We pump as needed. One is much more often than the others, but it is the right
size for a certain magnet or we would stop using it.
Also hitting it with a heat gun while pumping is a good idea. Any stuff that
has physisorbed on to the surfaces comes off much more easily due to the heat.
Best,
Jeff
On Jul 10, 2024, at 5:57 PM, JOHN RALPH via groups.io wrote:
We finally did our two after ~10 years!!! Apparently 5 years is good for most.
Yes, pumped on it overnight with a good turbo (and help from wonderful campus
NMR people).
Best!
John R.
On Jul 10, 2024, at 5:07 PM, Matthias Brandl wrote:
Hi all,
I handle it the same way as Karel - only re-evacuate if it is actually needed.
Talking to some engineers, I was told that it is useful to pump transfer
lines for quite a long time, and maybe also assist the process with a heatgun,
as they are such long and narrow vessels.
The cold spot shown in Bob's picture is present to some degree in (I think)
all the transfer lines I have ever used. I have rarely seen frost there, but
almost always liquid condensation.
One further thing to note about pumping transfer lines: I have had a case where
I could evacuate the line perfectly well, but every time I went to use it
afterwards, it would suddenly lose its vacuum after working fine for a couple
of minutes. Maybe some crack that only opens up when the line is cold? Anyway,
the lesson is to not rely on the warm line holding vacuum, as it might fail
when cooled down.
Cheers,
Matthias
On 10/07/2024 23:51, Karel Klika wrote:
Dear all,
I have never done a pre-emptive evacuation of a helium transfer line, normally
just wait till it is needed (once only in a previous location). The transfer
line I am using at the moment actually could do with a pump down, it frosts up
initially but once the adsorbent inside the transfer line gets cold enough the
vacuum is good enough and the outside of the transfer line defrosts. The reason
for my tardiness is that soon we will be getting a new system, and a new transfer
line will be coming with it.
Regards,
K. Klika
________________________________________
> From: main_at_ammrl.groups.io on behalf of Bob Berno
> Sent: 10 July 2024 23:49
> To: main_at_ammrl.groups.io
> Subject: [AMMRL] AMMRL: Helium transfer line gone soft.
Hello everyone,
Last August there was a useful email exchange here on the AMMRL about
repairing helium transfer lines. I have a related question.
Do any of you perform a preemptive pump-down of your transfer line(s),
or do you wait until you see signs that the transfer line has lost vacuum?
You can probably guess the reason I ask.
Last week we aborted a helium fill when I saw excessive
frost/condensation begin to form on the transfer line just as we started
cooling it down.
We aborted the transfer and then completed the helium fill using another
transfer line.
I asked the person in charge of our helium recovery system to test the
suspicious transfer line. When he connected it to his pumping station,
he could tell that it had indeed lost vacuum. He left it pumping
overnight. Then on Friday morning he closed the valve, disconnected the
pump and let the transfer line sit over the weekend. When he come in
Monday morning, he found that it had maintained a good vacuum over the
weekend, so he doesn't think there is a leak, but that it simply had
lost vacuum.
To test that, this morning we did a scheduled helium fill of another
magnet to see how the freshly pumped transfer line performs.
The fill went smoothly enough, but we did see a small cold spot on one
of the welds (photo found at this link:
https://photos.app.goo.gl/377BQgQZWrqyV4tj8 ) which we didn't see before.
Strangely, this is the newest of my transfer lines - about 4-years
old. The second transfer line I used last week is more than 14 years
old and it has never been pumped down since we received it. And I have
two other transfer lines that are each more than 20-years old and seem
to be fine.
And so I was wondering if anyone performs a preemptive pump-down of your
helium transfer lines. If you do, how frequently do you do so? Every
year? Every five years? Or do you wait for the first signs that the
transfer line might be going soft?
Cheers,
Bob.
--
Dr. Bob Berno
Manager, NMR Facility
McMaster University, ABB-156
1280 Main St. West
Hamilton, ON L8S 4M1
T: 905-525-9140 x24158 or x24686
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