Magnet Safety
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4.4.1 Condensing Oxygen
Minimize contact with air. Be aware of the following facts and precautions, contact with air
occurs:
• Since liquid nitrogen is colder than liquid oxygen, the oxygen in the air will condense out.
• If this happens for a period of time, the oxygen concentration in the liquid nitrogen may
become so high that it becomes as dangerous as handling liquid oxygen. This applies
particularly to wide necked dewar due to the large surface area.
• Ensure that contact with air is kept to a minimum.
4.4.2 Nitrogen Flow System
A pressure relief valve is provided for the nitrogen vessel to ensure that at least the rear neck
tube cannot be blocked by the ingress of air or moisture.
This valve should be mounted at all times even when the vessel is being refilled!
4.4.3 Other General Rules
Other general rules include, but are not limited to:
• Do not allow liquid nitrogen to spill onto the room temperature bore closure flanges when
the refilling the nitrogen vessel.
• Place gum rubber tubes or Teflon tubes on the nitrogen neck tubes during refill!
• Stop the transfer immediately when the vessel is full. Failure to observe this can lead to
the freezing of the o-rings and a subsequent vacuum loss of the magnet cryostat.
4.5 Liquid Helium Refill
Please read this section carefully and make it accessible to anybody working with the magnet
system.
A shielded superconducting NMR Magnet System can be operated easily and safely provided
the correct procedures are obeyed and certain precautions observed.
The recommendations in this section cannot cover every eventuality and if any doubt arises
during the operation of the system, the user is strongly advised to contact the supplier.
Be aware of the following general rules including, but not limited to:
• Liquid helium is the coldest of all cryogenic liquids.
• Liquid helium will condense and solidify any other gas (air) coming into contact with it.
• Liquid helium must be kept in specially designed storage or transport dewar.
• Dewar should have a one way valve fitted on the helium neck at all times, in order to avoid
air entering the neck and plugging it with ice.
• Only vacuum insulated pipes should be used for liquid helium transfer. Breakdown of the
insulation may give rise to the condensation of oxygen.
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