What is small, contains a coil and a drop of rubber, and is capable of measuring very strong magnetic fields with incredible accuracy? An NMR probe, of course! Certain technicians and research scientists are very familiar with these 20 cm long copper tubes, indispensable when a magnetic field value has to be known with great precision. One of the uses is  the production and installation of MRI scanners (see our article on Siemens Magnet Technology). With a relative error of 0.1 ppm, NMR technology is by far more accurate than any other. It can be used for total field measurements for field strengths between 0.05 and 20 T, as long as the field is strictly homogeneous and varies slowly or not at all.

The measurement principle is well known to physicists: nuclei of atoms whose spins are aligned by a magnetic field are stimulated by an electromagnetic wave at their resonant frequency, which is directly proportional to the strength of the magnetic field (read more). The ingenuity lies in building an industrial-quality instrument that does all this simply, reliably and reproducibly. A technological tour de force that Metrolab has made one of its specialities: "For 20 years, we have been able to propose a reliable and industrialised solution – that’s made our success", says Metrolab's Pascal Sommer.

After a first series of probes known as 1040 and 1050 (no longer available), Metrolab developed a miniature probe called 1060. 8 probes cover the range of 0.043 to 13.7 T. The first five probes, from 0.043 to 2.1 T, are fitted with rubber samples, a rich source of hydrogen nuclei. The remaining three probes, designed to measure intense magnetic fields of 2.1 to 13.7 T, are based on heavy water; this contains deuterium, an isotope of hydrogen that has a much lower resonant frequency. In all the probes, the sample is surrounded by an RF coil that ensures the electromagnetic excitation of the nucleus, as well as a modulation coil to induce a slight variation of the magnetic field. A clever design allows the same probe head to be used for axial as well as transverse fields without any change or commutation. This series of probes is connected to the instrument itself via an amplifier (Type 1030), needed to amplify the RF wave going to the probe and the very weak NMR signal on its way back to the main unit.

In a next generation, the model 1062 probes, the external amplifier was integrated into the probes, providing an improved signal-to-noise ratio and less sensitivity to external perturbations – with no change in size! "The 1062 probes are highly recommended for measurements requiring a very long cable, up to 100 m, between the probe and the teslameter", adds Pascal Sommer.

Versions of the 1060 and 1062 series with miniaturised heads (external diameter of 10 mm) resulted in the 1080 and 1082 subminiature probes. These probes also cover field strengths between 0.043 and 13.7 T; however, they are only sensitive to transverse magnetic fields.