Hey there,
I’m not sure leak checking is what to look for, though I may have misunderstood the email. Increased boil off rate doesn’t mean helium is “leaking” ….it should mean that too much heat is getting into the helium bath causing accelerated boil off. The helium boil off isn’t the problem, it’s likely the symptom.
In this situation I would be worried something has penetrated the magnet’s insulating vacuum space compromising the shield with a worse vacuum.
Also a “touch” may have developed somewhere inside. This is usually considered some sort of “ice" bridge between layers through the vacuum shield etc. allowing direct thermal conduction.
You know… something along this line of reasoning…whatever the actual source the end result is the same.
Think you may want to check:
-is the magnetic field “drifting” more rapidly
-if N2 boil off has also increased
-if there’s been any recent damage
-is there a manostat for the helium bath and is it working?….etc.
Was there recently any N2 liquid spilled on the upper and/or lower O-ring during a fill?
Considering the age of the system, one of the O-Rings may have lost flexibility and the vacuum space is being penetrated.
Magnets can also build up material slowly in the vacuum space, freezing to the sides until it builds up enough to cause a touch.
On a more “first step” level…..is the liquid N2 actually filled? (i.e. don’t trust the meter) Do you see liquid gush out when filled to the brim?
The fastest way to increase helium boil off is to empty the N2 shield and allow heat directly into the helium dewar…..
I’ve seen a situation when the refilling dewar empties just before the magnet is filled, resulting in slightly lower N2 levels each week until eventually the magnet N2 shield is empty. The empty refilling dewar caused a “whoosh” of gas at the end of each week, confusing staff and misleading them into think it was full.
Sorry if these are dumb suggestions and I’ve totally misunderstood your post.
Best wishes,
Ryan
---------------------------------------------------------------------------------
Ryan T. McKay, Ph.D.
NMR Facility Supervisor
Room E3-17A Email: ryan.mckay_at_ualberta.ca <mailto:ryan_at_nanuc.ca>
Department of Chemistry Phone: 780.492.9950
University of Alberta Cell: 780.920.8871
Edmonton, Alberta
Canada T6G 2G2
Website: uab.ca/nmr
> On Jul 30, 2020, at 4:15 PM, Chuck Smithhart <csmithrt_at_deltastate.edu> wrote:
>
> Hi AMMRL List-
>
> The helium loss rate for our 14 year-old Oxford magnet (7 Tesla, 300 MHz) has increased to about 0.8 % LHe/day from a baseline of about 0.55% LHe/day.
>
> I tried to chase down the leak with a TCD-based thermal conductivity leak detector (Restek Electronic Leak Detector, for GCs), but that instrument does not
> seem to be stable enough when it is moved around the magnet’s fringe field.
>
> I do not have access to a mass spectrometer-type sniffer. Does any AMMRL member know of a reliable, inexpensive way to detect helium leaks?
>
> I was thinking that a sensitive IR camera might work. Or, I suppose that I could resort to soapy water. 😉
>
> I would be interested in anything that works.
>
> Thanks, and sorry if this has been covered before.
>
> Regards,
>
> Chuck
>
> Charles B. Smithhart, Ph.D.
> Delta State University
> Division of Mathematics and Sciences
> P.O. Box C-4
> Cleveland, MS 38733
Received on Fri Jul 31 2020 - 10:27:41 MST