Maintenance 3
400/54 Premium Shielded NMR Magnet System Product Overview 41
the original helium hold- time, and keeps the helium can
clean and free of ice. It is important that the helium can
remains leak tight at all times, so the compression seals
must be fitted correctly as shown.
Figure 5 Compression seal arrangement
1 Determine the amount of liquid helium required to
top- up the helium can. The volume is determined from
the helium level chart provided. Arrange to have enough
liquid helium in a NONMAGNETIC storage Dewar
available to fill the helium can entirely, plus transfer
losses (normally transfers are 75% to 85% efficient). The
liquid level in the helium transfer Dewar (NOT the
magnet) is determined with a “dipstick” or
“thumper- tube”. This part is only included with selected
products, however they are available for purchase from
Agilent Technologies.
Make sure all the nitrogen ports are well sealed before starting to
helium fill. The liquid nitrogen can become supercooled due to the
thermal linking within the cryostat. This results in zero boil off,
which can allow air to cryopump into the nitrogen can, forming
air-ice and blocking the exhaust ports, and creating a serious
safety hazard.
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