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Inspecting Small-Diameter, Austenitic Steel Pipes using a Dual Linear Array™ (DLA) Probe

Small-Diameter Pipe Inspection


Small-diameter, thin-walled, austenitic steel inspection of pipes, where standard linear shear-wave inspection is not possible.


Standard linear shear-wave inspection cannot adequately meet the demands of this application. The acoustic characteristics of small-diameter, thin-walled, welded pipes create unique requirements for dual linear arrays. To ensure that focusing occurs in the desired zone, a different wedge roof angle is required for each diameter.


To address the need for different roof angles, we created a Dual Linear Array™ (DLA) probe with variable roof angles, assembled in one standard housing. This probe adapts to wedges included in a package covering a range of 1 in. OD to 4.5 in. OD, attaches to the COBRA small-diameter pipe weld scanner, and can be operated using the OmniScan™ SX flaw detector. This package offers a solution for inspections where linear shear waves cannot detect flaws in noisy or attenuative materials.

dual linear array probe: bottom view

Item Number Part Number Description
Q3301132 5DL16-12X5-A25-P-2.5-OM Standard phased array probe, 5 MHz, dual 16 element arrays, 12 × 5 mm total active aperture, 0.75 mm pitch, 5 mm elevation, A25 case type, impedance matching to Rexolite®, PVC sheathing, 2.5 m cable, one (1) OmniScan connector.
Q7201159 SA25-DN70L-Kit One (1) SA25-DN70L flat wedge and nine (9) SA25-DN70L curved wedges to cover nominal pipe sizes (NPS) from 0.84 in. OD to 4.5 in. OD. Features IH option (irrigation and scanner holes). Fits A25 dual array probes. IMPORTANT NOTE: Focal laws for the OmniScan SX using this solution cannot be created in the OmniScan flaw detector; they must be created using NDT SetupBuilder (TomoView can also be used). This solution will increase minimum height clearance required for use of the COBRA scanner.
U8750063 COBRA-HALF One-sided COBRA scanner package for 0.84 in. OD to 4.5 in. OD pipe inspection using one probe including irrigation parts and setup templates. 2.5 m encoder cable with a LEMO connector compatible with the OmniScan X3 and SX. NOT INCLUDED: Wedges and probes.
Q1000036 OMNISXPA1664PR-A25-SA25 OmniScan SX and A25 COBRA scanner DLA promotional kit. Includes OmniScan SX portable 16:64PR phased array acquisition unit (including one UT conventional channel) with: AC adaptor, battery, small carrying case, SD card, USB flash drive, two (2) anti-glare screen protectors, hard copy of User’s Manual, USB key including OmniScan software User's Manuals, 1-year warranty. Also includes the following items: (1) OmniPC HASP key with OmniPC and NDT SetupBuilder (OMNIPC-A) -(1).
+ One-sided COBRA scanner package for 0.84 in. OD to 4.5 in. OD pipes (COBRA-HALF) -(1) 5 MHz, 16-element dual linear array probe, A25 housing for COBRA scanner (5DL16-12X5-A25- P-2.5-OM) (1) A25 series wedge kit, including one (1) flat wedge and nine (9) contoured wedges (SA25-DN70L-KIT) -(1) ES BeamTool version 8 HardLock (HASP key) package for PA technique development (SOFT-ESBEAM8HL).

Set Up

To set up the solution

  1. Select the wedge and LAW file for the desired outside diameter.
  2. Couple the probe and wedge.
  3. Load the LAW file for the specific focus depth and outside diameter. (A USB key with Advanced Calculator Setup and LAW files are provided with each probe.)
  4. Set up Part and Weld for inspection.
  5. Calibrate ID and OD notches using the same base material:
    • Manually adjust the wedge delay (~ 0.6μs is typically sufficient).
    • Use reflectors that have the desired index offset to complete a two point sensitivity calibration in the OmniScan wizard.

Solution setup diagram

The result should be two peaked reflectors at the weld centerline of the inspection.

Phased Array probe setup on small diameter pipescreen view of the results with proper setup

Below is a diagram of the possible signal paths of the longitudinal dual linear array. If the gates are set up properly, the A-gate will display techniques 1 and 2 traveling at the longitudinal compression velocity and will plot in the weld overlay at the correct volumetric position. The B gate will display techniques 3 and 4 containing at least one shear-wave leg and, even though it will not plot correctly, it is still useful for flaw detection and length sizing.

Transducer echo paths


Using the calibrated setup, we scanned and recorded data. We found that the A gate recorded the weld geometry along with signal responses from four flaws in the pipe. Changing the C-scan to display the B gate revealed that the flaws were easily identified in positions along the scan axis when viewing the mode converted signal.

Results: C-scan changed to display the B gate

We then observed each flaw in the A-C-R-S view. This allowed us to view each indication along the scan access, the A-scan where the cursor is placed, and the R/S scan showing where the indications are positioned* within the weld. We observed the data in the A gate to verify whether it plotted accurately, keeping in mind that additional indications appear from mode converted signal.

*The position of the indication is only accurate if the direct longitudinal wave breaks the threshold of the A gate.

Flaw 1 is a centerline crack, and it was detected in both the A and B gates. We can observe the centerline crack, plotted accurately, in the weld volume just above the root geometry. There is also a secondary mode converted signal that is out of position but useful for flaw detection and length sizing on the scan axis.

Flaw 1: centerline crack diagramFlaw 1: centerline crack scan results

Flaw 2 is another centerline crack that is clearly detected with the mode converted signal but appears very weak in the direct longitudinal. In this case, you can see that the flaw is displayed at 23.5% amplitude in the center of the weld.

Flaw 2: centerline crack diagram (deeper crack)Flaw 2: centerline crack scan results

Flaw 3 is an outside fatigue crack that is along the weld bevel. The indication is displayed very clearly in the mode converted data but only appears at 19.6% amplitude for the direct longitudinal wave.

Flaw 3: outside fatigue crack diagramFlaw 3: outside fatigue crack scan results

Flaw 4 is an inside fatigue crack along the weld root. It shows up just inside the second leg of the longitudinal signal and plots correctly on the weld overlay.

Flaw 4: inside fatigue crack diagramFlaw 4: inside fatigue crack scan results


The A25 dual linear probe is able to inspect thin-walled, small-diameter austenitic welds. Using gates positioned to display all modes of signal ensures good detection and length sizing of flaws. Advanced understanding of echo dynamics, dual linear signal modes, and probe calibration are critical to successful inspection.

Olympus IMS

Products Used for This Application

Our Dual Linear Array (DLA) and Dual Matrix Array (DMA) probes consist of two linear or matrix array probes wired to the same connector with the capacity to perform transmit-receive longitudinal (TRL) inspection. They are particularly helpful when testing cladded pipes or highly attenuating materials, or for applications with high-sensitivity requirements.

The single group, lightweight OmniScan SX flaw detector features an easy-to-read 8.4-inch (21.3 cm) touch screen and provides cost-effective solutions. The OmniScan SX comes in two models: the SX PA and SX UT. The SX PA is a 16:64PR unit, which, like the UT-only SX UT, is equipped with a conventional UT channel for P/E, P-C, or TOFD inspections.

The COBRA™ manual scanner combined with the OmniScan X3 flaw detector is used to perform circumferential welding inspections on small-diameter pipes. The COBRA small-diameter scanning unit can hold two PA probes to inspect pipes with ODs ranging from 0.84 to 4.5 in.

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