Hello All,
We have an Oxford 18.8T pumped magnet and it was installed in early 2003. Last weekend, all of a sudden the fridge pressure went up to about 34 mbar (usual operating range is 16-18 mbar on this magnet) and when I tried to adjust the needle valve, it was tight to begin with and it started to move freely both ways without increasing or decreasing the fridge pressure. Since it was a weekend, I turned the second pump as well to bring down the fridge pressure to the operating range and also not to let the fridge flooded with helium. After about 40 hours or so, the fridge pressure was close to zero and I was back on one pump and whatever I did to the first needle valve didn't do anything to the fridge pressure. However, I when I opened the second needle valve, it too was tight initially but I could open it and the fridge pressure went up to bout 3 mbar and after some time when opened a little bit more it went up to 8 mbar for a bout 30 mins. and from there all of a sudden it went up to about 34 mbar. When I tried to close the second valve, it too became ineffective. The knob moves but I couldn't feel the movement of the needle valve inside or see the fridge pressure go down. So, I am back with 2 pumps that is running currently keeping the fridge pressure in the right range. Since it is pulling off more helium gas than is necessary, the top bath is starting to cool down as well. The bottom of the top bath is at 4.00 K instead of usual 4.2 K.
In the past, after a helium fill, the needle valve used be smoother to adjust and since I had to do a helium fill yesterday I tried to adjust both valves and both of them wouldn't respond (the knobs move but there is no response in terms of the fridge pressure). So, it appears that the valves are stuck but probably , fortunately, in the open position.
I am talking to John Davidson of CSI. I am wondering whether any of the 800 MHz Oxford pumped magnet users have had any such experience and how they went about fixing this issue. It may be few weeks before John could get here and is there anything that I could do to keep the magnet operating safely? I would really appreciate your input with this issue. Thank you all for going through this long message. Please reply to me directly and I can post summary later for the benefit of the Oxford pumped magnet users.
Sam Arumugam
NMR Facility Manager
529 S. Jackson St.
JG Brown Cancer Center
University of Louisville
(502)562-6832 (Office)
Received on Thu Jul 26 2018 - 08:48:26 MST