We have a QD system that is essentially a closed loop that operates in direct
recovery 90% of the time. And so there are some similarities here. I don’t
know the QT system so this may not be directly comparable. Also we are at altitude
in Denver and there are probably things that we can do that others can’t.
Any system that has a differential pressure regulator (DPR/BPR) that many of us
use will show oscillations due to atmospheric pressure. This is an age old problem
and so we use a manostat (APR) that regulates absolute pressure on each magnet
cryostat and it is set above header line pressure.
If you have a BPR on your recovery line setting operating pressure it will
be subject to atmospheric oscillations and in our case the liquifier it self
may also regulate operating pressure. Ours does this primarily through regulating
gas inflow but it also has an internal PRV that it uses when it is not liquifyin.
Our liquifier is set at 0.05 psi during direct recovery. So our differential
pressure downstream of the magnets for the line is 0.25 PSI. (See below)
To maintain magnet stability it is better to have a manostat on each magnet.
It does mean raising pressure even higher. We can do it as we are at altitude
and have a lot of room to work with so I am not making suggestions for how you
should run but our example.
We have our header line at 0.2 PSI above the liquifier pressure which is close to
local atmospheric pressure (830 mBar) . This pressure is set by the input check
valve to the liquifier. Our magnets are at ~0.5 PSI (885 mBar) above average
atmospheric pressure. And we have a BPR in the line set at 0.65 PSI acting as
a relief valve in case of something going wrong. Our check valve into the recovery
bags/cylinders is 0.33 PSI and can be switched to 0.15 PSI during fills.
During fills the line pressure never gets above this and so the other magnets do
not experience any significant pressure swing as they are always above line
pressure.
Most helium dewars come with a 0.5 PSI safety valve and I believe that even
then the helium temperature is still below the critical current temperature
for most modern magnet coils, maybe someone can provide insight into this so
it may be possible to do this at lower elevations.
My 2c.
David
Associate Professor
Dept. Of Pharmacology
University of Colorado
School of Medicine
Tel (303)-724-3600
On Feb 6, 2026, at 8:16 AM, Ulrike Werner-Zwanziger via groups.io wrote:
Dear all,
I echo Sameer's and Tara's statements. Our system is similar to Sameer's and Tara's.
We regularly observe that the magnet's boil-off, as instantaneously measured by
the flow meters on the magnet exhausts, varies depending on the atmospheric pressure
and the state of the system (transfer happening, just happened, or all settled down).
Happy resonating
Ulli
On Fri, Feb 6, 2026 at 10:13 AM Tara Sprules via groups.io wrote:
Hello,
Despite the closed loop and no bag- we also have a QT system- the pumped magnet
pressure during a fill is directly correlated to the atmospheric pressure. I
don't know if having an APD related to the pressure control for the pumped magnet
is what affects that (Sameer?).
I've never calculated it out, but have always felt that after a few days of
higher or lower than usual atmospheric pressure there is a small difference in
liquid levels. We recover around 90% and assume that we are still losing
around the old flowmeters on the oxford/varian magnets that we've never
removed- so I guess the system isn't 100% closed...
Tara
*********************************************
Tara Sprules
0.6 FTE, Working days: Monday, Wednesday and Thursday
QANUC
Quebec/Eastern Canada High Field NMR Facility
http://www.nmrlab.mcgill.ca
phone: (514) 398-1721
fax: (514) 398-8254
3420 University St., Rm 023
McGill University
Montreal, QC, H3A 2A7
SHIPPING ADDRESS:
Department of Chemistry, McGill University
Rm. 41 801 Sherbrooke St. W
Montreal QC
H3A 0B8
-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#3195): https://urldefense.com/v3/__https://ammrl.groups=
.io/g/main/message/3195__;!!PvDODwlR4mBZyAb0!QUuMbB78EqgDswJ6AFW51HWzNILvfR=
t2dkDLaKbf8CK9Dxe5cmVIYfizSZDnlB70MshWscyymp0zf215Y31y9VrnZ-xs$
Mute This Topic: https://urldefense.com/v3/__https://groups.io/mt/117661513=
/7559972__;!!PvDODwlR4mBZyAb0!QUuMbB78EqgDswJ6AFW51HWzNILvfRt2dkDLaKbf8CK9D=
xe5cmVIYfizSZDnlB70MshWscyymp0zf215Y31y9SYKZSrR$
Group Owner: main+owner_at_ammrl.groups.io
-=-=-=-=-=-=-=-=-=-=-=-
Received on Fri Feb 06 2026 - 09:29:13 MST