Phased Array Ultrasonic System

What is a Phased Array Ultrasonic System

A phased array ultrasonic system is an electronic non-destructive testing instrument that excites a probe made of many small ultrasonic elements, each pulsed individually with a precisely computed time delay. The set of delays applied to the elements is called a focal law. By advancing or retarding the firing of each element by fractions of a microsecond, the combined wavefront can be tilted to a chosen refraction angle, focused to a chosen depth, or scanned along the probe length, all without any mechanical movement of the transducer. The same delay pattern is applied in reverse on reception, so the array also listens preferentially along the steered, focused direction.

This electronic beam control is the single feature that separates PAUT from conventional single-element ultrasonic testing. A conventional flaw detector fires one fixed beam at one angle set by a fixed wedge, so covering a weld across a range of angles requires several separate probes and multiple manual passes. A phased array probe sweeps an entire fan of angles, for example 40 to 70 degrees, from one position in one pass, and renders the result as a cross-sectional image rather than a single oscilloscope trace. The aperture, meaning the active group of elements used at any instant, sets how much the beam can be steered and focused: a larger aperture with more active elements gives a wider steering range and a deeper usable focal range.

The physics of ultrasonic flaw detection is older than the electronics. Pulse-echo ultrasonic testing emerged in the 1940s, and medical phased array imaging matured through the 1970s and 1980s. Industrial phased array remained costly and laboratory-bound until portable, battery-powered field instruments arrived in the early 2000s, after which PAUT spread quickly into oil and gas, power generation, pipeline, aerospace, and structural fabrication. The technique is now standardized: ASME, ASTM, ISO, AWS, and API all publish PAUT requirements, and encoded PAUT has become the default for documented, replayable weld examination where a recordable archive is required.

A complete phased array ultrasonic system is more than the handheld unit. It comprises the instrument, which contains the pulser and receiver electronics, focal-law processor, and display; the probe, which holds the element array; the wedge, which couples the beam into the part and sets the nominal angle; and, for documented work, an encoder and scanner that record probe position. Software ties these together by generating a scan plan that maps the focal laws to the weld geometry, then displaying the live A-scan, S-scan, and C-scan views during inspection.

Four engineering attributes determine whether a given PAUT system fits a job: the channel configuration (how many elements the instrument can pulse and receive at once), the imaging modes it supports (sectorial, linear, TOFD, and TFM), the probe and wedge range it can drive, and the recordability and traceability of its data. These attributes, not headline price, govern whether the system can satisfy the applicable code and produce an archive that survives third-party review years after the inspection.

Scan Types and Imaging Modes

A phased array system is defined as much by the images it produces as by its hardware. The instrument processes raw element signals into several standardized views, and a single examination often runs more than one mode at once. The table below summarizes the main scan and imaging modes, how the beam is controlled, and where each is used.

The A-scan is the fundamental signal: amplitude of the returned echo plotted against time of flight, which the instrument converts to depth once the material sound velocity is known. Every higher-level image is built from many A-scans. An inspector still reads the live A-scan to set gates, gain, and time-corrected gain, even when the primary record is an image.

The sectorial scan, or S-scan, holds the active aperture fixed and increments the beam angle in small steps, sweeping a fan-shaped cross section through the part. Because the fan covers many angles from one stationary probe position, the S-scan is the workhorse view for weld inspection: a single placement at the correct standoff can interrogate the root, fill, and cap of a bevel weld across the full angular range. Sizing on an S-scan uses the cursor against a calibrated angular and depth grid.

The electronic scan, or E-scan (also called a linear scan), keeps the refraction angle constant and multiplexes a fixed-size aperture element by element along the length of a long array, electronically raster scanning a rectangular zone without moving the probe. This suits thick-section welds where a single fixed angle is swept along the weld axis, and ASME Section V Article 4 Mandatory Appendices describe both encoded E-scan and S-scan procedures.

The C-scan is a plan-view map built by combining beam data with encoded probe position, presenting amplitude or depth as a color map over the scanned surface. It is the standard deliverable for corrosion mapping and composite inspection. TOFD, time-of-flight diffraction, is a distinct two-probe pitch-catch technique that measures tip-diffracted signals to size the through-wall height of a flaw with high accuracy, and is very commonly paired with PAUT so the S-scan detects and locates while TOFD sizes. TFM, the total focusing method, is covered in Chapter 3 because it depends on a specific acquisition mode.

Probes, Wedges, and Instrument Hardware

The performance envelope of a phased array system is set jointly by the probe, the wedge, and the instrument channel architecture. Choosing the wrong combination is the most common cause of an inspection that cannot reach the required coverage or sensitivity. The table below compares the principal probe and instrument variables and their selection consequences.

