Ohm's Law

Impedance

Phase Angle

Magnetic Permeability 

Where:
Z 
= 
Impedance (ohm) 
R 
= 
Resistance (ohm) 
X_{L} 
= 
Inductance Reactance(ohm) 
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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:
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
m 
= 
Any Given Magnetic Permeability (H/m) 
m_{o} 
= 
Magnetic Permeability in Free Space (H/m), which is 1.257 x 10^{6} H/m 
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Where:
s 
= 
Electrical Conductivity Siemens/m 
r 
= 
Electrical Resistivity (ohmm) 
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When resistivity is known
Where:
s_{%}_{IACS} 
= 
Electrical Conductivity (% IACS) 
r 
= 
Electrical Resistivity (mohmcm) 
s_{m}_{S/cm} 
= 
Electrical Conductivity (mSiemens/cm) 
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When conductivity in S/m or mS/cm is knownor
Where:
s_{%}_{IACS} 
= 
Electrical Conductivity (% IACS) 
s_{S/m} 
= 
Electrical Conductivity (Siemens/meter) 
s_{m}_{S/cm} 
= 
Electrical Conductivity (mSiemens/cm) 
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Current Density

Standard Depth of Penetration

Standard Depth of Penetration

Standard Depth of Penetration

Where:
Jx 
= 
Current Density (amps/m^{2}) 
J_{o} 
= 
Current Density at Surface (amps/m^{2)} 
e 
= 
Base Natural Log = 2.71828 
x 
= 
Distance Below Surface 
d 
= 
Standard Depth of Penetration 
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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) 
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When electrical conductivity (%IACS) is known.
In mm 

In inches 

Where:
d 
= 
Standard Depth of Penetration (mm or in) 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
s 
= 
Electrical Conductivity (%IACS) 
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When electrical resistivity (mohmcm) is known.
In mm 

In inches 

Where:
d 
= 
Standard Depth of Penetration (mm or in) 
r 
= 
Electrical Resistivity (mohmcm) 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
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Eddy Current Field Phase Lag

Eddy Current Field Phase Lag

Eddy Current Field Phase Lag

Eddy Current Field Phase Lag

Where:
q 
= 
Phase Lag (Rad or Degrees) 
x 
= 
Distance Below Surface (in or mm) 
d 
= 
Standard Depth of Penetration (in or mm) 
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When electrical conductivity (S/m) is known. Where:
q 
= 
Phase Lag (degrees) 
x 
= 
Distance Below Surface (m) 
p 
= 
3.14 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability 
s 
= 
Electrical Conductivity (Siemens/m) 
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When electrical conductivity (%IACS) is known.
In mm 

In inches 

Where:
q 
= 
Phase Lag (degrees) 
x 
= 
Distance Below Surface (mm) 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
s 
= 
Electrical Conductivity (%IACS) 
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When electrical resistivity (mohmcm) is known.
In mm 

In inches 

Where:
q 
= 
Phase Lag (degrees) 
x 
= 
Distance Below Surface (inch) 
r 
= 
Electrical Resistivity (mohmcm) 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
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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 
e^{0}=100% 
0 rad = 0^{o} 
d 
e^{1}=37% 
1 rad = 57.3^{o} 
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 
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Selecting a frequency that produces a standard depth of penetration that exceeds the material thickness by 25% will produce a phase angle of approximately 90^{o}between the liftoff signal and the material thickness change signal.
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Defect Detection A test frequency that puts the standard depth of penetration at about the expected depth of the defect will provide good phase separation between the defect and liftoff signals. Nonconductive Coating Thickness Measurement To minimize effects from the base metal the highest practical frequency should be used. 