A 32:128 phased array instrument carries twice the pulsers of a 16:64 unit (32 vs 16) while both share 128 receive paths, and that pulser delta directly controls maximum active aperture, focal-law count, and Total Focusing Method (TFM) capability. In portable UT flaw detectors this XX:YY naming convention is the first filter when matching an instrument to a probe footprint and inspection code [S1][S5].
For a 128-element linear-scan workflow, both 16:64PR and 32:128PR configurations are field-deployed, and the 32:128 form is required when the full aperture is exercised simultaneously; a 16:64 unit in the same scenario must step the aperture in smaller groups. Eddyfi's TOPAZ64, an ultra-intelligent portable PAUT unit, ships in 32 or 64 channel variants with TFM, and the OmniScan X3 family supports 16:64PR, 16:128PR, and 32:128PR PA configurations alongside two conventional UT channels [S2][S6].
What XX:YY actually means on a spec sheet
The XX:YY convention specifies the number of pulsers (XX) and the number of receive paths (YY), with receivers always greater than or equal to pulsers [S1]. Pulsers set the maximum number of elements that can be grouped into one active aperture, and receivers set the element count available for sequencing apertures across a probe footprint, which is what drives coverage speed in linear scanning.
Portable field units span 16:16 to 32:128, while higher pulser/receiver counts (e.g. 64:128, 128:128) are generally reserved for in-line inspection systems using larger element-count probes [S1]. The practical takeaway: XX is the active-aperture ceiling, YY is the sequencing ceiling, and any 32:128 instrument lets you fire 32 elements in parallel where a 16:64 unit is limited to 16.
Focal-law load: 30 laws vs 128 laws for a 128-element scan
Focal-law count scales with the configuration. Evident's reference example shows a 16:16 instrument paired with a 16-element transducer in a sectorial scan of three side-drilled holes from 40 to 70 degrees, producing only 30 laws at 1 degree steering steps, 15 laws at 2 degree steps, or 7 laws at 4 degree steps over a 2 inch (50 mm) metal path [S1].
That same source explicitly notes that a 16:128 or 32:128 instrument running linear scan mode with a 128-element transducer can require up to 128 focal laws to cover the full aperture sequence [S1][S5]. More focal laws does not always mean more functionality: a coarse 4 degree sectorial scan can still detect the SDHs, and finer steering only marginally improves sizing unless focusing also reduces beam diameter [S1]. For a direct comparison across use cases, the OmniScan X3 line demonstrates this contrast in practice: a 32-channel X3 unit paired with a 5L64-A32 64-element probe produces a high-quality S-scan, but the image resolution reflects that only 32 of the 64 elements are fired per shot, while the X3 64 variant exploits the full 64-element aperture for sharper focal points and 128-element aperture TFM [S4].
16:64 vs 32:128: a criterion-by-criterion comparison
For a portable PAUT decision matrix, the four criteria that matter most are active aperture, focal-law budget, TFM support, and cost/portability weight. On aperture, 16:64 fires 16 elements per pulse, 32:128 fires 32 elements per pulse, and both sequence across all 128 receive paths; on a 128-element probe this translates to either 8 or 4 aperture steps to traverse the full footprint. [S1]
On focal laws, a 16:64 linear scan over 128 elements tops out around the same 128-law envelope as a 32:128 instrument when steering the full aperture, but the per-step sub-aperture is half the size, so defect sizing at the focal point is coarser [S1][S4]. On TFM, the 32:128 architecture is the standard platform for full-aperture TFM and FMC acquisitions: the OmniScan X3 64 supports TFM/FMC with 64-element aperture support and 128-element aperture TFM, configurations only reachable when the pulser count matches the probe geometry [S4][S6]. On portability, both 16:64 and 32:128 fit in field-portable enclosures, and instruments such as the TOPAZ64 demonstrate that 32- to 64-channel PAUT plus TFM is feasible in a hand-carried chassis [S2]. Higher pulser counts (64:128, 128:128) generally push back into rack-mount or in-line inspection systems [S1].