Probe frequency is the primary penetration-versus-resolution trade. Frequencies near 1 to 2.25 MHz penetrate thick, attenuative, or coarse-grained material such as cast and austenitic stainless steel, while frequencies near 5 to 10 MHz give finer near-surface resolution for thin sections and small defects. General carbon-steel weld inspection most often uses 5 MHz. Standard angle-beam phased array probes span roughly 0.5 to 18 MHz with element counts from about 10 to 128; a representative weld probe might carry 32 elements at 0.5 mm pitch with a 10 mm passive aperture (elevation).

The wedge is not an accessory but part of the beam model. For shear-wave weld inspection the array is mounted on a plastic wedge, typically cross-linked polystyrene (Rexolite), that couples the longitudinal wave from the elements into the part and, through mode conversion at the interface, produces the refracted shear wave at the design angle. The wedge fixes the standoff, the first index point, and the angular window over which the focal laws stay valid, so the probe, wedge, and scan plan must be specified as a matched set.

Instrument channel configuration is written as pulser-to-receiver, for example 16:64PR, 16:128PR, or 32:128PR on the Evident OmniScan X3. The first number is how many elements can be pulsed simultaneously and the second is the total addressable aperture across which that group can be multiplexed; a 32:128 instrument can therefore slide a 32-element aperture across a 128-element probe. The OmniScan X3 also provides two conventional UT channels for TOFD or pulse-echo and integrates TOFD into the same scan. Pulser voltage on that platform is selectable in steps of 10, 20, 40, 80, 120, and 160 Vpp bipolar square pulse, over a usable frequency band of about 0.2 to 26.5 MHz, with a maximum pulse width near 1,000 ns.

TFM and FMC push the hardware hardest. Full matrix capture (FMC) fires each element individually while every element receives, so a 64-element probe yields 64 firings and 4,096 element-to-element A-scans per position. The total focusing method (TFM) then synthetically focuses at every pixel of the image region, giving an image that is optimally resolved across the whole frame rather than only near a programmed focal depth as in standard PAUT. The data and compute load are large: the OmniScan X3 reconstructs TFM frames up to 1,024 by 1,024 pixels with a 64-element aperture and can run multiple live propagation modes at once. TFM improves small-flaw resolution and characterization but demands more instrument capability and disciplined setup.

Standards, Codes, and Acceptance

Phased array inspection is a regulated activity. The applicable code dictates how the technique is qualified, how the equipment and probes are characterized, how the scan plan is built, and how indications are accepted or rejected. Specifying a system without first identifying the governing code is the most expensive mistake in the procurement chain, because an instrument that cannot produce the required encoded, traceable record may pass the demonstration yet fail the code audit. The table below maps the principal PAUT standards to their scope.

In the ASME framework, phased array of pressure-retaining welds is performed under ASME Boiler and Pressure Vessel Code Section V, Article 4. Mandatory Appendix IV and Mandatory Appendix V set out the requirements for manual raster, encoded electronic E-scan, and sectorial S-scan examination, and the code defers much of the contact technique detail to ASTM E2700, Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays. ASTM E2700-20 gives worked example scan setups for thin butt welds, thick butt welds, corner joints, and T-joints, and addresses shear-wave angle selection and index offset so the focal laws cover the full weld volume and heat-affected zones.

In the ISO framework, the technique itself is run to ISO 13588, which defines PAUT techniques, examination levels, and the basis for assessment of indications. Acceptance for full-penetration welds in ferritic steel of 6 mm minimum thickness is judged against ISO 19285:2017, whose acceptance levels correspond to the weld quality levels of ISO 5817, with indications classified per ISO 13588. The equipment and probes used must be characterized and verified to the ISO 18563 series (instruments, probes, and combined systems). A typical European project procedure therefore cites ISO 13588 together with ISO 18563 and ISO 19285 as a set rather than any one in isolation.

Sector-specific codes extend coverage beyond pressure vessels. AWS D1.1, the structural welding code for steel, includes an annex recognizing phased array as an alternative ultrasonic method for structural welds. API 1104, governing welding of pipelines and related facilities, recognizes automated and phased array ultrasonic examination for girth welds, frequently paired with engineering critical assessment and alternative acceptance criteria based on flaw height. NORSOK and several nuclear in-service inspection codes add their own qualification regimes, and offshore and nuclear work commonly require formal technique qualification on representative test blocks before any production inspection.

The practical consequence for a buyer is that the code drives the feature list. A code that mandates an encoded, replayable record rules out unencoded manual-only operation; a code that requires TOFD sizing demands an instrument with conventional UT channels; and a code that requires probe characterization to ISO 18563 obliges the supplier to provide traceable characterization data. Confirm the governing code before comparing instrument feature sets, not after.

Key Specification Parameters

Reading a phased array datasheet is a core procurement skill. Spec sheets list many numbers, but a manageable set actually drives selection: channel configuration, conventional UT channels, pulser voltage and frequency band, focal-law and PRF capacity, imaging modes, display, encoder support, and environmental rating. The table below decodes the headline specifications of a representative portable instrument, the Evident OmniScan X3, as a worked reference; verify the exact figures for any specific model against the current manufacturer datasheet.

