Precision measurement of the magnetic field in terms of the free-proton NMR frequency

We have developed a NMR standard probe using a spherical pure water sample of 1 cm diameter to determine the absolute magnetic field B in terms of the free-proton NMR frequency fp with an accuracy of 3.4 × 10−8. Our standard probe can be used conveniently to calibrate other probes in the field range from 1.45 to 1.7 T and can readily be employed over a much wider field range. The probe design and the tests carried out to verify its precision and accuracy are presented.