T.O. 33B-1-1 4-16 Table 4-4.  Effect of Changes in Test Variables on the Surface Intensity and Depth of Penetration of Eddy Currents. Test Variable Change in Variable Effect on Surface Intensity Effect on Depth of Penetration Conductivity Increase Increase Decrease Decrease Decrease Increase Relative Magnetic Increase Increase Decrease Permeability Decrease Decrease Increase Geometry of Part — Variable Variable Discontinuity — Variable Variable Frequency Increase Decrease Increase Decrease Decrease Increase Electromagnetic Increase Increase None Coupling Decrease Decrease None Coil Increase Increase None Current Decrease Decrease None Temperature Increase Decrease Increase Decrease Increase Decrease 4.2.3.3.2 Effective Depth of Penetration. Effective depth of penetration is the depth in the inspection article at which the magnetic field strength or the intensity of the induced eddy currents is reduced to 5 percent of the value at the surface.  This depth is approximately 3 times the standard  depth  of  penetration.    The  effective  depth  of  penetration  is  used  to  determine  test  frequency  when  working with thin materials, so the overall electromagnetic field does not extend beyond the back surface of the test part so that thickness variation effects can be suppressed. 4.2.3.3.3 Temperature Effects. For most applications, temperature is not a major factor in determining depth of penetration.  However, if necessary the effects   of   temperature   would   be   included   as   adjustments   to   the   values   for   conductivity   and   relative   magnetic permeability used in the formula to calculate the standard depth of penetration. SECTION III ANALYSIS OF EDDY CURRENT SIGNALS 4.3 ANALYSIS OF EDDY CURRENT SIGNALS. 4.3.1 Overview of Signal Detection, Processing and Display. 4.3.1.1 Signal Sources. Changes in an electrically conductive material close to a coil containing an alternating current can affect the resultant electromagnetic field from that coil.  These material changes can be detected by monitoring the alternating current in the  coil  or  using  a  separate  sensing  coil  to  monitor  the  resultant  electromagnetic  field.    Changes  so  detected  are  the eddy  current  inspection  signals.    These  signals  can  be  analyzed  for  information  relevant  to  the  inspection  being conducted.