Table 4-8. Effects of Material and Inspection Variables on the Sensitivity and Range of Thickness Measurements

T.O. 33B-1-1 4-85 4.8.1.2 General Limitations of Plating Thickness Measurement. The use of eddy current techniques for thickness measurement is confined to thin materials. This limitation results from the inability of the eddy current field to penetrate deeply into conductive materials. The effective depth of penetration, and therefore the thickness that can be measured, decreases as the conductivity and/or permeability of the metal increases. To determine the thickness of plating or coatings on metallic substrates, a difference must exist in conductivity or permeability between the surface material and base material. Increased sensitivity is obtained, as the differences between plating and substrate conductivity or permeability become larger. For nonconductive coatings, the sensitivity improves with increasing frequency. Larger probe diameters provide greater sensitivity for measurement of thicker plating. A summary of the effects of an increase in material properties and inspection variables on the sensitivity and range of thickness measurements is presented in Table 4-8. Table 4-8. Effects of Material and Inspection Variables on the Sensitivity and Range of Thickness Measurements. Variable Increased Sensitivity of Measurement Range of Measurement Conductivity Increases for thin metallic parts and plating. Increases throughout effect range for nonconductive coatings. Decreases for metallic materials. Increases for nonconductive coatings. Permeability Increases for thin metallic parts and plating. Decreases for thick metallic parts and plating. Increases throughout for nonconductive coatings. Decreases for metallic materials. Increases for nonconductive coatings. Frequency Increases for thin metallic parts and plating. Decreases for thicker metallic parts and plating. Increases throughout the effective range for nonconductive coatings. Decreases for metallic materials. Increases for nonconductive coatings. Probe Diameter Increases for thicker metallic parts and plating and throughout effective range for nonconductive coatings. Increases for metallic parts, plating, and nonconductive coatings. 4.8.1.3 Test Systems. A wide variety of specialized equipment is manufactured for thickness measurement. Many such instruments are optimized for one or two types of applications. Examples include instruments designed to measure nonconductive coatings on nonmagnetic metals or instruments for measuring nonmagnetic plating on a magnetic substrate. Because of limited requirements, such specialized equipment is usually not available for use in the field. In most cases, general- purpose instruments may be adapted for thickness measurement. Many of the meter type instruments can be used for a wide variety of thickness measurement operations. Impedance plane analysis equipment is very useful for thickness measurement. Phase change is more nearly linear with increasing depth of penetration, thereby providing more consistent sensitivity and accuracy over the entire range of measurement. 4.8.1.4 Thickness Measuring Procedures. Before thickness measurement can be performed, the eddy current measurement procedures should be carefully established and proven to ensure accuracy and reliability. Curves should be prepared to relate instrument readings to known thickness standards. A sufficient number of samples within the thickness range to be measured must be used in preparing the curves to ensure that a smoothly increasing or decreasing curve will be obtained. The type and number of standards necessary for instrument standardization must be defined. The limitations of the procedures in terms of material and dimension applicability must be established and noted in the procedures. Because instrument settings for a specific inspection may vary slightly with test conditions, different probes, and variations between instruments, specific instrument settings are not usually provided for these applications. 4.8.2 Measurement of Total Metal Thickness. 4.8.2.1 Applications of Total Thickness Measurement. The primary use of eddy current techniques for measuring the total thickness of metal parts is to detect corrosion on the far sides or between layers of structure. However, this technique can also be employed to establish the thickness of thin