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sheet, to determine wear or thinning of sheet materials and to measure thickness, erosion, or corrosion of tubing walls.
Thickness measurement with eddy currents is generally used when mechanical methods of measurement cannot be
utilized and ultrasonic equipment is not available or applicable or if very thin materials are to be measured.
4.8.2.2
Total Thickness Limitations.
The accuracy and range of metal thickness measured with eddy currents are dependent upon the electromagnetic
properties of the material and the test system. Increasing conductivity and magnetic permeability increase accuracy in
measuring very thin specimens, but decrease the effective range of measurement and the accuracy at greater depths.
Therefore, at a specified frequency, greater thicknesses of metals with low conductivity and/or low magnetic
permeability can be measured than can be measured in materials of high conductivity and/or high permeability.
4.8.2.3
Frequency Effects In Total Thickness Measurement.
Just as decreasing frequency increases the depth of penetration of eddy currents in a conductor, decreasing frequency
also increases the thickness of a metal that can be measured by eddy current inspection techniques. Higher sensitivity
is obtained for the thinnest specimens with the higher frequency. For greater thicknesses (over 0.050 inch), the lower
frequency provides greater sensitivity and greater overall penetration. Sensitivity in any thickness range can be
determined by the plot. The greater the slope of the plotted line, the greater the sensitivity. Optimum frequency can be
estimated by calculating one standard depth of penetration.
4.8.2.4
Effects Of Probe Construction.
Probes designed specifically for thickness measurement have air cores and are generally larger in diameter than the
ferrite core probes used for flaw detection. Larger diameter probes average thickness measurements over a larger area.
Smaller diameter probes and probes with ferrite cores reduce the area of measurement and therefore can be used in
smaller areas and closer to edges. The larger air core probes can provide greater sensitivity for thickness measurements
than the ferrite core pencil probes.
4.8.2.5
Operating Procedures for Total Thickness Measurement.
All thickness measuring should be performed in accordance with pre-established procedures. In general, these
procedures will include the following steps:
a. Prepare part for thickness measurement.
b. Establish the presence of geometrical factors, which will limit or restrict thickness measurement.
c. Select appropriate test system, probe, and operating frequency.
d. Develop or verify a calibration curve and calibrate the test system using the specified standards.
e. Perform thickness measurements at designated points
f. Record thickness and report all rejectable values as required by the written procedure.
4.8.2.6
Prepare Part for Thickness Measurement.
Many thickness measurements must be performed through nonconductive coatings such as paint or anodic coatings.
Lift-off compensation must be used during the calibration. Any loose foreign material should be removed from the
surface where thickness is being determined. Any sharp edges, protrusions, or chemical, which is potentially damaging
to the probe, should be removed.
4.8.2.7
Presence of Geometrical Limitations.
Prior to measuring thickness by eddy current techniques, the presence and position of any structural features that could
restrict accessibility or reduce accuracy of measurement must be established. Thickness measurement must be
performed sufficiently far away from fastener and other conductive objects to prevent its influencing the meter reading.
Limited access may restrict the type of probe to be used. In most cases, written inspection procedures will define
geometrical limitations.
