T.O. 33B-1-14-86sheet, 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 beutilized and ultrasonic equipment is not available or applicable or if very thin materials are to be measured.4.8.2.2 TotalThicknessLimitations.The accuracy and range of metal thickness measured with eddy currents are dependent upon the electromagneticproperties of the material and the test system. Increasing conductivity and magnetic permeability increase accuracy inmeasuring 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 magneticpermeability can be measured than can be measured in materials of high conductivity and/or high permeability.4.8.2.3 FrequencyEffectsInTotalThicknessMeasurement.Just as decreasing frequency increases the depth of penetration of eddy currents in a conductor, decreasing frequencyalso increases the thickness of a metal that can be measured by eddy current inspection techniques. Higher sensitivityis obtained for the thinnest specimens with the higher frequency. For greater thicknesses (over 0.050 inch), the lowerfrequency provides greater sensitivity and greater overall penetration. Sensitivity in any thickness range can bedetermined by the plot. The greater the slope of the plotted line, the greater the sensitivity. Optimum frequency can beestimated by calculating one standard depth of penetration.4.8.2.4 EffectsOfProbeConstruction.Probes designed specifically for thickness measurement have air cores and are generally larger in diameter than theferrite 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 insmaller areas and closer to edges. The larger air core probes can provide greater sensitivity for thickness measurementsthan the ferrite core pencil probes.4.8.2.5 OperatingProceduresforTotalThicknessMeasurement.All thickness measuring should be performed in accordance with pre-established procedures. In general, theseprocedures 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 pointsf. Record thickness and report all rejectable values as required by the written procedure.4.8.2.6 PreparePartforThicknessMeasurement.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 thesurface where thickness is being determined. Any sharp edges, protrusions, or chemical, which is potentially damagingto the probe, should be removed.4.8.2.7 PresenceofGeometricalLimitations.Prior to measuring thickness by eddy current techniques, the presence and position of any structural features that couldrestrict accessibility or reduce accuracy of measurement must be established. Thickness measurement must beperformed 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 definegeometrical limitations.
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