Eddy Current Testing (ET) Level II Practice Questions

30 original questions in 6 topics, each with the correct answer, an explanation and its basis. Last reviewed October 16, 2026.

About this ET Level II practice test

These 30 multiple-choice questions are original study questions written by Atlantis NDT for eddy current technicians preparing for a Level II examination. They are not ASNT exam questions. They follow the Level II job: select probes and frequency, standardize on reference standards, read the impedance plane, carry out surface and tubing examinations, evaluate indications against a procedure and report the results.

Under ASNT SNT-TC-1A (2024 edition), the employer certifies Level II technicians after general, specific and practical examinations set out in its written practice. ASNT's central ASNT NDT Level II examinations do not currently include eddy current testing, so ET certification rests on the employer's own examinations. Use this set to find weak areas before them.

Atlantis NDT runs NDT training year-round, with theory online and hands-on practice on its Practical NDT simulator or in person at the client's own facility. Simulator practice supports training; it does not replace the practical examination or the experience hours the employer's written practice requires.

How to use this practice test

  • Answer each question before opening the explanation.
  • For the depth-of-penetration questions, check which unit convention the formula uses before you substitute numbers.
  • Sketch the impedance plane when a question asks about signal direction or phase; it is easier to reason from a picture.
  • Note the topics where you miss questions, study them against your employer's procedure, then re-test.

Topics in this set:

Practice questions, not exam questions

Atlantis NDT wrote these questions as original study material. They are not taken from ASNT or from any employer's examination, and real examinations differ in wording, emphasis and format.

Certification is a separate step: under ASNT SNT-TC-1A the employer certifies its technicians after the examinations its written practice sets, and ASNT runs central Level II examinations for several methods. A good score here is a study signal, not a qualification.

Principles and electromagnetic theory

Question 1. How are eddy currents produced in a test part?

  1. A. By the static field of a permanent magnet held near the part
  2. B. By induction from the alternating magnetic field of an AC coil
  3. C. By passing direct current through the part between two contacts
  4. D. By ultrasonic vibration of the part surface
Show answer

Correct answer: B. By induction from the alternating magnetic field of an AC coil

An AC coil produces a changing magnetic field that induces circulating currents in a nearby conductor. A static field or direct current does not induce continuous eddy currents in a stationary part.

Basis: General ET principle (electromagnetic induction)

Question 2. What effect do the induced eddy currents have on the test coil?

  1. A. They set up an opposing secondary field that changes the coil's impedance
  2. B. They raise the frequency of the alternating current in the coil
  3. C. They reinforce the coil's primary field and leave its impedance unchanged
  4. D. They only heat the part slightly and cannot be detected by the coil
Show answer

Correct answer: A. They set up an opposing secondary field that changes the coil's impedance

By Lenz's law the eddy-current field opposes the change that created it. Changes in that field, caused by flaws, conductivity, permeability or lift-off, alter the coil's resistance and inductive reactance, which the instrument displays.

Basis: General ET principle (Lenz's law)

Question 3. The standard depth of penetration (δ) is the depth at which the eddy current density has fallen to about what fraction of its surface value?

  1. A. 50% (1/2)
  2. B. 5% (1/20)
  3. C. 37% (1/e)
  4. D. 10% (1/10)
Show answer

Correct answer: C. 37% (1/e)

At one standard depth the density is 1/e, about 37% of the surface value. At about three standard depths it falls to about 5%, which is often used as the practical (effective) depth of penetration.

Basis: General ET principle (skin effect)

Question 4. Using δ (mm) = 50·√(ρ/(f·µr)), with ρ in µΩ·cm and f in Hz, what is the standard depth of penetration in a non-magnetic aluminum alloy with ρ = 4.0 µΩ·cm at 10 kHz?

  1. A. 0.1 mm
  2. B. 0.5 mm
  3. C. 2.0 mm
  4. D. 1.0 mm
Show answer

Correct answer: D. 1.0 mm

δ = 50 × √(4.0 ÷ (10,000 × 1)) = 50 × √0.0004 = 50 × 0.02 = 1.0 mm, with µr = 1 for a non-magnetic material. The constant 50 corresponds to 503 in the SI form δ (m) = 503·√(ρ/(f·µr)) with ρ in Ω·m.

Basis: Standard depth of penetration equation

Question 5. In a non-magnetic alloy, the standard depth of penetration is 1.0 mm at 10 kHz. What is it at 40 kHz?

