Phased Array Ultrasonic Testing (PAUT) and Time-of-Flight Diffraction (TOFD) are the two methods that quietly replaced radiography on most modern weld inspection programs. They are both advanced ultrasonic. They both produce permanent digital records. They both qualify against ASME, API, and CSA codes. And they answer fundamentally different questions.
If you treat them as interchangeable, you will overspend on the wrong scope, miss critical flaws, or fail an audit because the procedure could not detect what the weld actually contained. The decision matters.
This is the field-level guide we hand to project managers, fabricators, and integrity engineers when they ask which method to write into the QA plan — and why our crews on jobs like the Wood Fibre LNG powerhouse run both on the same weld.
The 60-second definition
PAUT — Phased Array Ultrasonic Testing. A multi-element probe (typically 16, 32, or 64 elements) that electronically steers an ultrasonic beam through a range of angles in a single pass. Produces sectorial scans (S-scans), linear scans, and encoded data. Excellent at detecting and characterizing discontinuities anywhere in the weld volume.
TOFD — Time-of-Flight Diffraction. A pair of probes (one transmit, one receive) that capture diffracted signals from the tips of discontinuities, not their reflective surfaces. Produces a grayscale D-scan image. Excellent at sizing through-wall flaws — the most accurate UT method available for through-wall measurement.
Both are encoded. Both produce auditable digital records. Both are governed under ASME Section V, ASME Code Case 2235 (UT in lieu of RT), API 1104, CSA W59, and CSA Z662 with the right written procedure.
Where each one wins decisively
PAUT wins when you need full-volume detection and characterization
- Pipe and vessel butt welds with thickness 6 mm and up
- Geometric or position-restricted welds — angle steering reaches what a single-angle UT cannot
- Coverage of the heat-affected zone with multiple beam angles in one pass
- Production environments where scan speed and one-sided access matter (PAUT typically scans 3–5x faster than encoded conventional UT)
- Flaw characterization — sectorial imaging shows shape, orientation, and location, which conventional UT inferred from amplitude
TOFD wins when sizing is the deciding question
- Through-wall sizing accuracy of 0.3–1.0 mm — better than any other UT method
- Detection of mid-wall planar flaws that PAUT angle limits can miss
- Fast volumetric coverage of long welds (TOFD can survey at >100 mm/s)
- In-service flaw monitoring — you can re-shoot the same weld and measure crack growth over time
When you need both
The reason most modern code-driven programs run PAUT + TOFD as a combined scope is that the two methods are complementary, not duplicative. PAUT finds and characterizes. TOFD verifies through-wall size with the highest accuracy available. Together they exceed the probability-of-detection of either method alone — and they exceed RT for almost every realistic flaw type without the radiation safety footprint, the shutdown of adjacent work, or the wait for film processing.
ASME Code Case 2235 specifically allows UT in lieu of RT when these methods are applied under qualified procedures. API 1104 Annex A recognizes mechanized UT (PAUT and TOFD) for new pipeline construction. CSA Z662 cross-references the same guidance. The standards landscape is settled. The execution discipline is what separates programs.
Field notes from a working LNG site
On the Wood Fibre LNG powerhouse scope, PSC's NDT team works to a project-specific QA plan inside an active EPC schedule, with code-driven advanced UT methods on structural and piping welds. A few patterns we see again and again on this kind of work:
- Procedure qualification is the long pole. The biggest delays on large LNG, pipeline, and pressure-equipment projects are not scanning time — they are getting the PAUT and TOFD procedures qualified to the project specification, the applicable code, and the client's QA gate. Plan for this in the schedule.
- CSWIP Level 3 oversight matters at the procedure stage. PAUT and TOFD give a Level 2 inspector the ability to collect rich data; a Level 3 specifies the focal laws, beam coverage, scan plan, and acceptance criteria that make the data defensible.
- Encoded scans pay for themselves on revisions. If a weld is rejected and reworked, the original encoded data is still in the file. The repair scan compares directly. No "trust me, it was different" arguments.
- TOFD has a near-surface dead zone. Always pair with PAUT or surface methods (MT, PT) for the first few millimeters under the cap and root.
- Couplant and surface prep are still the limiting factors. No amount of advanced UT compensates for a surface the probe cannot couple to.
These are the kinds of decisions that happen before the scanner ever touches metal — and they are the difference between a clean handover and a contested one.
How to specify the right scope on your project
A practical decision framework:
- What is the code? ASME, API, CSA, or project-specific? The code drives method eligibility and acceptance criteria.
- What is the wall thickness? Below 6 mm, conventional UT or radiography may still be the right call. Above 25 mm, TOFD becomes essential for sizing.
- What is the flaw of concern? Planar (cracks, lack of fusion) → TOFD-leading. Volumetric (porosity, slag) → PAUT-leading.
- What is the production rate? High weld counts on a tight schedule favor mechanized PAUT + TOFD over manual conventional UT.
- What is the audit requirement? If the client wants permanent digital records and re-shoot capability, encoded PAUT and TOFD are the answer.
When in doubt, get a Level 3 inspector on the call before the procedure goes for client review. The cost of getting the procedure right at the start is a fraction of the cost of getting it wrong on the first weld.
Why PSC's PAUT/TOFD scope is structured the way it is
PSC delivers PAUT and TOFD under CSWIP Level 3 oversight, with welding engineering support integrated at the procedure stage. That means the same team that writes the WPS and reviews the PQR is in the room when the PAUT scan plan is built. The advantage is operational: when an indication needs an engineering call — fitness-for-service, repair specification, or accept/reject under code — the answer comes from one team, not three.
Our advanced UT scope feeds the same programs as our ground-based and drone inspection services, and the same project deliverables as our Wood Fibre LNG and other major projects.
Frequently asked questions
Can PAUT or TOFD fully replace radiographic testing? For most weld geometries above 6 mm wall thickness, yes — under ASME Code Case 2235 and equivalent provisions in API 1104 and CSA Z662, with qualified procedures. The advantages over RT are no radiation hazard, no shutdown of adjacent work, faster turnaround, and permanent digital records.
Is PAUT alone enough, or do I need TOFD too? PAUT is excellent at detection. TOFD is the most accurate UT sizing method available. For high-consequence assets (pipelines, pressure vessels, LNG/process piping) the best practice is both — PAUT for coverage and characterization, TOFD for through-wall sizing.
What thickness range does TOFD work in? Practically 6 mm to 300+ mm. Below 6 mm the dead zones at the surfaces consume too much of the wall to be useful. Above 25 mm, TOFD becomes the dominant sizing method.
Do you need a Level 3 to interpret PAUT and TOFD? A Level 2 can operate the equipment and report under qualified procedures. A Level 3 writes the procedure, qualifies it, and signs off interpretation on contested indications. Most serious programs require Level 3 oversight on the procedure stage.
Need PAUT, TOFD, or a combined scope under a tight schedule? PSC's Level 3 inspectors are based in Coquitlam, BC and mobilize across Canada. Talk to a Level 3 inspector about your scope, your code, and your schedule.
PSC delivers PAUT and TOFD under CSWIP Level 3 oversight, with engineering support integrated from procedure qualification through field execution.
.png)
%20(1).png)
