Multi Frequency Impedance Plane Inversion for Inline Alloy Sorting
Multi-frequency impedance inversion isolates lift-off clearance vectors from conductivity shifts to achieve real-time inline alloy sorting down to 150 microseconds.
ISO 21968
Coating Thickness Measurement Standard
Standardized technical specifications govern non-destructive thickness testing of non-conductive coatings applied over non-magnetic, electrically conductive substrate materials using high-frequency eddy current instruments. Standard ISO 21968 establishes instrumentation requirements, calibration protocols, operational practices and measurement uncertainty formulations for amplitude-sensitive eddy current measurement systems. The methodology covers paint, plastic, ceramic and anodized protective coatings applied to non-ferrous metals like aluminum, copper, brass and austenitic stainless steels.
It ceases to apply to magnetic metallic substrates or to metallic coatings where electrical conductivity approximates or exceeds substrate conductivity.
Electromagnetic Sensing Principles
High-frequency excitation currents passing through a small probe coil generate an alternating primary magnetic field, inducing localized eddy currents within the conductive base metal below the coating. These eddy currents produce a secondary magnetic field that opposes the primary field, altering the coil’s electrical impedance. The coating layer acts as a physical barrier separating the probe coil from the conductive metal substrate.
As coating thickness increases, probe lift-off distance grows, weakening the electromagnetic coupling between the probe coil and the substrate and altering coil impedance. Instrumentation translates this complex electrical impedance shift directly into calibrated linear coating thickness units. High operating frequencies, usually ranging from hundreds of kilohertz to several megahertz, confine eddy currents to the substrate skin depth, preventing interference from underlying structural features.
Error Sources
Variations in substrate electrical conductivity alter baseline eddy current intensity, requiring probe recalibration whenever base alloy composition or heat treatment changes. Surface curvature of test components distorts the spatial distribution of induced eddy currents, introducing geometric errors if calibration was conducted on planar reference blocks. Proximity to component edges concentrates eddy currents, generating false thickness readings when measurements are taken closer to an edge than the probe coil’s magnetic footprint allows.
Operator probe tilt introduces artificial lift-off air gaps, causing thickness overestimation. Substrate thickness below the minimum electromagnetic skin depth allows electromagnetic fields to penetrate through the substrate, generating severe thickness calculation errors.
Calibration Verification
Instrument verification demands two-point calibration using bare substrate zero blocks matched to the component’s exact alloy and geometry, followed by certified non-conductive calibration foils of known thickness. Calibration foil standards carry traceability to national metrology institutes with defined thickness tolerances verified by mechanical interferometry or precision micrometers. Statistical uncertainty budgets require calculating probe repeatability, calibration foil uncertainty, substrate conductivity variability and surface roughness influences.
Field qualification protocols require periodic verification against reference check foils at designated inspection intervals to detect instrument drift or probe face wear.