Channel configuration is the first number that matters, written pulser-to-receiver such as 16:64PR or 32:128PR. The pulse number caps how large an active aperture can fire at once, and the aperture number caps the total array length addressable. A larger pulse-to-aperture ratio lets the system drive longer probes, sweep wider, and supply the aperture that TFM needs. Note that on some platforms the entry configuration limits how many simultaneous groups, such as TOFD, PA, and TFM, can run together, so confirm the group budget against your scan plan.

Conventional UT channels determine whether the unit can run TOFD or single-element pulse-echo alongside the array. An instrument with two UT channels can pair a transmit and receive probe for TOFD in the same acquisition as the PAUT S-scan, which is what most weld procedures require for combined detection and sizing.

Pulser voltage and frequency band set the drive envelope. Selectable bipolar square-pulse voltage, for example 10, 20, 40, 80, 120, and 160 Vpp, trades penetration against electronics stress and resolution; thick or attenuative material wants higher voltage and lower frequency. The usable band, around 0.2 to 26.5 MHz on the reference instrument, must encompass the probe frequency, and the maximum pulse width near 1,000 ns matters for driving low-frequency probes efficiently.

Imaging modes and TFM resolution describe what the instrument can render. Confirm that the unit supports the modes your code requires, including encoded S-scan, E-scan, TOFD, and, where specified, FMC and TFM. For TFM, the reconstruction grid (up to 1,024 by 1,024 pixels on the reference instrument), the supported aperture, and the number of simultaneous live propagation modes set both image quality and frame rate.

Encoder, display, and environment close the list. Documented examinations need encoder inputs (commonly one to three axes) to tie data to position; the field display size and brightness govern usability outdoors; and the ingress and temperature ratings determine where the unit can work. Battery runtime, weight, and data export format round out a practical comparison, since a lightweight encoded-capable unit such as the Eddyfi Mantis at roughly 3.6 kg (about 8 lb) is far easier to deploy on scaffolding than a rack instrument.

Selection Decision Factors

To turn the preceding chapters into a specific instrument and probe set, work the decision sequence below in order. Most selection failures come not from one wrong answer but from deciding hardware before the code and the scan plan are fixed. These steps can serve as a fixed RFQ template.

1. Governing code and acceptance basis: Identify the applicable code first, ASME Section V Article 4, ASTM E2700, ISO 13588 with ISO 19285, AWS D1.1, or API 1104, and confirm whether it mandates encoded data, TOFD sizing, and probe characterization to ISO 18563. The code defines the minimum feature set.
2. Material, thickness, and geometry: Wall thickness, joint type, and material grain structure set probe frequency and scan mode. Thick or coarse-grained austenitic material pushes toward lower frequency and may require dedicated focal laws; thin sections favor higher frequency and S-scan from a single position.
3. Imaging modes required: Decide which of S-scan, E-scan, C-scan, TOFD, and TFM the work needs, then require all of them in one instrument. TOFD needs conventional UT channels; TFM needs FMC capability and adequate aperture and compute.
4. Instrument channel configuration: Match the pulser-to-aperture ratio (16:64, 16:128, 32:128, or higher) to the longest probe and widest sweep in the scan plan, and check the simultaneous-group budget so TOFD, PA, and TFM can coexist.
5. Probe and wedge set: Specify frequency, element count, pitch, and passive aperture against Chapter 3, then pair each probe with a matched wedge of the correct nominal angle. Treat probe, wedge, and scan plan as one specification.
6. Encoding and scanning: Decide manual versus encoded. Documented, replayable examination needs an encoder and scanner with the right number of axes; confirm scanner compatibility and magnetic or vacuum coupling for the surface.
7. Software and scan-plan tools: Evaluate the scan-plan generator, calibration workflow, data export format, and analysis software, since these govern setup time, repeatability, and the auditability of the archive years later.
8. Total cost of ownership: Beyond purchase price, weigh probe and wedge consumables, annual instrument and probe characterization, operator certification, software updates, and the value of recordable data in reducing repeat inspections and disputes.

One dimension that buyers consistently underweight is serviceability and support: local calibration and characterization facilities, probe and wedge lead times, firmware and software update policy, and operator training availability. A PAUT archive may be revisited in a code dispute long after the inspection, so traceable calibration and a maintained instrument matter as much as the original specification. Among portable platforms, Evident (OmniScan X3 and X4), Eddyfi Technologies (Mantis, Gekko, Panther), Baker Hughes Waygate Technologies (Krautkramer Mentor UT), and Sonatest (Veo3) maintain global service and calibration networks, which makes them defensible choices for projects that must survive third-party audit.