Maintenance 3
400/54 Premium Shielded NMR Magnet System Product Overview 39
Make sure that the liquid nitrogen reservoir is refilled
regularly and liquid nitrogen remains in the reservoir at all
times. Refer to the Nitrogen Probe Calibration Chart for
information on when to refill the nitrogen can, typical refill
volumes, and minimum nitrogen hold times. See “Nitrogen
Probe Calibration Chart — 400/54 Premium Shielded ” on
page 56.
To refill the reservoir, the nitrogen fill adaptor and the
nitrogen transfer line (2.5m provided as standard) are
required, as well as a nonmagnetic nitrogen storage Dewar of
suitable capacity. If the nitrogen Dewar is not of the
self- pressurizing type, then a regulated nonmagnetic cylinder
of helium or nitrogen gas is also needed.
To improve transfer efficiency, fit insulation to the transfer
line. Simple, cheap, easily available, and replaceable
expanded foam pipe lagging works well. Trials showed that
Armaflex Tuffcoat
®
insulation (20mm internal diameter, and
13mm insulation thickness), although slightly more
expensive, was the most effective at improving nitrogen
transfer efficiency. Use a suitable waterproof tape to
preserve the integrity of the insulation, and to prevent
air/water ingress into the lagging at the joints and the ends.
All insulation freezes during the nitrogen transfer, and if
moved when cold, it cracks. Repeated freeze/thawing also
degrades the insulation. Allow the insulation to thaw before
removing, and replace as necessary.
The top- up procedure is simple.
1 Provide two free turrets for the fill by removing the heat
sinks and/or the nitrogen safety valve (small
silver- colored valve) from the turrets.
2 Connect the liquid outlet of the storage Dewar to one of
these ports via the transfer line with the nitrogen fill
adapter connected.
3 Connect a length of reinforced plastic tube (or similar) to
the other port, which will be used to exhaust. Position
the top of the tube to direct the exhausting nitrogen
away from the cryostat, personnel and sensitive
equipment.
4 Liquid nitrogen is transferred using a backpressure in
the storage Dewar of about 6 psig. Continue the transfer
until liquid starts to overflow from the free port of the
system nitrogen reservoir; this transfer takes about 30
minutes on most systems.
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