Low permeability reference standards with improved high magnetic field strength performance

Commercial instruments for determining the relative magnetic permeability of weakly magnetic materials require calibration. This is achieved using standards of known permeability. These materials suffer to some extent from the fact that their relative magnetic permeability varies with the applied magnetic field strength. When used to calibrate instruments, this can introduce significant errors. The authors report on the development of a material which improves this situation. By uniformly dispersing iron particles in an acrylic matrix their self demagnetising properties are substantially retained and, consequently, the permeability changes due to applied field are significantly smaller than for conventional materials. Furthermore, because the amount of iron added can be varied, the permeability can be tailored to the required value. This allows commercial instruments to be calibrated at various points of their ranges or at a particular value of interest. Using this technique it was possible to make reference materials with permeabilities in the range 1.005 to 1.100