Hi Piotr,
My first suspicion would be icing somewhere in the magnet. If the exhaust
is partially blocked then you'd get higher back pressure with a lower flow rate,
which sounds like what you're seeing. Or if the transfer line is running
into ice somewhere and getting partially blocked then you'd get less flow as
well. Is the o-ring on the fill port in good shape?
It sounds like your transfer line might be okay based on the lack of ice/wet
spots. You might also check the normal exhaust flow from the magnet
--if the boil-off has increased and more helium is boiling off between fills
then the fills would take longer. And this would show up in increased
helium exhaust from the magnet under normal operating conditions.
However I wouldn't expect that to cause higher back pressure--just longer
fills.
Cheers,Mike
On Friday, July 12, 2024 at 10:21:08 AM MDT, Piotr Dobrowolski wrote:
Dear NMR Managers,
Recently, we started having problem with helium fill on our old (about 25
years) 500 MHz Inova. Transfer line precooling takes a little longer than
before (I think) but there is no water condensation on the line. However,
from the beginning of the transfer the pressure in the transport dewar
(60 L) is high. We try to keep it below 2 psi by setting low gas flow
(about 2 L/min). I guess because of that the fill takes very long time -
about 1 hour to transfer 50-55 L. Unfortunately, our Oxford helium level
monitor does not work. We just fill-up every 3 months.
I've been taking care of this magnet for more than 6 years and the fill was
always efficient (20-25 minutes) with gas flow set to 5 L/min (pressure 1.5-2 psi).
Transfer line vacuum was pumped last year.
Has anyone experienced such issue? Does this indicate transfer line or magnet
problem?
Thank you,
Piotr
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Received on Fri Jul 12 2024 - 09:57:30 MST