MIL-STD-1537C
3.4 Eddy current instrument.
An electrical measuring instrument that induces and responds to eddy currents in metals through a probe. These currents are induced by means of time-varying electromagnetic fields. Note: also called an eddy current meter.
3.5 Probe.
Many turns of fine wire usually wound on a ferrite or laminated iron core in such a way as to produce a varying electromagnetic field when energized by an alternating current, usually 60
KHz or above for conductivity measurements.
3.6 Nonferrous conductivity standards.
Either primary (reference), secondary (laboratory), or instrument conductivity standards which have the following characteristics:
a. Primary or Reference Nonferrous Conductivity Standards:
Uniformly dimensioned rectangular or round bars of homogeneous nonferrous metal finished to a surface finish which will permit dimensional and resistance measurements such that they can be certified for conductivity values within ±0.2 %IACS or ±0.5 percent of stated value, whichever is less at 20°C (68°F). These standards must be National Institute of Standards and
Technology (NIST) traceable.
b. Secondary or Laboratory Nonferrous Conductivity Standards:
Blocks or coupons of nonferrous homogeneous metal of adequate thickness to prevent noticeable effects on the meter reading. The surface finish must be of sufficient quality so as to eliminate lift-off errors and of such an area as to preclude edge effect calibrated on alternating current by comparison with primary conductivity standards and certified within ±0.35 %IACS or
1 percent of stated value, whichever is less at 20°C (68°F).
c. Instrument Nonferrous Conductivity Standards:
Similar or identical to the secondary or laboratory standards in 3.6b above except they are fastened securely to an eddy current instrument and certified within ±0.85 %IACS at 20°C (68°F) when calibrated against secondary or laboratory standards using the eddy current instrument as tile transfer means.
3.7 Standard depth of penetration
3.7.1 English Units.
1
B = 26
fµa
or 26(fµa )
where B = standard depth of penetration (inches)
f = operating frequency (Hertz)
µ = relative permeability (1.0 for nonmagnetic materials)
a = conductivity in %IACS
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