Phased Array Level II Practice Questions With the Focal-Law Reasoning

Questions 1-2: What a phased array qualification actually is

Question 1. A client asks for your PAUT Level II certificate. What do you send? Answer: a UT Level II certification issued by your employer under its written practice, together with documented phased array technique training and a practical examination performed on phased array equipment. SNT-TC-1A does not create a separate phased array method. Phased array is a technique inside ultrasonic testing, so the recommended prerequisites are the ultrasonic ones — 40 hours of training beyond Level I, 630 hours of ultrasonic experience and 1,200 total NDT hours — with the technique training layered on top of that base.

Question 2. What is actually stored inside a single focal law? Answer: a set of per-element transmit delays and per-element receive delays, the list of elements forming the active aperture, and the amplitude weighting applied to each element. Those delays are computed from probe pitch, element count and frequency, wedge angle, wedge velocity and wedge height, and the velocity of the material. Change one of those inputs and every delay in every law shifts. That is why an incorrect wedge height entry does not produce an obviously broken image. It produces a plausible image pointing at the wrong place.

The examination that decides your competence is the practical one, and it is graded on whether the set-up survives scrutiny. Our plain-English walkthrough of ASME Section V Article 4 UT requirements covers the code layer these questions sit on. Phased array specifically is addressed in Article 4 through Mandatory Appendix IV for manual raster techniques and Mandatory Appendix V for E-scan and S-scan linear scanning, with Mandatory Appendix III covering TOFD and Mandatory Appendix XI covering full matrix capture.

Questions 3-5: Focal laws, steering and aperture

Question 3. A 0.6 mm pitch array is coupled directly to steel with no wedge, and the instrument applies a 100 ns delay increment between adjacent elements. What shear angle does that steer to? Answer: 32.6 degrees. The steering relationship is the sine of the angle equals velocity times the inter-element delay, divided by the pitch. Working it: 3,230 multiplied by 100 nanoseconds gives 0.000323, divided by 0.0006 gives 0.538, and the arcsine of 0.538 is 32.6 degrees. With a wedge in the path the same principle applies inside the wedge and the result then refracts through Snell's law at the wedge-to-steel interface, which is why wedge velocity is a mandatory input.

Question 4. A probe has 0.5 mm wide elements. Working at 5 MHz in shear, how far can it steer before sensitivity collapses? Answer: about 42 degrees either side of the wedge's natural angle. The minus 6 dB steering limit follows element directivity, where the sine of the limit equals 0.514 times the wavelength divided by the element width. Shear wavelength is 3,230 divided by 5,000,000, or 0.646 mm, so the sine is 0.514 times 0.646 divided by 0.5, giving 0.664 and an angle of 41.6 degrees. Evaluated inside the wedge the raw number is smaller and then refracts outward, which is why the limit is confirmed on a block rather than trusted from arithmetic.

Question 5. Sixteen elements at 0.6 mm pitch, 5 MHz shear. Can you focus at 60 mm depth? Answer: no. Focusing is achievable only within the near field of the active aperture, approximated by aperture squared times frequency, divided by four times velocity. A 9.6 mm aperture gives 0.0096 squared multiplied by 5,000,000, which is 460.8, divided by four times 3,230, which is 12,920 — a near field of 35.7 mm. Reaching 60 mm needs an aperture of at least 12.5 mm, or 21 elements at this pitch. Asked to focus beyond the near field, the instrument calculates a focal law that produces a beam which never converges.

Questions 6-7: Wedge delay and velocity calibration

Question 6. Why is wedge delay calibrated for every focal law rather than once for the probe? Answer: because every refracted angle takes a different path through the wedge. Wedge delay calibration compensates that path so measured depth and sound path are referenced to the exit point in the part rather than to the element face. On a sectorial scan spanning 40 to 70 degrees the shortest and longest wedge paths differ by a meaningful fraction of a millimetre of steel-equivalent depth. Left uncorrected, one reflector plots at a different depth on every angle and the S-scan smears it into an arc.

The calibration is performed on a radius, and the 100 mm arc of an IIW Type 1 block is the classic choice, because every refracted angle sees an identical metal path to the arc surface. The instrument peaks each focal law in turn, measures the discrepancy, and stores a delay for that law. Verify afterwards on a known reflector at a known depth. A wedge delay captured with a worn wedge or a poor couplant film is not detectably wrong on the calibration block itself, and only reveals itself when a real flaw is mis-depthed on a real weld.

Question 7. Focal laws were built for a shear velocity of 3,230 m/s. The part measures 3,100 m/s. What happens to a nominal 60 degree beam? Answer: it lands at about 56 degrees. A Rexolite wedge runs near 2,330 m/s, so 60 degrees in steel corresponds to 38.7 degrees inside the wedge. Keeping that wedge angle and substituting 3,100 m/s gives a sine of 0.625 multiplied by 3,100 divided by 2,330, which is 0.831, or 56.2 degrees. A 4% velocity error moved the beam nearly 4 degrees, which through 25 mm of weld thickness misplaces the reflector by around 6 mm along the surface.

Questions 8-9: TCG, DAC and angle-corrected gain

Question 8. Why does phased array use TCG where conventional ultrasonics uses a DAC curve? Answer: because a DAC curve is drawn for one beam. DAC fixes the gain and draws a falling curve on the display, and the operator compares each echo to the curve height at that sound path. TCG instead raises receiver gain as a function of time so equal reflectors at every depth display at the same height, turning the reference into a flat line across the screen. A sectorial scan contains dozens of beams, each with its own attenuation path and its own steering loss, so TCG is built per focal law and DAC has nothing to attach to.

