National Physical Laboratory

Magnetic signature measurements using the NPL low magnetic field facility

Magnetic signature measurements

A low magnetic field facility has been established at NPL in which the earth's magnetic field is cancelled to less than a nanotesla for a small volume in the centre of a 3 metre diameter triaxial Helmholtz coil system. Other coil systems placed inside the triaxial coils can be calibrated using a proton resonance magnetometer. Cancellation of ambient magnetic fields results in better repeatability and fewer uncertainty contributions to this type of measurement. Using the cancellation system for these calibrations results in a lower uncertainty. The triaxial Helmholtz coils are also used for the calibration of fluxgate magnetometers and other DF magnetic field meters and devices such as SQUID and Caesium magnetometers at low magnetic field strengths.

In space and defence applications, it is important that the magnetic moment of items are at levels that do not compromise a larger system or safety critical situation. When the items have large dimensions it is not possible to use a detection coil method and measuring the magnetic flux density at a distance is used to determine the magnetic moment. For the method cancellation of ambient magnetic fields is necessary and the triaxial coil system shown in the picture is used. Any measurements needing this low field environment should be discussed with us.

The calibrated three-axis fluxgate magnetometer used to establish the zero of the cancellation system can be used for magnetic signature measurements. The item under test is first removed to a position at which it can no longer be seen by the magnetometer and the zero condition established. The item is then brought up to a known distance for each of its three geometric axis and the magnetic flux density measured for forward and reverse directions. From these the magnetic moment can be calculated. Either of these quantities can be compared to the customer requirements to see if the item passes for magnetic signature. Work was recently completed on the magnetic signature of a battery bank to be used on a satellite involved with terrestrial magnetic field observations.

In addition, a set of high frequency Helmholtz coils have been produced for the calibration of magnetic field meters up to a frequency of 120 kHz. This system is used to calibrate portable magnetic field meters that are used for compliance testing. The magnetic field range has been selected to meet that of the majority of meters available on the market. Limited measurements for non-sinusoidal waveforms are also available.

Measurements on site

Certain measurements, e.g. the exploration of the ambient magnetic fields or calibrations of coil systems, can, in some circumstances, be made on site. Details of this service are available on request.

Contact

Customer Service tel: +44 20 8943 8681
E-mail: materials_enquiries@npl.co.uk

Last Updated: 18 Aug 2015
Created: 13 Aug 2007

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