the calibration trimmer with a suitable screw driver. The in-
strument is factory calibrated. A change of calibration is not
recommended. Further details concerning the calibration of the
instrument are described in the next section.
4 Measurement of Permeability
In order to measure the magnetic permeability of a workpiece or
sample of material the permeability probe is placed perpendic-
ularly on the surface of the sample. The tip of the probe should
slightly touch the sample but any mechanical load of the tip
should be avoided. The measured permeability value is shown
on the LC display.
The FERROMASTER is calibrated to precise permeability stan-
dards which have been manufactured and calibrated by the Na-
tional Physical Laboratory (NPL, Teddington, UK). For easy
check of the calibration a calibration standard is supplied with
each instrument. In order to check the calibration the perme-
ability probe is placed perpendicularly on the top of the stan-
dard so that the tip touches its center. The measured value
should correspond with the number printed on the standard.
The calibration can be readjusted by turning the trimmer CAL
on top of the instrument with a screw driver. The calibration
standard should be protected against mechanical load and ex-
posure to strong magnetic fields.
The magnetic permeability and the result of a permeability mea-
surement depend on a number of factors which must be taken
into account if a precise measurement is to be made. In partic-
ular, the following points should be considered:
4.1 Dependence on the magnetic field strength
The magnetic permeability of a material depends on the strength
and frequency of magnetizing field. The permeability probe of
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