Ulli,
Don't mind long emails, but decided to write without reading in detail. Sorry
if you said smt about the following. I suspect your magnet simply need a rebuild
with new primary rings. These require the magnet warmed, taken apart, cleaned,
new oring, and put back together cooled and reenergized. Needs to be done
every 15-25 yrs.
Odd the Bruker eng didn't discuss this....
Charlie
On Dec 6, 2023, at 2:54 PM, Ulrike Werner-Zwanziger <Ulli.Zwanziger_at_dal.ca> 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
Tel. 1 902 494 8085
FAX: 1 902 494 1310
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Received on Wed Dec 06 2023 - 12:48:02 MST