Procedure for Filling Cryomagnet with Liquid Nitrogen for the Bruker NMR
(2008 Version)
Barbara White
Equipment:
Liquid nitrogen tank (low pressure tank only—gauge will read 22 psi, NOT 230 psi)
Silicone hose and fittings for the liquid nitrogen tank’s liquid (inlet tube is 6’ long, exhaust tube very short---replacement tubing available from the Chemistry stockroom)
Cryogenic gloves
Safety goggles or face shield (preferable)
IMPORTANT SAFETY NOTES:
· Remove all metal objects, credit cards, etc., before entering the NMR room.
· You must use safety goggles/face shield and cryogenic gloves at all times when performing this procedure.
· You must never leave the magnet unattended while the filling procedure is being performed.
· Only those trained by Chemistry Department personnel are allowed to perform the nitrogen filling procedure.
1. Move the liquid nitrogen tank within about 3-4 feet of the magnet.
2. Make sure the liquid nitrogen tank is not empty before beginning the procedure. A fresh tank is located in the corridor of the first floor. The empty tank should be placed where the fresh one originally was.
3. The six foot long silicone tubing (“filling tube”) for the liquid nitrogen tank should be connected to the liquid valve of the full tank. Make sure the proper Swagelok fittings are used for the liquid valve. Do not connect to the valves labeled “vent”, “gas”, or “pressure building”.
4. Remove the large cap on the right side of the cryomagnet.
5. Attach the silicone tubing that is connected to the liquid nitrogen tank to the inlet on the right side of the cryomagnet.
6. On the cryomagnet, remove the large cap on the left side.
7. Attach the short rubber tubing to the outlet (and make sure that it is facing the wall instead of you or another person). This is called the “exhaust tubing”. Ensure that this is the short tubing---using the longer tubing for the exhaust will result in a magnet quench.
8. Open the liquid valve on the liquid nitrogen tank slightly.
9. Loop inlet tube so that there’s a loop in the upward direction, roughly in the Ώ shape; hold the tubing against the top part of the magnet until the tubing freezes. Once is it frozen, it is not necessary to keep holding it.
10. After the tubing is frozen, it is safe to turn up the liquid nitrogen pressure a small amount. When the tube begins to shake, do not increase the pressure any further.
11. The cryomagnet is filled when liquid nitrogen falls from the exhaust tubing attached to the left hand side outlet. The liquid should pour from the exhaust tubing, not simply drip. Prior to this, vapor will be discharged through the rubber tubing.
12. Once filling is complete, shut off the liquid valve on the liquid nitrogen tank.
13. Remove tubing when thawed. Snap rings can be used to make this easier.
14. Replace the cap on the left side.
15. Repeat Steps 13 and 14 for the right side.
16. Return all items to their original places, but leave the silicone tubing attached to the liquid nitrogen tank unless it is empty.
17. Check the helium boil-off rate each time to ensure that it is similar to those of previous fills. If this number changes drastically, then there is a problem which requires immediate attention from the Chemistry Department.
18. Initial the log after performing the nitrogen filling.
Additional notes:
· Occasionally vacuum grease should be added to outlet and inlet fittings.
· Do not hit or jar the magnet.
· No one should be conducting an analysis on the NMR during a nitrogen filling.
· Helium filling should be scheduled with the Chemistry Department every eight weeks (do not go by the gauge, follow the calendar!)
· Limit access to NMR room to trained personnel only. If Facilities personnel must enter, they should be escorted and apprised of the above safety precautions.
· Sometimes the silicon tubing can split and the liquid nitrogen will pour onto the floor. If this should occur, immediately shut off the liquid valve on the liquid nitrogen tank.