measurements. It is recommended that numerous measurements be taken on
rough samples to average out the effects of the surface roughness. Any foreign
particles on the surface will also affect the accuracy of the measurements, since
the contact between the probe and the surface will be obstructed.
Sample Shape
The size and shape of the part is also an important consideration when using the
lift-off method.
Edge Effects
Since the magnetic field may extend past the edges of the sample, the magnetic
induction technique is sensitive to abrupt changes in the surface contour of the
sample area. Measurements should not be made near an edge unless that is the
specific area on the sample to be measured.
Curvature
Sample curvature has a large affect on the measurements. It is recommended
that the base sample used to calibrate the magnetic probe have the same radius
as the samples to be measured. If there is more than one type of sample
curvature, a separate calibration will be necessary on each radius of curvature
being measured.
Probe Orientation
The correct orientation of the probe is perpendicular to surface or tangent to the
curved surface. It is necessary to have proper probe orientation and consistent
probe pressure to ensure accurate and repeatable results. There are several
probe guides and stands available to help with the proper orientation and
pressure of the probe.
Magnetized Steel
The magnetic induction method should not be used to measure coating thickness
on highly magnetized steel. Measurements on slightly magnetized steel are
correct if the user calibrates with a bare sample magnetized to the same extent
as the samples to be measured.
Stainless Steel Base
If a stainless steel substrate is magnetic (400 Series), then the magnetic
induction method can be used in measurements. If the stainless steel in non-
magnetic (300 Series), no measurements can be made.
Oxford Instruments Contact Measurement Page 24 of 24
233 USER GUIDE
The information in this document is private and confidential and must not be copied.
Oxford Instruments plc
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