ASNT Level III Electromagnetic Testing (ET) Exam Preparation

By Anoop Rayavarapu, ASNT NDT Level III · Published September 29, 2026

Who ET Level III exam prep is for

ET Level III candidates are a smaller, more specialised group than UT or RT candidates, and they usually come from one of three backgrounds. Tubing specialists inspect heat exchanger, condenser and boiler tubes with bobbin probes in refineries, chemical plants and power stations, and are expert at reading Lissajous signals from supports, dents and wall loss. Aerospace technicians use surface probes and bolt-hole probes for crack detection around fasteners and on wheels, and run conductivity checks to verify heat treatment. Manufacturing technicians run encircling-coil systems on bar, tube and wire lines, or sort materials by conductivity and hardness.

Each group knows its own application deeply and the others hardly at all. The Level III exam spans all of them and goes underneath them into the physics. A tubing analyst who has never set up a surface-crack test, or an aerospace technician who has never seen a bobbin signal, needs a structured plan to cover the unfamiliar half.

ASNT Level III ET exam structure

ASNT's eligibility routes are the same for every Level III method: 12 months (252 days) of NDT experience with a four-year engineering or physical science degree, 24 months (504 days) with two years of such study, or 48 months (1,008 days) with no college requirement.

What the ET Level III exam covers

We organise ASNT's electromagnetic testing body of knowledge, which follows the ANSI/ASNT CP-105 topical outline at Level III depth, into six domains. Eddy current carries most of the weight. Related electromagnetic techniques, such as remote field testing and alternating current field measurement, can also appear, so we cover their principles.

1. Electromagnetic principles

Faraday's law of induction, Lenz's law, magnetic fields around a coil, self and mutual inductance, inductive reactance, resistance and impedance, and phase. You should be comfortable with how coil impedance changes when a conductor enters the field, and why.

2. Material properties and depth of penetration

Electrical conductivity (and the %IACS scale), magnetic permeability, and the standard depth of penetration, which falls as frequency, conductivity or permeability rise. Expect calculation questions and questions on the phase lag with depth that lets an analyst estimate how deep a flaw lies. Ferromagnetic materials get special attention: why permeability variations swamp flaw signals, and how magnetic saturation or other techniques address that.

3. The impedance plane and signal analysis

The normalised impedance plane, lift-off and fill-factor effects, the conductivity curve, the characteristic frequency, and the direction and phase of signals from cracks, wall loss, dents, deposits, support plates and conductivity changes. Signal analysis questions often show a signal and ask what it represents or which variable produced it.

4. Probes, coils and instrumentation

Surface (pancake and pencil) probes, encircling coils, internal bobbin probes, absolute and differential configurations, bridge and reflection (driver-pickup) arrangements, shielding and focusing, array probes, and rotating probes. Instrument settings: frequency, gain, phase rotation, filtering (high-pass and low-pass), and display modes.

5. Techniques and applications

Frequency selection for a given depth and material, multifrequency operation and mixing to suppress support-plate or other unwanted signals, calibration and reference standards (flat-bottom holes, through-wall holes, EDM notches, conductivity standards), tubing inspection and sizing, surface-crack detection, bolt-hole inspection, conductivity measurement, coating-thickness measurement, and sorting.

6. Codes, standards and procedures

ASME Section V Article 8 for eddy current examination of tubular products, the relevant ASTM practices, procedure content and essential variables, and the reporting of results.

A 12-week ET Level III study plan

Key references for the ET Level III exam

• ASNT Nondestructive Testing Handbook, Electromagnetic Testing volume.
• ASNT Level III Study Guide: Electromagnetic Testing.
• ASNT Question & Answer Book for ET.
• ANSI/ASNT CP-105 electromagnetic testing topical outline.
• ASME BPVC Section V, Article 8 (eddy current examination of tubular products).
• ASTM E243 (copper and copper-alloy tubes), E426 (stainless and similar alloy tubular products), E309 (steel tubular products using magnetic saturation).
• ASTM E1004 (electrical conductivity), E376 (coating thickness), E2096 (remote field testing of ferromagnetic tubes) and E2261 (alternating current field measurement of welds).