Question 9. Your S-scan is TCG-calibrated and a side-drilled hole still reads 12 dB lower at 70 degrees than at 45 degrees. What is missing? Answer: angle-corrected gain. TCG corrects amplitude against depth; ACG corrects amplitude against angle. Sensitivity falls at the extremes of a sweep because element directivity drops away, the wedge path lengthens, and the transmission coefficient at the wedge-to-steel interface changes with incidence. ACG is built by directing the beam at a radiused surface so every focal law sees an identical reflector at an identical metal path, and the instrument then equalises each law's receiver gain.

Both calibrations anchor to the same reflectors as conventional ultrasonics. The ASME basic calibration block, Figure T-434.2.1 in Section V Article 4, carries side-drilled holes at one quarter, one half and three quarters of the block thickness, plus notches 2% of thickness deep and at least 1 in long, with hole diameter between 3/32 in and 3/16 in according to block thickness. Peak the reference hole to 80% full screen height, record that gain as the reference level, and build the TCG from the remaining holes without touching the gain again.

Questions 10-11: Sectorial versus linear scanning

Question 10. When do you choose an E-scan over an S-scan? Answer: when the geometry is thick and parallel-sided and you need constant beam characteristics across the whole bevel. An E-scan holds one refracted angle and steps the active aperture electronically along the array, so every beam is identical in focus, sensitivity and steering loss. It is a mechanical raster performed at electronic speed. An S-scan holds the aperture fixed and sweeps through angles, so no two beams share the same characteristics, but a single probe position interrogates a reflector from many directions at once.

Question 11. A narrow-gap weld in 40 mm plate is inspected with an S-scan from 40 to 70 degrees at one index offset. What is the likely failure? Answer: the root and the upper bevel fall outside the covered volume. A single S-scan from one offset covers a fan, not a rectangle. The fan narrows against the far wall on one side and against the near-surface dead band on the other. The fix is multiple index offsets, an E-scan at a fixed angle matched to the bevel, or both — settled on a scan plan before anyone couples a probe to the part.

Questions 12-13: Encoders, scan plans and provable coverage

Question 12. What does an encoder change about the data, beyond convenience? Answer: it converts a live image into a positional record that can be re-analysed and audited months later. Without an encoder the C-scan axis is time, not distance, so no indication carries a location that survives the shift. ASME Section V Article 4 requires a scan plan for encoded examination, and that plan must depict the required examination volume coverage. Encoder calibration is its own step: travel a known distance, confirm the readout, and repeat after any wheel or belt change, because an encoder error scales every length you report.

Question 13. How do you prove coverage rather than assert it? Answer: with a scan plan showing the beam envelope of every focal law against the required examination volume, a calibration block demonstration at the extremes of the depth range, and recorded index offsets that match the plan. Simulation on its own is not proof; the block confirms that the beams the software drew actually exist at the sensitivity the procedure claims. Procedure development is where this is settled once rather than argued every job — see NDT technical procedure development.

Questions 14-15: Grating lobes, wedge noise and mode conversion

Question 14. A 1.0 mm pitch array working at 5 MHz in shear shows a second indication 40 degrees away from a known reflector. What is it? Answer: a grating lobe. Grating lobes appear where the sine of the off-axis angle equals an integer multiple of the wavelength divided by the pitch. With a wavelength of 0.646 mm and a pitch of 1.0 mm, the first order sits at 40.2 degrees off the main beam. Reduce the pitch to 0.3 mm and that ratio exceeds 1, so no real solution exists and no grating lobe forms. A pitch at or below half a wavelength, 0.32 mm here, guarantees none at any steering angle.

Question 15. An indication sits at short sound path, appears on the low-angle laws only, and does not move as you translate the probe along the weld. What is it, and how do you confirm? Answer: a wedge echo, sound reflecting inside the wedge and returning through the array. Confirm by lifting the probe off the part — a wedge echo persists, a part reflector disappears. Wedges with attenuating backing and machined serrations suppress it, and the gate is set to start beyond it. Mode-converted longitudinal signals behave differently: they move with the probe but plot shallower than the true reflector, because the instrument applies shear velocity to a faster wave.

Every one of these artefacts is separable by a single habit: move something and watch what the indication does. Translate the probe, change the index offset, narrow the sweep, lift the wedge. A real reflector moves with the geometry in the way the scan plan predicts. Artefacts move in ways it does not, or refuse to move at all. Writing that reasoning into the report is what makes the call reviewable a year later, and our note on what makes an NDT report defensible sets out the structure.

What the phased array band is worth

Phased array sits at the top of the ultrasonic pay band because it compresses inspection time and produces auditable data, and because the population of technicians who can build a defensible scan plan is far smaller than the population who can drive the instrument. The US market pays for the first skill, not the second. Current figures are broken down on the NDT Level II salary page, and the step above it on the NDT Level III salary page.

The Level III step is where you stop executing procedures and start writing, qualifying and signing them. That is also where phased array money concentrates, because a written procedure that survives a client's technical audit is worth more than a scan. Working through the UT Level II practice questions first is the sensible order — phased array does not replace conventional ultrasonic reasoning, it multiplies that reasoning across forty beams at once and punishes any gap in it forty times over.

Atlantis delivers ASNT method training and phased array technique training for technicians and teams. Affordable, accessible, fully customizable. If your phased array procedures already exist but have never been challenged, an independent review against Article 4 and its Mandatory Appendices is faster than rewriting them, and outsourced ASNT Level III consulting covers exactly that. Ask for a demo or a quote through contact.

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