Note that superconducting NMR magnets contain liquid helium and liquid nitrogen and are filled with these liquids on a regular basis.
The sudden boil-off of these cryogens (and loss of magnetic field) is called a quench. A quench is an extremely unlikely occurrence under normal circumstances. However, please note the following:
There will be a loud venting of the gases at the top of the magnet and a dense white fog.
This vapour cloud will seek the highest point in the room as it warms and expands over 700 times in volume.
Nothing can stop a magnet quench once it begins.
There is a serious asphyxiation hazard so leave the room immediately, keeping underneath the vapour cloud if you can.
Contact NMR or safety staff urgently (security if out of office hours).
Do not re-enter the room until NMR staff have given permission.
Using liquid Nitrogen for VT NMR experiments
Please refer to the safety rules in the Department of Chemistry if handling liquid N2.