Eddy current lift-off: a study guide to signal context
A direction on an eddy current display needs a caption and a reference. Use two fictional cases and a worked note correction to separate a spacing change from the way a response is presented.
Understand the spacing variable
For a surface probe, changing the separation between the coil and a conductive part changes electromagnetic coupling and can change the coil’s impedance. The IAEA’s Eddy Current Testing at Level 2 manual calls this proximity effect lift-off.
The same manual identifies material properties, frequency, geometry and probe handling among the variables that affect a response. A changed signal alone does not establish its cause.
Sources: IAEA TCS-48: variables and proximity effects, pages 112–114.
Compare two fictional study cases
Read these as supplied lesson descriptions, not instructions to operate equipment. Neither case includes a measured trace or a component acceptance decision. In Case A, the stated change identifies the variable being studied. In Case B, a visible response is all that has been supplied.
| Study case | What is established | What remains unproved |
|---|---|---|
| A: The description says only probe separation changed. | A spacing change is part of this example. | A universal signal direction or numerical sensitivity loss. |
| B: The response changes, but the description omits what changed in the setup. | An observation is available; its context is incomplete. | The cause of the response or whether a discontinuity exists. |
Read the display context, then correct the note
In the IAEA manual’s impedance-plane description, amplitude is distance from a balance point, and phase angle is an angle relative to that point. Evident’s introductory tutorial also presents a horizontal lift-off response within a specified example. A screen direction needs that context before it can support an explanation.
Extend Case A with this fictional caption: “Surface-coil study. Only coil-to-part separation changes; the probe, material and frequency remain unchanged. The balance point is marked, and the display orientation stays fixed for this example. The spacing-related response is shown horizontally.” This is a reading exercise with supplied facts, not a recorded inspection.
A learner writes: “The response is horizontal, so it is lift-off and the part has no discontinuities.” The caption identifies a spacing change. The learner has instead used direction as proof and added an unsupported conclusion about the part.
A supported replacement is: “The caption identifies a spacing change, so this example illustrates lift-off. Horizontal describes its presentation here; it does not identify every horizontal response or establish the condition of a part.” The explanation now rests on the supplied change.
Sources: IAEA TCS-48: impedance-plane display, page 131; Evident: introduction to eddy current testing, equipment and display example.
Check the orientation and reference point
Now imagine a second illustration explicitly labelled “the same response, with the display rotated.” Does its different direction establish a different physical cause? No. The caption accounts for the presentation change. Without that caption, the direction alone would leave the relationship between the illustrations unresolved.
Where is the balance point identified? A distance from an image corner is not a distance from that reference. If a cropped illustration omits it, note the missing information instead of estimating an amplitude from the crop.
Check your note for the change stated in the caption, the reference and display convention, and the conclusion those facts support. Apply that check to Case B: its missing setup context still leaves the cause unresolved.
Sources: Evident: impedance-plane example with display rotation; IAEA TCS-48: impedance-plane display and phase rotation, pages 131 and 136.
Connect the exercise with ET Level I and II training
The IAEA’s training guidelines include lift-off within coupling effects at both Level 1 and Level 2. It is a topic to revisit as your study progresses.
NDT Academy’s ET course includes Reading the Impedance Plane, Lab: Generating Liftoff Curves and Lab: Liftoff and Frequency Effects in Level I, followed by a Liftoff lesson in Level II. Start with Reading the Impedance Plane if the reference point or display orientation is unclear in your notes; use the lift-off topics to connect the spacing variable with the example’s stated conditions.
The online course provides formal training and completion records. Personnel certification and work authorization depend on the applicable certification program and employer requirements; completing this article or course does not by itself award them.
Sources: IAEA training guidelines: ET outlines, pages 154 and 162.
Sources and further reading
- IAEA TCS-48: impedance-plane display and phase rotation, pages 131 and 136
- Evident: introduction to eddy current testing, equipment and display example
- Evident: impedance-plane example with display rotation
- IAEA training guidelines: ET outlines, pages 154 and 162
Sources checked on October 26, 2026. Follow the current requirements from your employer or certification body when planning qualification or an examination.