  1. A. 0.25 mm
  2. B. 2.0 mm
  3. C. 4.0 mm
  4. D. 0.5 mm
Show answer

Correct answer: D. 0.5 mm

δ is proportional to 1/√f. Raising the frequency by a factor of 4 reduces δ by √4 = 2, giving 0.5 mm. A fourfold change in δ would need a sixteenfold change in frequency.

Basis: Standard depth of penetration equation

Question 6. Relative to the surface, what is the phase lag of the eddy currents at one standard depth of penetration?

  1. A. About 57° (1 radian)
  2. B. 0° (no lag)
  3. C. 90° (a quarter cycle)
  4. D. 180° (half a cycle)
Show answer

Correct answer: A. About 57° (1 radian)

Phase lag increases linearly with depth as x/δ radians, so at one standard depth it is 1 radian, about 57°. This phase lag is what lets signals from flaws at different depths be told apart by their phase.

Basis: General ET principle (phase lag)

Probes, coils and instrumentation

Question 7. Compared with an absolute probe, how does a differential probe (two coils comparing adjacent areas) behave?

  1. A. It is more sensitive to gradual changes such as slow wall thinning and conductivity drift
  2. B. It cannot detect small cracks because the two coils cancel
  3. C. It largely ignores gradual changes and drift but responds well to small, abrupt flaws
  4. D. It can only be used for conductivity measurement
Show answer

Correct answer: C. It largely ignores gradual changes and drift but responds well to small, abrupt flaws

A differential probe responds to the difference between its two coils, so changes common to both, such as gradual wall loss, slow conductivity variation or drift, largely cancel. Absolute probes are better for gradual changes and conductivity measurement.

Basis: General ET principle (probe types)

Question 8. Why is a standard bobbin coil insensitive to circumferential cracks in tubing?

  1. A. Bobbin coils cannot operate at the high frequencies cracks need
  2. B. Circumferential cracks are always too shallow to give a signal
  3. C. Circumferential cracks only occur in ferromagnetic tubes, which need other methods
  4. D. Its currents circulate parallel to the crack, so the crack barely disturbs them
Show answer

Correct answer: D. Its currents circulate parallel to the crack, so the crack barely disturbs them

A flaw is detected when it interrupts the eddy-current path. Bobbin-coil currents circulate around the tube, so axial cracks interrupt them strongly while circumferential cracks lie along the current path. Rotating or array probes are used for circumferential cracking.

Basis: General ET principle (bobbin coils)

Question 9. A bobbin probe with a coil diameter of 16.0 mm is used in a tube with an inside diameter of 18.0 mm. What is the fill factor?

  1. A. 89%
  2. B. 79%
  3. C. 63%
  4. D. 94%
Show answer

Correct answer: B. 79%

For an internal (bobbin) coil the fill factor is (coil diameter ÷ tube inside diameter)² = (16.0 ÷ 18.0)² = 0.889² ≈ 0.79, or 79%. Using the diameter ratio without squaring gives the wrong value of 89%.

Basis: General ET principle (fill factor)

Question 10. A flat pancake (surface) probe is placed on a plate. Which discontinuity is it least likely to detect?

  1. A. A surface crack perpendicular to the surface
  2. B. A pit open to the surface
  3. C. A surface-breaking fatigue crack running across the probe path
  4. D. A lamination lying parallel to the surface
Show answer

Correct answer: D. A lamination lying parallel to the surface

The eddy currents from a surface probe flow in loops parallel to the surface. A lamination parallel to the surface lies in the plane of the current flow and causes little disturbance, whereas cracks perpendicular to the surface interrupt the currents.

Basis: General ET principle (current flow and flaw orientation)

Question 11. What is the main advantage of a shielded (or ferrite-focused) pencil probe for inspection near edges, fasteners or geometry changes?

  1. A. It concentrates the field, reducing edge effect and nearby-feature signals
  2. B. It increases the standard depth of penetration directly under the probe tip
  3. C. It allows testing of non-conductive materials such as plastics
  4. D. It removes the need for a reference standard near fasteners
Show answer

Correct answer: A. It concentrates the field, reducing edge effect and nearby-feature signals

Shielding or a ferrite core concentrates the field under the probe tip, so the probe can work closer to edges and fasteners before their signals swamp a flaw response. It does not increase penetration or make non-conductors testable.

