MIL-STD-1537C
3.7.2 Metric Units.
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B = 656.5
fµa
or 656.5(fµa )
where B = standard depth of penetration (millimeters)
f = operating frequency (Hertz)
µ = relative permeability (1.0 for nonmagnetic materials)
a = conductivity in %IACS
3.8 Lift-off effect.
The change in eddy current meter indication when the spacing between the surface probe and the test object changes from an "in contact" position to a position away from the surface of the test object. The resulting gap between the probe and the object surface is filled with nonconductive air, coatings, or shims.
3.9 Direct reading eddy current instrument.
An analog or digital electronic instrument and probe which displays the conductivity of a nonferrous metal under test in percent IACS.
3.10 Indirect reading eddy current instrument.
An electronic instrument and probe with variable frequency and gain characteristics requiring graphic interpolation of pointer and scale indication for any probe frequency and gain combination to obtain values of conductivity of a nonferrous metal under test in percent IACS.
4 GENERAL REQUIREMENTS
4.1 Equipment.
4.1.1 Instruments.
Electrical conductivity of aluminum alloys shall be determined using either a direct or an indirect reading eddy current instrument. Whenever possible, direct reading instruments shall be used for conductivity measurements. Where indirect reading eddy current instruments are required
for conductivity measurements, calibration curves are necessary to correlate instrument readings with percent IACS values, since indirect reading instruments typically have non-linear characteristics and read in microamperes (µA) or are read by relative signal position on an instrument display. Indirect reading eddy current instruments require at least three certified standards to verify the calibration curve being used. These standards must be within +/-10 percent IACS of the value being measured, with at least one above and one below the value to be measured.
4.1.2 Instrument sensitivity.
Direct and indirect reading instruments shall have sensitivity such that changes of at least 0.5
%IACS are clearly distinguishable over the conductivity range of the aluminum alloys under test.
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