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Eddy Current Inspection Formula
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Ohm's Law
Impedance
Phase Angle

Magnetic
Permeability Where:

 I = Current (amp) V = Voltage (volt) Z = Impedance (ohm) Where:

 Z = Impedance (ohm) R = Resistance (ohm) XL = Inductance Reactance(ohm) Where:

 f = Phase Angle (deg) XL = Inductance Reactance(ohm) R = Resistance (ohm) Where:

 m = Magnetic Permeability (Henries/meter) B = Magnetic Flux Density (Tesla) H = Magnetizing Force (Am/meter)
Relative
Magnetic Permeability
Conductivity & Resistivity
Electrical
Conductivity
(%IACS)
Electrical
Conductivity
(%IACS) Where:

 mr = Relative Magnetic Permeability (dimensionless) m = Any Given Magnetic Permeability (H/m) mo = Magnetic Permeability in Free Space (H/m), which is 1.257 x 10-6 H/m Where:

 s = Electrical Conductivity Siemens/m r = Electrical Resistivity (ohm-m)

When resistivity is known Where:

 s%IACS = Electrical Conductivity (% IACS) r = Electrical Resistivity (mohm-cm) smS/cm = Electrical Conductivity (mSiemens/cm)

When conductivity in
S/m or m
S/cm is known or Where:

 s%IACS = Electrical Conductivity (% IACS) sS/m = Electrical Conductivity (Siemens/meter) smS/cm = Electrical Conductivity (mSiemens/cm)

Current Density
Standard Depth
of Penetration
Standard Depth
of Penetration
Standard Depth
of Penetration Where:

 Jx = Current Density (amps/m2) Jo = Current Density at Surface (amps/m2) e = Base Natural Log = 2.71828 x = Distance Below Surface d = Standard Depth of Penetration

When electrical conductivity (S/m) is known. Where:

 d = Standard Depth of Penetration (m) p = 3.14 f = Test Frequency (Hz) m = Magnetic Permeability (H/m) (1.257 x 10-6 H/m for nonmagnetic mat'ls) s = Electrical Conductivity (Siemens/m)

When electrical conductivity
(%IACS) is known.

 In mm In inches Where:

 d = Standard Depth of Penetration (mm or in) f = Test Frequency (Hz) mr = Relative Magnetic Permeability (dimensionless) s = Electrical Conductivity (%IACS)

When electrical resistivity
(mohm-cm) is known.

 In mm In inches Where:

 d = Standard Depth of Penetration (mm or in) r = Electrical Resistivity (mohm-cm) f = Test Frequency (Hz) mr = Relative Magnetic Permeability (dimensionless)

Eddy Current
Field Phase Lag
Eddy Current
Field Phase Lag
Eddy Current
Field Phase Lag
Eddy Current
Field Phase Lag
 In Radians In Degrees Where:

 q = Phase Lag (Rad or Degrees) x = Distance Below Surface (in or mm) d = Standard Depth of Penetration (in or mm)

When electrical conductivity (S/m) is known. Where:

 q = Phase Lag (degrees) x = Distance Below Surface (m) p = 3.14 f = Test Frequency (Hz) mr = Relative Magnetic Permeability s = Electrical Conductivity (Siemens/m)

When electrical conductivity
(%IACS) is known.

 In mm In inches Where:

 q = Phase Lag (degrees) x = Distance Below Surface (mm) f = Test Frequency (Hz) mr = Relative Magnetic Permeability (dimensionless) s = Electrical Conductivity (%IACS)

When electrical resistivity
(mohm-cm) is known.

 In mm In inches Where:

 q = Phase Lag (degrees) x = Distance Below Surface (inch) r = Electrical Resistivity (mohm-cm) f = Test Frequency (Hz) mr = Relative Magnetic Permeability (dimensionless)

Standard Depth of Penetration and
Phase Angle

Material Thickness Requirement for
Resistivity or Conductivity Measurement
Frequency Selection
for Thickness Measurement of Thin Materials
Frequency Selection
for Flaw Detection
and Nonconductive
Coating Thickness Measurements
 Std. Depth Relative Strength of EC Phase Lag 0 e0=100% 0 rad = 0o d e-1=37% 1 rad = 57.3o 2d e-2=14% 2 rad = 114.6 o 3d e-3=5% 3 rad = 171.9 o 4d e-4=2% 4 rad = 229.2 o 5d e-5=0.7% 5 rad = 286.5 o

When measuring resistivity or conductivity, the thickness of the material should be at least 3 times the depth of penetration to minimize material thickness effects Where:

 t = Material Thickness d = Standard Depth of Penetration

Selecting a frequency that produces a standard depth of penetration that exceeds the material thickness by 25% will produce a phase angle of approximately 90obetween the liftoff signal and the material thickness change signal.  