Basis: General ET practice (probe selection)

Standardization and reference standards

Question 12. Why should an eddy current reference standard be made, as far as practical, from the same material, heat-treat condition and dimensions as the part under test?

  1. A. Because the standard is used only to check the probe cable and connectors
  2. B. Because the coil response depends on conductivity, permeability and geometry
  3. C. Because codes forbid artificial flaws in standards made from other materials
  4. D. Because the instrument gain cannot be adjusted between different materials
Show answer

Correct answer: B. Because the coil response depends on conductivity, permeability and geometry

Signal amplitude and phase from a given flaw depend on the material's electrical and magnetic properties and on the geometry. A mismatched standard gives a misleading set-up for amplitude, phase and depth estimation.

Basis: General ET practice; ASME BPVC Section V, Article 8

Question 13. Why is the impedance-plane display usually rotated so that the lift-off signal runs horizontally?

  1. A. So flaw signals, out of phase with lift-off, show mainly as vertical deflections
  2. B. Because the instrument cannot display the lift-off signal in the vertical direction
  3. C. To increase the standard depth of penetration at the test frequency
  4. D. To switch the instrument from absolute to differential mode for crack detection
Show answer

Correct answer: A. So flaw signals, out of phase with lift-off, show mainly as vertical deflections

Setting lift-off horizontal is a phase adjustment. With probe wobble along the horizontal axis, signals with a different phase, such as cracks, show mainly as vertical components, and the vertical channel or an alarm can then be used with less interference from lift-off.

Basis: General ET practice (phase set-up)

Question 14. In bobbin-coil examination of non-ferromagnetic tubing, how is the depth of an outside-surface flaw commonly estimated?

  1. A. From the signal duration on the strip chart at the probe speed used
  2. B. From the probe's travel speed when the signal appears
  3. C. By reading its phase against a phase-depth curve from reference flaws
  4. D. From the fill factor alone
Show answer

Correct answer: C. By reading its phase against a phase-depth curve from reference flaws

Reference standards with machined flaws of known depth, such as through-wall and flat-bottom holes, give a curve of phase angle against depth at the test frequency. The phase of a field signal is read against that curve; ASME Section V Article 8 uses this approach for tubing.

Basis: ASME BPVC Section V, Article 8

Question 15. Using %IACS = 172.41 ÷ ρ (with ρ in µΩ·cm), what is the conductivity of a material with a resistivity of 3.45 µΩ·cm?

  1. A. About 29% IACS
  2. B. About 50% IACS
  3. C. About 59% IACS
  4. D. About 100% IACS
Show answer

Correct answer: B. About 50% IACS

172.41 ÷ 3.45 ≈ 50% IACS. For comparison, 100% IACS corresponds to annealed copper at about 1.724 µΩ·cm.

Basis: IACS conductivity conversion

Question 16. During conductivity measurement of aluminum parts, why must the temperature of the part and the conductivity standards be considered?

  1. A. Because conductivity varies with temperature, causing measurement error
  2. B. Because the probe frequency changes with room temperature
  3. C. Because aluminum becomes ferromagnetic when cold
  4. D. Because temperature has no effect and is recorded only for information
Show answer

Correct answer: A. Because conductivity varies with temperature, causing measurement error

Electrical conductivity of metals falls as temperature rises. Measurements are made with the part and standards at similar temperatures, or corrected, as the procedure requires.

Basis: General ET practice (conductivity measurement)

Impedance-plane interpretation

Question 17. On a normalized impedance plane, how does the response when an absolute coil is placed on ferromagnetic carbon steel differ from placing it on aluminum?

  1. A. Both move in the same direction by the same amount
  2. B. On steel the coil impedance does not change at all
  3. C. On aluminum the inductive reactance rises above the air point, and on steel it falls
  4. D. On steel, permeability raises the reactance; on aluminum, the reactance falls
Show answer

Correct answer: D. On steel, permeability raises the reactance; on aluminum, the reactance falls

High permeability concentrates the coil's magnetic flux and raises inductive reactance, which outweighs the eddy-current effect in most steels. In a non-magnetic conductor the opposing eddy-current field lowers the inductive reactance.

Basis: General ET principle (impedance plane)

Question 18. On the normalized impedance plane of a non-ferromagnetic material, which change moves the operating point along the conductivity curve in the same direction as an increase in conductivity?

