Dear Ulli
it is also possible that the absorbers that are installed in the vacuum chamber
of all magnets are saturated. These absorbers can deal with small amounts of
some gases that might be present in magnets.But eventually they saturate and
can no longer do their job.
The situation can be improved by pumping with a turbo pump for at least 24
hours. This will not regenerate these absorbers but it will buy you at least
6 month of time to plan and schedule a rebuild of the magnet. The magnet
needs to be warmed up and opened. At this point you should replace the O-rings.
Regards
Clemens
On 12/6/2023 2:46 PM, Ulrike Werner-Zwanziger wrote:
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<mailto:Ulli.Zwanziger_at_Dal.ca>
Tel. 1 902 494 8085
FAX: 1 902 494 1310
--
==========================
==========================
====================
Clemens Anklin Ph.D. Vice President
Bruker BioSpin NMR Applications & Training
15 Fortune Drive NEW Phone: 978-313-5NMR(5667)
Billerica MA 01821 twitter: _at_canklin
web: http://www.bruker.com
e-mail: clemens.anklin_at_bruker.com<
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==========================
====================
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Received on Thu Dec 07 2023 - 03:45:01 MST