application is capable of tracking from 0 to 40 mil if thicker standards are used in
the calibration.
Magnetic Induction
Theory
The magnetic induction method is used for measuring the thickness of any
nonmagnetic coating such as zinc, cadmium, paint, etc. on a steel substrate.
This measurement method is also called the “lift-off” method. Magnetic principles
are also used to measure magnetic nickel on a nonmagnetic substrate. In this
technique, the probe system is essentially a transformer. The primary winding
for the probe is driven with an alternating current. The secondary winding (the
output of the probe) reacts to the presence of a magnetic material. As the probe
is brought closer to the magnetic material, the efficiency steadily increases and
progressively more voltage is output. Thickness readings are based on the
output voltage of the probe. As the coating gets thicker, the distance between
the probe and the magnetic substrate increases, decreasing the output voltage.
Base Variations
Calibrating a magnetic probe consists of placing nonmagnetic shims of known
thicknesses directly over the bare conductive substrate. The magnetic probes
are very susceptible to changes in the base including different grades of steel,
hardening processes, or shapes of the sample. Because of this, calibrations
must be made on a bare sample of the same part to be measured. If the
company has several parts of different types of steel or configurations, they will
need a calibration for each type of part.
Base Metal Thickness
If the base metal thickness is less than infinite, any variations in the thickness of
the base will affect the coating measurements. The magnitude of this effect
decreases with increasing coating thickness. Limit the effect of the base
variations as follows. Take numerous readings on the bare sample during the
calibration to average out the base variations. When taking measurement on the
coated samples, several measurements must be also taken over the entire
surface of the sample to determine the average coating thickness.
Inherent Base Variation
Base variation can also be due to inherent variations in electrical conductivity or
magnetic permeability due to the manufacturing processes. To determine the
effect of these variations on the sample measurements, calibrate the unit and
then take several measurements across the bare sample. If the range of
readings is found to be large, then calibrate in the same method as described in
Base Metal Thickness.
Surface Roughness
Since the magnetic method measures coating thickness as the probe distance
from the surface of the base material, surface roughness will cause variations in
surface to probe contact and affect both the repeatability and the accuracy of
Oxford Instruments Contact Measurement Page 23 of 24
233 USER GUIDE
The information in this document is private and confidential and must not be copied.
Oxford Instruments plc
To Next Page
Loading...