  1. A. A decrease in test frequency
  2. B. An increase in lift-off
  3. C. An increase in test frequency
  4. D. A decrease in coil diameter
Show answer

Correct answer: C. An increase in test frequency

The position on the curve depends on the product of frequency, conductivity and permeability, together with coil size. Raising the frequency therefore moves the point the same way as raising the conductivity. Lift-off moves the point along a different path, toward the air point.

Basis: General ET principle (impedance plane)

Question 19. Compared with a surface-breaking crack, how does the signal from a similar flaw located below the surface of a non-magnetic material typically differ?

  1. A. It is larger and has the same phase as the surface signal
  2. B. It has smaller amplitude and a greater phase lag
  3. C. It is identical, since depth does not affect eddy current signals
  4. D. It always appears exactly along the lift-off line
Show answer

Correct answer: B. It has smaller amplitude and a greater phase lag

Eddy-current density decreases and phase lag increases with depth, so deeper flaws give weaker signals rotated in phase relative to surface flaws. This phase change is what allows depth estimation.

Basis: General ET principle (phase lag with depth)

Question 20. As a differential probe passes over a short, abrupt flaw, what impedance-plane trace is typically produced?

  1. A. A single straight line along the lift-off direction
  2. B. No signal, because differential probes cancel all flaws
  3. C. A dot that does not move
  4. D. A figure-eight with two opposing lobes, one per coil
Show answer

Correct answer: D. A figure-eight with two opposing lobes, one per coil

Each coil of the pair sees the flaw in turn, so the difference signal swings one way and then the opposite way, tracing two lobes. Only changes seen equally by both coils cancel.

Basis: General ET principle (differential probes)

Question 21. A large signal appears as a surface probe approaches the edge of a plate. What is this called, and why does it matter?

  1. A. Fill-factor effect; it means the coil is too large for the tube
  2. B. Edge effect; it can mask flaws close to the edge
  3. C. Skin effect; it shows the test frequency is too low for the plate
  4. D. Lift-off; it shows the coating is too thick
Show answer

Correct answer: B. Edge effect; it can mask flaws close to the edge

Near an edge the eddy currents cannot complete their normal loops, which produces a large signal unrelated to a flaw. Shielded probes, edge guides or reference scans along the edge help separate edge effect from cracks.

Basis: General ET principle (edge effect)

Question 22. For which crack orientation is the eddy current response greatest?

  1. A. Crack plane parallel to the direction of eddy current flow
  2. B. Crack lying parallel to the surface below the coil
  3. C. The response is the same for all orientations
  4. D. Crack plane perpendicular to the direction of eddy current flow
Show answer

Correct answer: D. Crack plane perpendicular to the direction of eddy current flow

A crack is detected because it interrupts the current path. Maximum interruption, and so maximum response, occurs when the crack lies across the current flow; a crack parallel to the flow causes little disturbance.

Basis: General ET principle (flaw orientation)

Applications and techniques

Question 23. Eddy current lift-off measurement is commonly used to measure what?

  1. A. The thickness of paint or anodizing on non-ferrous metals
  2. B. The hardness of ceramic parts after firing
  3. C. The wall thickness of plastic pipe from the outside surface
  4. D. The grain size of austenitic stainless steel welds
Show answer

Correct answer: A. The thickness of paint or anodizing on non-ferrous metals

A non-conductive coating holds the probe away from the metal, so the lift-off signal can be calibrated against coating thickness. Ceramics and plastics are not conductive and cannot be tested by eddy currents directly.

Basis: General ET application (coating thickness)

Question 24. Compared with a surface-crack inspection, what change is needed to detect corrosion on the far side of an aluminum skin a few millimeters thick?

  1. A. Use a higher frequency to increase the depth of penetration
  2. B. Use a lower frequency so the currents reach the far side
  3. C. Use a smaller lift-off compensation only
  4. D. Use a differential probe at the highest frequency available
Show answer

Correct answer: B. Use a lower frequency so the currents reach the far side

Penetration increases as frequency decreases (δ is proportional to 1/√f). Low frequencies are therefore used for far-side and second-layer corrosion, accepting lower sensitivity to small surface cracks.

Basis: General ET practice (frequency selection)

Question 25. Why is multi-frequency mixing used in heat-exchanger tube examination?

