T.O. 33B-1-14-904.8.3.3 ImpedanceEffectsofNonconductiveCoatings.NOTEUS Navy: follow PD-214 instructions for nonconductive coating thicknessmeasurement.When an eddy current probe is placed on bare metal, the impedance of the coil is changed by an amount that isdependent on the frequency of the oscillating current, the conductivity, magnetic permeability, and geometry of the testpart, and the geometry and construction of the test coil. When impedance measuring eddy current instruments areused, the measurement of nonconductive coating thickness is determined from variation in current or voltage across thecoil as the coil impedance changes due to increase or decrease in lift-off.4.8.3.4 InfluenceOfMaterialPropertiesandFrequency.Increases in the conductivity or magnetic permeability of the base metal or in the operating frequency improve thesensitivity of the thickness measurement of nonconductive coatings.4.8.3.5 TestSystemsforNonconductiveCoatingMeasurement.Nonconductive coating thickness can be measured with almost any eddy current inspection system. Sensitivity islimited by the frequency obtainable with available test instruments. Accuracy and range of measurement are increasedwith increasing frequency. The size and construction of available probes, and instrument circuit design affect theaccuracy of measurement. Accuracy decreases with increases in coating thickness. Sometimes probes are spring-loaded to prevent variations in readings caused by inconsistent pressures.4.8.3.6 ProceduresforMeasuringNonconductiveCoatings.The following steps should be followed in performing thickness measurements of nonconductive coatings or the totalthickness of nonconductive materials.a. Establish the range of thickness to be measured and the accuracy required.b. Select test system capable of performing required thickness measurement to specified tolerances.c. Prepare the part or area for thickness measurement.d. Prepare calibration curve or verify calibration curve with existing standards. A calibration curve isrequired for each combination of instrument and probe and for each base metal.e. Perform thickness measurement.4.8.3.7 StandardsforMeasurementofNonconductiveCoatings.Standards for measurement of nonconductive coatings may be obtained from a number of sources. Layers of paper,plastic, and tape are three of the most available standards. Standards should be uniform in thickness and conform tothe surface of the bare metal representing the part to be measured. When standards are stacked layers of material, nogaps or pockets should exist between the layers. Standards can also be actual sections of parts with known thicknessesof the nonconductive coating applied. These standards usually require more effort and expense to prepare. Whenpossible, standards should be measured to an accuracy 10 times greater than the accuracy required for the measurementof the nonconductive coating. This may not always be possible under field conditions.
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