Bruker AC-250 FT-NMR Liquid Nitrogen Fill Procedure
by F.J. Matthews - last updated 12/18/98
Weekly liquid nitrogen fill consists of
(a) liquid nitrogen fill
(b) % liquid helium check
(c) air filter check.
Liquid nitrogen fill includes the following
(a) marking portable liquid nitrogen dewar (PLND) as to begin and end levels
(b) determining empty and filled levels of magnet liquid nitrogen dewar (MLND)
(c) transferring liquid nitrogen from PLND to MLND
To perform all of these tasks most efficiently the following procedure is provided.
1. roll PLND to appropriate point near magnet
2. turn magnet feet-air dampers to OFF position using toggle switch on floor at rear of magnet
3. mark beginning level on PLND using top of plunger as reference
4. approximate current level of liquid nitrogen in MLND using logbook and adjust alligator clips on thumping rod using meter stick for measurements (one clip is placed at approximated level and the second clip placed 10 cm further up thumping rod as a "stop" clip) - check to ensure that magnet feet-air dampers are deflated and magent is resting solidly on floor
5. remove cap from nitrogen fill port of MLND, insert thumping rod quickly to first alligator clip, and watch for liquid nitrogen squirting from upper end of thumping rod
a. if no liquid nitrogen squirts from upper end of thumping rod, slowly lower rod into MLND until nitrogen level determined (liquid nitrogen squirting from upper end of thumping rod), remove thumping rod, replace cap onto nitrogen fill port
b. if liquid nitrogen squirts from upper end of thumping rod at first alligator clip, remove rod, replace cap onto nitrogen fill port of MLND, move alligator clips down 10 cm, allow rod to warm, wipe off end, and repeat step 5
6. measure current level of liquid nitrogen in MLND from thumping rod using meter stick and enter value in logbook
7. attach tubing from PLND (liquid port) to MLND fill port, large insulation tubing should be on horizontal portion of latex tubing, smaller section of insulated tube should be on latex tubing at MLND fill port to help support tubing
8. using gloves, turn knob of "liquid port" on PLND 1/4 turn (note time in minutes-seconds) and support horizontal insulation section while liquid nitrogen fill begins and latex tubing freezes - also at this time, ensure that nitrogen exhaust port tubings on MLND are not touching the MLND and are pointed away from you and your path of travel
9. increase rate of liquid nitrogen flow 1/4 turn every 4 to 5 minutes (enter times in logbook) to a maximum of 1 full turn of knob on liquid port of PLND
10. while nitrogen fill is progressing
a. check % liquid helium level on NMR console by pressing HE LEVEL button once and record value in logbook
b. check helium boil-off level at flow meter mounted on inside of left helium stack on top of MLND
c. check air filter by turning single-pole switch located on right side of rightmost timer box to ON position, then opening leftmost timer box and turning grey/orange turn-style switch conterclockwise one click (liquid/air should begin to be exhuasted from copper tubing into large plastic beaker) - when only air exhausts from copper tubing, turn both switches off (grey/orange turn-style switch counterclockwise one click first, then single-pole switch to OFF position), measure liquid level of plastic beaker, enter value in logbook, empty beaker and replace for daily automatic draining of air filter
11. when liquid nitrogen drains from nitrogen exhaust port tubings on MLND (note time in minutes-seconds) turn knob on "liquid port" of PLND back to only 1/4 open turn, allow MLND to complete fill (approximately 1 minute required) - when liquid nitrogen drains from nitrogen exhaust port tubings on MLND (again note time in minutes-seconds) turn knob on "liquid port" of PLND to OFF position - record all times (at each 1/4 turn setting of knob on "liquid port" of PLND) into logbook
12. when knob on liquid port of PLND is turned OFF, remove smaller section of insulated tube from latex tubing at MLND fill port (place smaller section of insulated tube on top of PLND on grey protrusion/exhaust port) and allow latex tubing to thaw - blow frozen condensate from inlet and exhaust latex tubings to minimize water drip down outside of magnet
13. adjust alligator clips on thumping rod using meter stick to 33 and 35 cm depth
14. when latex tubing on MLND fill port has thawed, remove and determine fill level of MLND by inserting thumping rod quickly to first alligator clip and watching for liquid nitrogen squirting from upper end of thumping rod as it is slowly depressed into fill port of MLND - remove thumping rod, replace cap on fill port of MLND, and turn magnet feet-air dampers to ON position using toggle switch on floor at rear of magnet - measure filled level of MLND on thumping rod with a meter stick and record in logbook
15. after magnet feet-air dampers inflate, move PLND to storage area and mark ending level on PLND using top of plunger as reference
16. check all three areas of NMR computer to ensure all modes are set to PO (power off)
17. dry all tubing on MLND with paper towel