National Physical Laboratory

Low relative magnetic permeability (feebly magnetic) measurements and reference materials: relative magnetic permeability in the range 1.002 to 1.6

Magnetic measurement systems

The determination of the relative magnetic permeability of weakly magnetic materials is an important requirement in the manufacture of devices and components for space, particle physics and defence applications. The relative magnetic permeability of a material can be measured by the solenoid method or by the comparator method in accordance with BS 5884:1999. A number of types of commercial instruments are now available for determining the relative magnetic permeability of such materials.

NPL has developed reference materials, based on iron particles dispersed in acrylic, for the calibration of these commercial instruments. A very important aspect of this technique of producing permeability standards is that the value can be tailored which allows the test instrument to be calibrated at the value of interest. Reference standards of size 40 mm diameter by 25 mm long, in four permeability values are presently available; 1.005, 1.024, 1.050 and 1.100. The range has been extended to include 1.63 by using a commercially available material based on the same technology.

The ideal magnetic behaviour for a permeability standard is paramagnetism. However, this effect is too small to be of practical use. Such behaviour can be approached by using self-demagnetized iron particles which are fixed in a matrix at a spacing such that interactions between them do not occur. As a result of the demagnetizing field produced by such a particle geometry, this system behaves in a paramagnetic manner for fields higher than the particle coercivity. Consequently, the permeability remains relatively constant as the applied field increases above HcB. In the figure, the variation of the permeability with applied magnetic field strength for an iron powder sample with a target µr of 1.05 is compared with that of a brass reference material. The change in permeability is much reduced in the new material.

Improved performance for the Fe powder based material
Figure showing improved performance for the Fe powder based material

The NPL iron powder samples are prepared by blending a predetermined quantity of iron with commercial acrylic powder. The blended powder is then hot pressed into cylindrical blocks which can be assembled into a long bar for determining the limiting value. This approach also allows each of the blocks to be placed in the centre of the bar so that uniformity and repeatability can be carefully checked.

Conventional materials are also available in the form of bars approximately 300 mm long and 25 mm x 25 mm section. Such dimensions avoid significant errors due to demagnetising effects. However, conventional materials have the drawback that the measured permeability depends on the applied magnetic field strength.

The three conventional reference materials available cover permeability values, measured at a magnetic field strength of 10 kA/m, as follows:

  • Stainless steel AISI type 316, µr of approximately 1.0030
  • Naval brass Type CZ114, µr of approximately 1.060
  • Ni /Al / bronze Type DGS 1043, µr of approximately 1.40

Both iron/acrylic and conventional reference materials are available complete with a certificate of calibration accredited by UKAS.

References

A E Drake and C I Ager: Reference materials for calibrating magnetic permeability measuring equipment, IEEE Transactions on Magnetics, 1990, 26, (5), 2050-2051.

M J Hall, A E Drake, S A C Harmon, and C I Ager: Low permeability reference standards with improved high magnetic field strength performance, IEE Proc.-Sci. Meas. Tecnol., Vol. 145, No. 4, PP. 181-183 (1998).

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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