T.O. 33B-1-14-874.8.2.8 SelectionofTestSystem.The test system selected for thickness measuring must be based on thickness measuring requirements, frequency of theeddy current instrument, and the types of probes available.4.8.2.9 SelectionofTestFrequencyforThicknessMeasurement.For each thickness measurement task to be performed by eddy current techniques there is an optimum frequency orrange of frequencies that will provide optimum sensitivity at the depth to be measured. The product of the materialconductivity in percent IACS and the relative magnetic permeability is plotted along the vertical axis, and frequency inkilohertz is plotted along the horizontal axis. Lines representing optimum thicknesses are plotted on the graph. Todetermine the recommended frequency, the product of material conductivity and relative permeability of the material tobe measured is found on the vertical axis. Follow this point horizontally to the diagonal line representing the thicknessto be measured. The recommended frequency is found on the horizontal axis by extending a line vertically downwardfrom the established point. Considerable variation from this frequency value will still provide sufficient sensitivity formost applications. When in doubt, the adequacy of a frequency may be determined by establishing a trial calibrationcurve.4.8.2.10 CalibrateInstrument.Because the general-purpose instruments are not specifically designed for thickness measuring, correlation must beestablished between instrument readings and thickness dimensions. Therefore, the thickness range over whichmeasurements are to be performed should be defined as closely as possible to minimize the number of data points to beestablished. Where applicable, lift-off compensation should be employed to minimize the effects of variations insurface finish on thickness readings.4.8.2.11 RecordThicknessandReportRejectableValues.Most written procedures provide acceptance limits for the thickness dimension. When a rejectable value is obtained, itis advisable to recheck the calibration of the instrument against the standards. The written procedure usually providesmethods for reporting rejectable values.4.8.2.12 StandardsforTotalThicknessMeasurement.The standards used for calibration in total thickness measurement must have the same electrical conductivity, magneticpermeability, and geometry as the material being measured. The same electrical conductivity is usually obtained byrequiring the standards to be fabricated from the same alloy and temper as the inspection material. In magneticmaterials, permeability can vary to such a degree within a single alloy and temper that selection of representativestandards can be difficult. The high permeability of iron and ferromagnetic steel restricts the use of eddy currentthickness measurement to very thin metals. The curvature of the standards should be the same as the part beinginspected. All standards should be uniform in thickness and the accuracy of the standard thickness should be at least10 times that required for the accuracy of the thickness measurement. For example if thickness measurement isrequired to the nearest 0.001 inch, the standards should be accurate to the nearest 0.0001 inch. All standards should beclearly identified with alloy, temper and thickness.4.8.2.13 AccuracyofThicknessMeasurement.The accuracy obtained in total metal thickness measurement varies widely depending on material properties, thickness,frequencies employed, and system noise level. With higher frequencies (500 KHz and up) and thin materials (0-010inch and less), thicknesses may be measured to the nearest 0.0001 inch. As frequencies are lowered and thicknessesincrease, accuracy decreases. For maximum accuracy, variations in lift-off, conductivity, geometry and magneticpermeability must be reduced to the lowest possible level.4.8.2.14 ApplicationofConductiveCoatingMeasurement.Eddy current inspection techniques are commonly used to measure the thickness of conductive platings on metallicmaterials. These measurements may be used as a process control to determine that the proper thickness of platings orconductive coatings has been applied to a substrate. The thinning of such platings and coatings, because of erosion orcorrosion, can also be established. Eddy current methods have also been employed to determine the presence andthickness of surface layers which have been altered in composition from the metal deeper within the part. Thisapplication includes the measurement of carburized cases in steel and the depth of oxygen or hydrogen contamination