Where experienced ET technicians lose marks

• Physics without an instrument. Analysts who read signals expertly on a screen often cannot explain why a signal rotates the way it does. The exam asks why.
• Depth of penetration direction. Raising frequency reduces penetration, and so do higher conductivity and permeability. Under time pressure candidates reverse one of the three.
• Absolute versus differential. Knowing which configuration responds to gradual changes and which to abrupt ones, and what each does with long, uniform wall loss, is standard content.
• Mixing. Why two frequencies are needed to cancel a support-plate signal, and what the mix does to other signals, is often the weakest topic on a diagnostic.
• The other half of the method. Tubing specialists lose marks on surface, conductivity and coating questions. Aerospace technicians lose them on bobbin analysis and tube standards.

Choosing a test frequency: a worked example of Level III reasoning

Frequency selection is the decision ET questions return to most often, because it links the physics to the result. Take a non-ferromagnetic heat exchanger tube where the concern is wall loss starting from the outside surface. Too high a frequency concentrates the eddy currents near the inside surface, so outside-diameter defects give weak signals with poor phase separation. Too low a frequency gives deep penetration but poor sensitivity and poor phase discrimination between defect depths. The usual reasoning sets the primary frequency so the phase separation between inside and outside surface indications on the reference standard falls in a useful range, then adds a lower frequency that responds strongly to support plates so the two can be mixed and the support signal suppressed. A higher frequency may be added for inside-surface conditions.

Switch the problem to a surface crack in an aluminium aircraft skin and the logic changes. Now high frequency is an advantage, because the currents crowd at the surface where the crack is and lift-off can be separated by phase. For a crack in a second layer under a fastener, the Level III goes the other way again and chooses a low frequency to reach the subsurface layer, accepting reduced resolution. The exam asks versions of all three problems. Candidates who understand why the choice changes do far better than those who have memorised a single application.

Other electromagnetic techniques: what a Level III should know

Ferromagnetic tubing defeats conventional eddy current because permeability variations swamp flaw signals. Two answers come up. Magnetic saturation uses a strong DC field to push permeability down so conventional eddy current works. Remote field testing uses a low-frequency exciter and a detector spaced well apart, so the field passes through the tube wall twice, giving through-wall sensitivity in carbon steel tubing without saturation. Alternating current field measurement induces a uniform current in a surface and measures field disturbances around surface-breaking cracks, often through coatings, which makes it useful for weld toe inspection on structures. Magnetic flux leakage, which ASNT now lists as its own Level III method, detects field leakage from wall loss in magnetised steel. Knowing when each technique fits, and what limits it, is enough for most exam questions on them.

What an ET Level III does day to day

In a tubing programme, the ET Level III writes the technique and calibration requirements, approves the analysis guidelines that tell analysts how to call and size indications, resolves disputed calls, and reports tube condition to the plant engineer. In aerospace and manufacturing, the Level III develops techniques for specific parts and cracks, designs or approves reference standards, and validates conductivity and coating measurements. In every setting, the Level III writes and grades the company's ET Level I and II examinations under the written practice. The weight the exam puts on physics, signals and procedures matches that work.

How Atlantis ET Level III exam prep works

The course is led by Anoop Rayavarapu, ASNT NDT Level III, in live online sessions, recorded for shift and outage workers.

• Impedance-plane workshops that build the plane from first principles, so signal questions become reasoning rather than memory.
• Cross-application sessions that take tubing specialists through surface work and surface specialists through tubing.
• Timed practice questions with explanations, and two full 135-question mocks.
• Procedure review of an ET procedure outline against ASME Section V Article 8 and the relevant ASTM practice.
• Method refresh on Practical NDT: the Practical NDT simulator, the first-ever online practical NDT simulation portal, includes eddy current scenarios across its 12 methods and 5 virtual environments. It is practice only and does not replace hands-on work.

Request an ET Level III prep schedule, or warm up with our free ET Level II practice questions.

Employer sponsorship for ET Level III candidates

ET Level IIIs are scarce, and outage schedules do not wait for an outside Level III's availability. Companies that run tube inspection or aerospace eddy current work often sponsor a senior analyst so that analysis guidelines, calibration standards and examinations are controlled in-house. The ASNT certificate is the credential. Under SNT-TC-1A, the employer still certifies its Level III under its written practice. We support both the candidate and the company's written practice. Ask about a private ET Level III cohort or use the training enquiry form.