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Magnetics

Low permeability reference materials

Reference materials for accurate measurements

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 EN 60404 Part 15. A number of commercial instruments are 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. Values routinely supplied are:

Nominal value
Shape
Dimensions
1.005
Block
40 mm diameter by 25 mm long
1.024
Block
40 mm diameter by 25 mm long
1.050
Block
40 mm diameter by 25 mm long
1.100
Block
40 mm diameter by 25 mm long
1.27
Bar
300 mm long and 25 mm x 25 mm section
1.63
Bar
300 mm long and 25 mm x 25 mm section

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-demagnetised iron particles which are fixed in a matrix at a spacing such that interactions between them do not occur. As a result of the demagnetising field produced by such a particle geometry, this system behaves in a paramagnetic manner for fields higher than the particle coercivity (HcB). Consequently, the permeability remains relatively constant as the applied field increases above HcB.

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.

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