A low magnetic field facility has been established at NPL in which the Earth's ambient magnetic field is reduced to less than a nanotesla at the centre of a 3m diameter triaxial Helmholtz coil system. Field-generating coil systems can then be located inside the triaxial coils to characterise magnetic sensors. Sensor parameters including gain, offset, linearity, gradient and frequency response can be measured in addition to the sensor’s noise performance. The Power Spectral Density (PSD) of the cancelled environment has demonstrated an existing noise floor of 12 pT/√Hz at 1 Hz, and the system allows for noise measurements to be made down to 0.1 mHz. The facility also has the capability to characterise magnetic sensors at the operational temperatures encountered in a range of industrial applications. A temperature control system has been integrated into the cancellation system that allows calibration of such sensors over the temperature range of – 55°C to +125°C.
Measurements performed in the cancellation system exhibit a better measurement repeatability and lower uncertainty contributions for this type of measurement. The field-generating coil systems are calibrated using a proton resonance magnetometer to obtain the lowest uncertainties with traceability gained via frequency. The lowest uncertainty offered over the range 20 μT to 90 μT is ± ± 0.0030% (k=2, 95%). The majority of magnetic measurements made at NPL can be issued on calibration certificates covered by NPL’s UKAS (ISO 17025) accreditation.