T.O. 33B-1-1
4-87
4.8.2.8
Selection of Test System.
The test system selected for thickness measuring must be based on thickness measuring requirements, frequency of the
eddy current instrument, and the types of probes available.
4.8.2.9
Selection of Test Frequency for Thickness Measurement.
For each thickness measurement task to be performed by eddy current techniques there is an optimum frequency or
range of frequencies that will provide optimum sensitivity at the depth to be measured. The product of the material
conductivity in percent IACS and the relative magnetic permeability is plotted along the vertical axis, and frequency in
kilohertz is plotted along the horizontal axis. Lines representing optimum thicknesses are plotted on the graph. To
determine the recommended frequency, the product of material conductivity and relative permeability of the material to
be measured is found on the vertical axis. Follow this point horizontally to the diagonal line representing the thickness
to be measured. The recommended frequency is found on the horizontal axis by extending a line vertically downward
from the established point. Considerable variation from this frequency value will still provide sufficient sensitivity for
most applications. When in doubt, the adequacy of a frequency may be determined by establishing a trial calibration
curve.
4.8.2.10
Calibrate Instrument.
Because the general-purpose instruments are not specifically designed for thickness measuring, correlation must be
established between instrument readings and thickness dimensions. Therefore, the thickness range over which
measurements are to be performed should be defined as closely as possible to minimize the number of data points to be
established. Where applicable, lift-off compensation should be employed to minimize the effects of variations in
surface finish on thickness readings.
4.8.2.11
Record Thickness and Report Rejectable Values.
Most written procedures provide acceptance limits for the thickness dimension. When a rejectable value is obtained, it
is advisable to recheck the calibration of the instrument against the standards. The written procedure usually provides
methods for reporting rejectable values.
4.8.2.12
Standards for Total Thickness Measurement.
The standards used for calibration in total thickness measurement must have the same electrical conductivity, magnetic
permeability, and geometry as the material being measured. The same electrical conductivity is usually obtained by
requiring the standards to be fabricated from the same alloy and temper as the inspection material. In magnetic
materials, permeability can vary to such a degree within a single alloy and temper that selection of representative
standards can be difficult. The high permeability of iron and ferromagnetic steel restricts the use of eddy current
thickness measurement to very thin metals. The curvature of the standards should be the same as the part being
inspected. All standards should be uniform in thickness and the accuracy of the standard thickness should be at least
10 times that required for the accuracy of the thickness measurement. For example if thickness measurement is
required to the nearest 0.001 inch, the standards should be accurate to the nearest 0.0001 inch. All standards should be
clearly identified with alloy, temper and thickness.
4.8.2.13
Accuracy of Thickness Measurement.
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 (0-010
inch and less), thicknesses may be measured to the nearest 0.0001 inch. As frequencies are lowered and thicknesses
increase, accuracy decreases. For maximum accuracy, variations in lift-off, conductivity, geometry and magnetic
permeability must be reduced to the lowest possible level.
4.8.2.14
Application of Conductive Coating Measurement.
Eddy current inspection techniques are commonly used to measure the thickness of conductive platings on metallic
materials. These measurements may be used as a process control to determine that the proper thickness of platings or
conductive coatings has been applied to a substrate. The thinning of such platings and coatings, because of erosion or
corrosion, can also be established. Eddy current methods have also been employed to determine the presence and
thickness of surface layers which have been altered in composition from the metal deeper within the part. This
application includes the measurement of carburized cases in steel and the depth of oxygen or hydrogen contamination