Who each configuration is for, and who should skip it
A 16:64 instrument is the right pick for standard phased array weld inspection on 32- to 64-element probes, corrosion mapping scans, and code work where the procedure is written around a sub-aperture, such as AWS D1.1 structural welds and most ASME Section V Article 4 pipe girth welds. The lower pulser count keeps the unit cost and weight down, and most inspectors do not need more than 16 simultaneous channels when the probe geometry matches. [S1]
A 32:128 instrument is required when the procedure calls for TFM/FMC, when a 64-element probe must be fired across its full 64-element aperture without stepping, and when high-resolution sizing of small flaws in thick or attenuative materials is on the procedure card [S4]. The OmniScan X3 64 is explicitly aimed at thick and attenuative materials and at developing new TFM-based procedures, applications where the 16:64 pulser count would be the binding constraint [S4]. A related consideration is the broader phased array UT instrument ecosystem, where 16:64 sits in the mid-tier and 32:128 sits in the high-resolution tier for portable work. Skipping 32:128 makes sense for shops whose procedures are fully migrated to conventional PA and whose probes top out at 32 elements; skipping 16:64 in favor of stepping straight to 32:128 wastes budget if TFM is not on the near-term roadmap.
Display rate, multi-group, and other real-world constraints
Beyond the pulser/receiver split, three operational parameters are equally worth checking on a data sheet. PRF and display update rate: instruments vary widely across image modes, and in some designs the A-scan PRF of a single focal law is capped by the maximum image display update, so a 60 Hz image display may throttle the underlying A-scan rate; verifying the A-scan PRF inside the focal-law sequence is necessary for high-speed scanning [S1].
Multi-group support: capable instruments sequence multiple focal-law groups on one or more connected transducers, which is essential for collecting volumetric data offline; a 5 MHz, 64-element probe can be programmed, for example, to use elements 1-16 for a 40 to 70 degree sectorial scan while the same unit runs a second group on the same connector [S1]. Waveform storage and probe recognition: storing raw RF waveforms enables offline re-analysis over large scan areas, and probe recognition cuts operator setup time by auto-loading element count and geometry [S1]. For inspection teams balancing a new TFM-capable unit against a conventional PA-only fleet, the broader analytical instrument selection logic is similar: match the most demanding procedure card on the shop floor, not the average one, because the binding constraint, here 32 pulsers for 64-element TFM, dictates the spec.
Sourcing and standards to anchor the decision
The XX:YY definition, focal-law scaling examples, and 16:16/16:128/32:128 reference cases are documented in Evident's phased array specifications tutorial, mirrored on the Wabtec IMS knowledge base, and are consistent with the OmniScan X3 product literature [S1][S4][S5]. Eddyfi's TOPAZ64 product page confirms that 32- and 64-channel portable PAUT with TFM is a current shipping category, and the OmniScan X3 launch release confirms 16:64PR, 16:128PR, and 32:128PR as fielded PA configurations with TOFD and TFM/FMC support [S2][S6]. NDT.net forum discussion of 16, 32, 64, and 128-element probes (thread of 12 Sep 2014) is a useful practitioner cross-check on the element-count trade-offs, though it predates the latest TFM hardware generations [S3].
Trackable signals for the next planning cycle: (1) confirm whether your procedure card specifies full-aperture TFM on a 64- or 128-element probe, which forces 32:128 or higher; (2) re-check A-scan PRF under the heaviest focal-law sequence you actually run, since display-update caps can throttle throughput; (3) for new pressure transmitter skids or flow meter stations entering a TFM inspection program, specify the instrument and probe pair (e.g. 32:128 with a 64-element 5L64-A32, or 16:64 with a 32-element probe) in the procurement document so the channel count is locked before field deployment.
Related analysis: 14-bit raw radiometric vs 8-bit AGC video: when radiometry wins over display-ready output.