot of the increased evaporation.
> (Thanks! We would be in even more trouble than we are in already).
>
> 6) Overpressure: Because of the helium recovery system the 700 MHz magnet
> was always at slightly elevated pressure, compared to atmosphere, due to
> the liquifier header pressure. I don't see how internal frost could have
> built up when the system was bleeding helium out rather than getting air
> in.
>
> 7) SLCB sensor board, Yes, our sensor board in the BSMS was flaky, for
> years already. We always had to take a big average (5 readings) to get a
> sensible state of the helium level. However, even completely unhooking the
> sensor line from the magnet (terminating the cable with the black plug) did
> not make any difference to the evaporation rate (testing for about one
> week). We recently exchanged the board and I am impressed how reliable the
> measurements are now. But also, the evaporation rate has not improved.
>
> 8) O-rings: A Bruker engineer was here to install a new spectrometer for
> another lab. He came to our lab in preparation for a new installation,
> which we will get soon. He looked at the O-rings on the fill stack and
> found them to be in good shape. We did not look at O-rings on other
> stacks.
>
> 9) Unfortunately, he also mentioned that our ceiling is not high enough to
> take out and defrost the sensor. Maybe we can move it up a bit and try to
> defrost it then, in the hope that that does not introduce more frost than
> can be removed. It seems to me a risky maneuver overall, not just because
> of the electrical impact.
>
> 10) Leak testing: We have tested for helium leaks as much as we could and
> found that the Bruker one way valve leaked out some helium. We have now
> added teflon around the joints. Since the system is on helium over-pressure
> to the outside air pressure that should not have caused frost to get in?!
> A handheld leak tester was purchased recently, but had to be returned . We
> hope to use it for detailed tracing when we get it back.
>
> 11) What happened in March that could have brought about such a change?
> Frankly, we don't know. Around that time, we had our one year helium
> liquifier service. BTW, our other magnet behaves just like before, so we
> don't think that the liquifier system has introduced a problem. Also, we
> did Variable Temperature experiments, which for years we have not done, but
> the temperatures were not dramatic. Since then, the VT unit is not
> connected to the probe heads.
>
> Do you have any suggestions of what we can do to get back to the much
> better evaporation rates? It baffles us that the rise in evaporation rate
> is so slow over more than half a year by now. Do you have any suggestions
> on what could go on here and how to remediate it?
>
> Has anybody seen anything like this before and what did you do about it?
>
> Thanks for your insights.
>
> Happy resonating.
>
> Ulli
>
> --
>
> Dr. Ulrike Werner-Zwanziger
> Adjunct Professor
> Department of Chemistry, NMR-3,
> Dalhousie University
> 6274 Coburg Road
> PO BOX 15000
> Halifax, NS B3H 4R2
>
> e-mail: Ulli.Zwanziger_at_Dal.ca
>
> Tel. 1 902 494 8085
>
> FAX: 1 902 494 1310
>
>
>
>
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Received on Wed Dec 06 2023 - 12:47:43 MST