2026-07-07 · EN
What Is Phased Array Ultrasonic Testing (PAUT)? How It Differs from Conventional UT
Phased array ultrasonic testing (PAUT) is an advanced form of ultrasonic examination (UT). Conventional UT works with a single crystal; a PAUT probe carries many small elements instead. The instrument fires those elements electronically, with controlled delays, to steer, focus, and sweep the sound beam. The result is a faster examination with broader coverage and a permanent record.
1. How it works: phase delay
A PAUT probe typically carries 16, 32, or 64 small piezoelectric elements. The instrument does not fire them at the same instant. It fires them with delays on the order of nanoseconds — typically tens to hundreds of ns. Each element sends out its own wavefront, and the wavefronts combine into a single beam. Change the delay law and you can:
- steer the angle (direction) of the beam electronically,
- adjust the focal depth,
- cover different zones without moving the probe.
In shear wave applications the array sits on a wedge. Electronic steering works within a range around the nominal refracted angle of that wedge, and steering too far from it costs beam sensitivity. Conventional UT changes the beam direction only by swapping probes of different angles. PAUT does all of it with one probe, electronically.
2. Scan types and imaging
PAUT produces cross-sectional images that go well beyond the conventional A-scan. The beam-forming modes come first, then the data displays:
- Sector scan (S-scan): The beam sweeps through an angular range, for example 40°–70°, and produces a fan-shaped image of the weld cross-section.
- Linear/electronic scan (E-scan): The beam holds a fixed angle and steps electronically along the length of the probe.
- Additional displays (C-scan, D-scan): A C-scan gives a plan-view area map; a D-scan gives a side view along the scan direction. Both views map the position and the size of a discontinuity.
Add an encoder (position sensor) and the instrument records all data together with probe position. That is what makes the examination reviewable and auditable.
3. Differences from conventional UT
| Feature | Conventional UT | PAUT |
|---|---|---|
| Probe | Single crystal | Multi-element array |
| Beam angle | Fixed (change the probe) | Electronic, adjustable |
| Imaging | A-scan | Sector/cross-sectional image |
| Recording | Limited | Full data recording (with encoder) |
| Speed | Slower | Wide coverage in a single pass |
| Operator interpretation | Heavy | Supported by imaging, but setup and interpretation demand more expertise |
4. Advantages
- Speed: One pass covers a large volume at multiple angles.
- Recordability: The data is stored, so a second specialist can review it later.
- Visualization: The cross-sectional image shows the position and the size of a discontinuity clearly.
- Match to the weld preparation: Select the sector range to suit the joint geometry (the bevel angle) and scan perpendicular to the fusion face. In narrow-gap welds this improves detection of sidewall lack of fusion (LOF) running parallel to the bevel.
- Access: A small probe works in narrow or restricted areas.
- Repeatability: Examinations run with the same setup can be compared directly.
5. Typical applications
- Weld seam examination, in fabrication and in service
- Corrosion mapping and vessel wall thickness scanning
- Nozzles, nodal joints, and complex geometries
- Automated girth weld examination on pipelines, usually together with TOFD; automated pipeline UT runs under API 1104
6. Calibration and verification
Calibration in PAUT means verifying angle, delay law, and sensitivity on reference blocks. TCG (Time-Corrected Gain) compensates for the amplitude loss that comes with depth. In sector scanning each angle travels a different sound path, so on top of TCG you also run angle-corrected gain (ACG) and wedge delay calibration. Run an element check before the examination to confirm that every element fires; dead elements degrade beam quality.
7. Related standards
- ASME BPVC Section V, Article 4: Ultrasonic examination; the relevant Mandatory Appendix provisions apply to PAUT (E-scan/S-scan). The appendix number can change with the edition in use.
- EN ISO 13588: Ultrasonic testing of welds — phased array technique.
- EN ISO 19285: Acceptance levels for phased array ultrasonic testing.
Acceptance criteria come from the construction or in-service code that governs the examination.
From the field
PAUT is a powerful method, but more data does not automatically mean a more accurate answer. A wrong delay law, the wrong wedge angle, or an incomplete calibration will hide a real discontinuity under a good-looking image. The equipment is not what creates the value. A properly built examination plan and a competent operator are.
This article is for educational purposes and does not replace the official standard, the code, or the written examination procedure — the final decision always rests with the code in force and the responsible inspector.
Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.
At DoaWise we carry out advanced non-destructive testing services, including phased array ultrasonic testing (PAUT) and TOFD, to international standards. Our weld examination and corrosion mapping work produces recordable, auditable results.