Commonly used formulae in MKS Units:

Formula 1

Formula 2

Formula 2-3

Formula 3

Converting to inches:

Formula 4

Where:

r = Resistivity in ohm-meters
tm = Thickness in meters
ti = Thickness in inches
F = Frequency in Hertz
µ = Permeability µrµ0
µr = 1 for non-magnetic alloys
mu0
Multiply meters by 39.37 to get inches.
Divide inches by 39.37 to get meters.

Formulae (3) and (4) calculates the frequency at which there is 1 radian (57.3) phase change in the eddy current flow for a given thickness and resistivity. This is the frequency at which this thickness of material has thickness of one skin depth. There will be two radians (114.6) phase shift as viewed on an impedance plane screen for defects this distance through a material because energy from the probe must travel from the probe, to the defect and back again, a distance of 2t.

F is calculated for several values of r and t using Formulae (3) and (4) and is tabulated (in kHz) in Section 2 of this report. This value is not an “optimum” frequency. Formulae to calculate an “optimum” frequency have been published by many. A good frequency to use for tubing inspection is usually between the value F and 2 times F calculated using Formulae (3) and (4). Many factors effect choice of Frequency; therefore, the best frequency or frequencies for a given application should be determined experimentally using Formulae (3) or (4) or the value listed in the table as a starting point.