Penetration Measure
Electromagnetic decay defines the distance below the surface of a conductor where the current density falls to a specific fraction of its surface value. In inductive sensing and high-frequency cabling, the skin effect depth determines how far the alternating magnetic field penetrates the metal target or conductor. This distance is inversely proportional to the square root of the operating frequency and the electrical conductivity of the material.
Frequency Influence
High-frequency excitation forces the sensing current to flow almost entirely along the outer boundary of the conductor. This behavior reduces the effective conductive cross-section, causing the skin effect depth to shrink to fractions of a millimeter at megahertz frequencies. The resulting rise in alternating-current resistance limits the depth of the metal target that participates in the inductive interaction.
This characteristic is used to design sensors that are sensitive only to surface layers or thin foil targets.
Target Selection
Material properties like resistivity and permeability dictate the choice of sensor operating frequency. For high-conductivity metals like copper or aluminum, the skin effect depth is very small, requiring high frequencies to generate sufficient signal amplitude. Low-conductivity alloys require lower frequencies to achieve a similar distribution of eddy currents.
Technicians measure this response by testing different foil thicknesses to ensure that the sensor can distinguish between base metals and superficial coatings.
Sensor Performance
Severe reduction in penetration depth can cause a loss of sensitivity if the metal target has a non-uniform surface layer or oxides. This limitation must be managed during calibration by setting the sensor-to-target distance to match the selected operating frequency and alloy type.
