Eddy Current Testing Simulator: Null, Scan and Read the Impedance Plane
Eddy current testing (ET) is learned on the impedance plane: the skill is telling a crack from lift-off, an edge or a change in material, and that takes repetition with feedback. The eddy current simulator in Atlantis Practical NDT has 33 jobs using absolute, differential, bobbin and bolt-hole techniques. You null the instrument on the reference, scan, and read a response computed from skin depth, lift-off, phase and gain.
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The four eddy current techniques in the catalogue
- Absolute: a single coil compared with a reference, which shows gradual changes as well as abrupt ones.
- Differential: two coils compared with each other, which emphasises a short, sharp change such as a crack and suppresses slow variation.
- Bobbin: an encircling probe pulled through a tube, the standard technique for tube inspection in heat exchangers.
- Bolt-hole: a rotating or hand-turned probe in a fastener hole, a classic airframe inspection.
In every technique the sequence is the same as the rest of Practical NDT: work order, applicability, equipment, preparation, null on the reference, scan, review data quality, interpret, post-check, report.
What the eddy current model computes
The response is a complex impedance computed from skin depth, lift-off, phase and gain. Skin depth depends on frequency and on the material's conductivity and permeability, so the depth the field reaches is a choice you make; lift-off moves the signal along its own direction on the impedance plane; and phase and gain decide how a flaw signal sits relative to that lift-off line. Nulling on the reference sets the starting point, which is why a careless null makes every later reading harder to interpret.
What eddy current reliability studies found
In an FAA eddy current reliability experiment at nine airline facilities, the crack length detected with 90% probability was 0.14 to 0.18 inch for the poorest inspectors and 0.06 to 0.07 inch for the best, and differences between facilities were significant. One or two of the most experienced inspectors got the worst performance scores. Years in the job are not the same as skill, and skills decay without use: a technician who has not done bolt-hole eddy current for a year can rehearse it before mobilising for the job.
Where the eddy current jobs run
Tube work runs on the dedicated heat exchanger, a bundle of 2,000 tubes for eddy current, IRIS, remote field and MFL tube inspection. The eddy current model there is not a validated probability-of-detection model, so use it to practise technique, nulling and signal reading, not to estimate what a real bundle would reveal. Airframe and component work runs in the aircraft hangar, and bench specimens in the NDT lab.
What the ET simulator cannot teach
A screen cannot give you the feel of keeping a pencil probe square to a curved surface, the drag of a bobbin probe in a long tube, or the noise and drift of a real instrument on a real part. Real conductivity and permeability variation, paint and coatings, and the geometry of real structure produce signals no teaching model fully reproduces. Interpretation of real eddy current data needs real reference standards and real components.
How ET simulator practice fits SNT-TC-1A
For eddy current testing, SNT-TC-1A's recommended figures are 40 training hours for Level I and 40 more for Level II, with 210 and 630 hours of experience in the method, and the employer's written practice sets the binding numbers. Simulator sessions can count as organised training where the written practice allows computer-based training. The FAA's advisory circular on NDI personnel defines experience as work in the work environment and excludes classroom and laboratory training, so simulator time is recorded as training, never as experience.
Check the theory with the free ET Level II practice questions.
Frequently asked questions
Which eddy current techniques can I practise?
Absolute, differential, bobbin and bolt-hole, each with nulling on the reference and a computed impedance-plane response.
Can I practise heat exchanger tube inspection?
Yes. The dedicated heat exchanger workplace has 2,000 tubes for eddy current, IRIS, remote field and MFL tube inspection. Its eddy current model is not a validated probability-of-detection model.
Does the ET simulator replace practice on reference standards?
No. It builds the impedance-plane habits; real reference standards and real components are still needed to interpret real signals.
Next steps
Practical NDT is a training simulation, not a validated engineering tool. It runs in a standard browser, works offline once downloaded, extends to standalone headsets and is integrated with the Atlantis LMS and ERP, so learners reach it from the client portal with one sign-in. Practice in it does not replace the practical examination or the experience hours your written practice requires.
Speak to an ASNT NDT Level III
Discuss your inspection requirements with Atlantis NDT. Include the asset, method, location and documents you need reviewed.