  1. A. To increase the probe travel speed through each tube
  2. B. To measure the hardness of the tube material
  3. C. To suppress support-plate or tubesheet signals so flaws there can be seen
  4. D. To make a bobbin coil respond like a rotating probe to circumferential cracks
Show answer

Correct answer: C. To suppress support-plate or tubesheet signals so flaws there can be seen

Signals from different sources change differently with frequency. Combining two or more frequencies lets the instrument cancel a known unwanted signal, such as a support plate, while keeping the flaw response.

Basis: General ET practice (multi-frequency mixing)

Question 26. Conductivity measurement on aluminum alloys is commonly used for which purpose?

  1. A. Detecting laminations in thick plate before machining
  2. B. Measuring residual stress directly in welded parts
  3. C. Verifying heat treatment and sorting mixed alloys
  4. D. Measuring ultrasonic velocity for thickness gauging
Show answer

Correct answer: C. Verifying heat treatment and sorting mixed alloys

Alloy and temper both affect electrical conductivity, so conductivity readings against known limits are used to verify heat treatment and to sort mixed material.

Basis: General ET application (conductivity sorting)

Codes, procedures and limitations

Question 27. Why is conventional bobbin-coil eddy current testing of carbon-steel tubing difficult without special techniques?

  1. A. High, variable permeability limits penetration and masks flaws
  2. B. Carbon steel does not conduct electricity well enough for ET
  3. C. Bobbin coils cannot be made small enough to fit inside steel tubes
  4. D. Steel tubes are always too thin-walled to give a usable signal
Show answer

Correct answer: A. High, variable permeability limits penetration and masks flaws

Permeability variations in ferromagnetic material give large signals and greatly reduce the standard depth of penetration. Magnetic saturation (as in ASTM E309) or techniques such as remote field testing are used instead.

Basis: General ET principle; ASTM E309

Question 28. Which ASTM standard covers eddy current examination of steel tubular products using magnetic saturation?

  1. A. ASTM E426
  2. B. ASTM E243
  3. C. ASTM E309
  4. D. ASTM E2700
Show answer

Correct answer: C. ASTM E309

ASTM E309 covers steel tubular products using magnetic saturation. ASTM E426 covers tubular products of austenitic stainless steel, titanium and similar alloys, ASTM E243 covers copper and copper-alloy tubes, and ASTM E2700 covers phased array UT of welds.

Basis: ASTM E309

Question 29. Which statement is a fundamental limitation of eddy current testing?

  1. A. It cannot detect cracks that break the surface of the part
  2. B. It needs a liquid couplant between the probe and the part
  3. C. It cannot be automated for high-volume production testing
  4. D. Only conductive materials can be tested, and skin effect limits depth
Show answer

Correct answer: D. Only conductive materials can be tested, and skin effect limits depth

Eddy currents need a conductive path and concentrate near the surface, so ET suits surface and near-surface examination of metals. It detects surface cracks well, needs no couplant and is widely automated.

Basis: General ET principle (limitations)

Question 30. During heat-exchanger tube examination, a periodic check on the reference standard shows the system response has changed beyond the procedure tolerance. What is normally required?

  1. A. Continue, and adjust only the results of the remaining tubes
  2. B. Restandardize and re-examine tubes done since the last good check
  3. C. Average the readings from before and after the check for each tube
  4. D. Record the change and take no further action
Show answer

Correct answer: B. Restandardize and re-examine tubes done since the last good check

If the system drifted beyond tolerance, tubes examined since the last good check may have been evaluated wrongly, so the system is restandardized and those tubes are re-examined. ASME Section V Article 8 contains this type of requirement; the procedure sets the tolerance.

Basis: ASME BPVC Section V, Article 8

Next step: ET Level 2 course

Practice questions check recall. Structured training adds the classroom and hands-on hours your certification route requires. The ET Level 2 course covers impedance-plane analysis, reference standards with machined notches, and surface, bolt-hole and tubing signal interpretation. Completing a course is not certification: under an employer-based scheme such as SNT-TC-1A, your employer certifies you after its own examinations.

For employers: ASNT Level III consulting covers procedures, the written practice, qualification examinations and audit support for your technicians.

Where to go next

Atlantis NDT courses run year-round, with theory online and hands-on practice on the Practical NDT simulator or in person at the client's own facility. From here:

Enquire about ET Level II training for yourself or for a team. Tell us the method, the level and where the trainees are based.

Practise before the exam

Written papers test what you know; employers and practical examinations test what you can do with a probe in your hand. Use the free practice tests to find your gaps, then practise the technique.