Students sometimes ask whether or not they can collect NMR data during a liquid helium refill. The answer depends on the type of NMR experiment. For most solids experiments, where high resolution ( <~ 100 Hz) is not required, one can often collect data without a problem. In experiments for liquids, where high resolution is almost always required, the quality of data are affected drastically by the disturbance from the liquid helium refill and it is not advisable to collect data during this time. Furthermore, after a liquid helium refill, the magnet must recover in order to obtain the same resolution prior to the fill. The figure below illustrates this point. A 300 MHz NMR spectrometer was set up to collect an NMR spectrum every minute before, during and after a liquid helium refill. One can see that the resolution during the fill is much worse than before the fill. Even 15 minutes after the fill, the magnet has not fully recovered the resolution attained before the fill. The last spectrum of the series was collected after re-shimming the magnet and is comparable to the spectrum collected before the fill.
The resolution profile during a fill will probably be slightly different for every magnet. For this particular magnet, it is advisable not to collect high resolution data for at least 30 minutes after a helium fill.