Key takeaways
- PAUT images the whole weld volume with steered beams and stores every scan, so results can be reviewed later.
- TOFD gives the most accurate through-wall height of a flaw, but has blind zones close to the surfaces.
- RT shows volumetric flaws such as porosity and slag very clearly and leaves a permanent image, but needs radiation safety and access from both sides.
- On most modern projects the answer is a combination: PAUT + TOFD for thick welds, RT where the code or the client requires it.
Choosing between PAUT vs TOFD vs RT is one of the first decisions in any weld inspection plan. The method you pick decides how quickly you get results, which flaws you will actually find, whether work around the weld has to stop, and how much documentation you hand over at the end. This guide compares the three methods on the criteria that matter on site, so you can agree the right approach with your client, your third-party inspector and your NDT contractor before welding starts.
PAUT vs TOFD vs RT: how each method works
Phased array ultrasonic testing (PAUT)
PAUT uses a probe made of many small piezoelectric elements. By firing the elements with precise time delays, the instrument steers and focuses the ultrasonic beam through a range of angles without moving the probe. The result is a sectorial scan: a cross-sectional image of the weld that an operator can read almost like a drawing. Because the data is recorded, every indication can be re-analysed, measured and audited after the inspection. Read more on our phased array testing service.
Time of flight diffraction (TOFD)
TOFD places a transmitter and a receiver on either side of the weld. Instead of relying on how strong a reflection is, it measures the time it takes for signals diffracted from the upper and lower tips of a flaw to reach the receiver. That timing gives a very accurate measurement of the flaw’s height through the wall, largely independent of its orientation. The weakness is a dead zone near the scanning surface and near the back wall, which is why TOFD is usually paired with PAUT. See our TOFD inspection page.
Radiographic testing (RT)
RT sends X-rays or gamma rays through the weld onto film or a digital detector. Denser or thinner areas show up as differences in grey level, which makes porosity, slag inclusions and lack of penetration easy to recognise. The radiograph is a permanent record that any inspector can review. The trade-offs are radiation safety, controlled areas that stop other work, and access from both sides of the component. Learn more about radiographic testing personnel.
Comparison table: PAUT vs TOFD vs RT
| Criterion | PAUT | TOFD | RT |
|---|---|---|---|
| Best at detecting | Planar flaws (lack of fusion, cracks) at many angles | Planar flaws, accurate height sizing | Volumetric flaws (porosity, slag, incomplete penetration) |
| Flaw height sizing | Good | Very good | Not possible from a single image |
| Access needed | One side | One surface, both sides of the weld cap | Both sides (source and detector) |
| Radiation hazard | No | No | Yes, controlled area required |
| Speed on long welds | Fast | Very fast (single pass) | Slower, exposure per section |
| Record | Digital, re-analysable | Digital, re-analysable | Film or digital image |
| Near-surface coverage | Good | Limited (dead zones) | Good |
Which flaws each method finds best
The flaws that matter most for the integrity of a weld are planar: lack of sidewall fusion and cracks. Ultrasonic methods detect them well because the beam reflects or diffracts strongly from a flat surface. Radiography can miss a tight planar flaw if the radiation beam is not aligned with it.
Volumetric flaws are the opposite case. Porosity and slag inclusions appear clearly on a radiograph but give weaker, scattered ultrasonic signals. They rarely govern the strength of a weld, but many codes still limit them, which is one reason RT remains common on pressure equipment.
Standards and acceptance criteria
Each method has its own ISO technique standard and its own acceptance levels, so the method must be chosen together with the acceptance criteria in the specification:
- PAUT: technique in ISO 13588, acceptance levels in ISO 19285.
- TOFD: technique in ISO 10863, acceptance levels in ISO 15626.
- RT: technique in ISO 17636-1 (film) and ISO 17636-2 (digital), acceptance levels in ISO 10675-1.
- Manual UT: technique in ISO 17640, acceptance levels in ISO 11666.
Projects built to ASME codes follow ASME Section V for the techniques, and the construction code states when ultrasonic testing may replace radiography. Personnel performing and interpreting any of these methods must be certified, typically to ISO 9712. The TWI knowledge base is a good independent reference for the background of each technique.
When to choose PAUT, TOFD or RT
- Pipeline girth welds in production: automated ultrasonic testing combining PAUT and TOFD keeps pace with welding and gives results within minutes. See AUT for pipelines.
- Thick-walled pressure equipment and structural nodes: PAUT + TOFD, where the code allows ultrasonic testing, for accurate sizing and no radiation downtime.
- Thin walls, small diameters and complex geometry: RT is often the practical choice, or manual UT where access allows.
- Busy sites and offshore: ultrasonic methods avoid controlled areas, so other trades can keep working.
- Client or code requires radiographs: RT, performed by qualified operators under the site’s radiation protection arrangements.
Practical tip: agree the NDT method, extent and acceptance criteria in the Inspection and Test Plan before production welding starts. Changing method halfway through a project usually means re-inspecting welds that were already accepted.
Cost and schedule considerations
RT has a low set-up cost, but each exposure takes time and the controlled area can stop other work nearby. PAUT and TOFD need more expensive equipment and a qualified set-up, but they inspect long welds quickly, need no evacuation and produce data that can be reviewed remotely. On large projects the saved schedule usually outweighs the higher daily rate of an advanced ultrasonic crew.
WeldInPro provides NDT personnel certified to ISO 9712 Level 2 and Level 3, onshore and offshore, with AUT equipment available for pipelines.
Frequently asked questions
Can PAUT replace radiography?
Often, yes. Many construction codes allow ultrasonic testing in place of radiography when the procedure is qualified and the acceptance criteria are defined. The specification and the client must approve the substitution before inspection starts.
Is TOFD enough on its own?
Usually not. TOFD has dead zones near the scanning surface and the back wall, so it is normally combined with PAUT or pulse-echo UT to cover the whole weld volume.
Which method is the fastest?
TOFD and automated PAUT are the fastest on long straight welds and girth welds, because a single scan covers the whole weld length and results are available immediately.
What certification should NDT operators have?
Operators should hold a valid certificate for the method and industry sector, usually to ISO 9712: Level 2 to perform and interpret tests, Level 3 to approve procedures and techniques.
