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
