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) 

Phase lag is a parameter of the eddy current signal that makes it possible to obtain information about the depth of a defect within a material. Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface. Phase lag can be calculated using the equations to the right. The second equation simply converts radians to degrees by multiplying by 180/p or 57.3.
The phase lag calculated with these equations should be about 1/2 the phase rotation seen between the liftoff signal and a defect signal on an impedance plane instrument. Therefore, choosing a frequency that results in a standard depth of penetration of 1.25 times the expected depth of the defect will produce a phase lag of 45^{o} and this should appear as a 90^{o} separation between the liftoff and defect signals.
The phase lag angle is useful for estimating the distance below the surface of discontinuities that concentrated at a specific depth. Discontinuities such as a crack must be divided into sections along its length and a weighted average determined for phase and amplitude at each position below the surface. For more information see the page explaining phase lag.