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 lift-off method (used to measure nonconductive coatings over
conductive bases) 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
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 field may extend past the edges of the sample, the eddy current
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 to be measured.
Curvature
Sample curvature has a large affect on the measurements. It is recommended
that the base sample used to calibrate the eddy current 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 the 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.
Chrome over Copper
The Cr/Cu application can be done using the nonconductive/conductive method,
but the calibration must be done on a Cr/Cu plated standard. There are two
reasons for this. First Cr is slightly conductive, therefore, the application will not
work as a straightforward nonconductive over conductive calibration. Secondly,
Cr plating generally has many cracks in the surface that would cause inaccurate
readings. The Cu base must be at least 0.030” thick. When calibrating with the
0.5 mil standard the recommended tracking range is 0 to 1.0 mil, but the
Oxford Instruments Contact Measurement Page 22 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...