Impedance Plane
The impedance plane is a 2D display in eddy current testing showing the probe coil's resistive (X-axis) and inductive reactive (Y-axis) components, on which signals from lift-off, defects, and conductivity changes trace characteristic vectors.
Definition
The impedance plane is the analytical core of ET. Defect signals appear at specific phase angles relative to the lift-off signal; experienced inspectors recognize and discriminate signals by their position and shape on the plane.
The principle
The display space of eddy current testing: coil impedance plotted as resistance against reactance. Lift-off, conductivity, permeability and defects each move the operating point along characteristic trajectories, and phase separation between them is what makes the method interpretable.
How it shows up in practice
Setup rotates the display so lift-off runs horizontally; defect responses then rise at an angle that grows with depth. Reading the plane — angle and amplitude together — is the core ET skill.
Where the codes address it
ASME Section V Article 8 requires phase-based setup on reference standards.
Where practitioners get caught
Interpreting amplitude alone. Two signals of equal height with different phase angles are different things — one may be lift-off wobble, the other a crack.
Where Impedance Plane fits in an inspection programme
A term is only useful when it connects to a decision. Impedance Plane appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Lift-Off — Lift-off is the change in eddy current signal caused by a variation in the distance between the probe coil and the test surface, used both as a calibration tool and as a nuisance variable to be minimized.
- Edge Effect — The edge effect is the distortion of an eddy current signal as the probe approaches a part edge, hole, or geometric discontinuity, which can mask defect signals near edges.
- Eddy Current Testing (ET) — Eddy Current Testing (ET) is an electromagnetic NDT method that induces circulating currents in conductive materials using an AC coil; changes in coil impedance reveal surface and near-surface defects, conductivity variations, and coating thickness.
Further reading
eddy current testing complete beginner guide
More physics terms
Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.