Types of Ultrasound Transducer Probes for Industrial NDT

In industrial non-destructive testing (NDT), ultrasonic inspection plays a vital role in ensuring structural integrity without damaging the test object. The heart of this technique lies in the transducer probe you use. Different probes offer varied performance in-depth penetration, resolution, and inspection flexibility. This guide will help you navigate the types of ultrasound transducer probes used in NDT, so you can match the right probe to the right job with confidence.

Why the Right Probe Matters in NDT

Ultrasound transducers aren’t one-size-fits-all. The probe you select determines your ability to reach defects, measure thickness accurately, or scan complex geometries. For example, inspecting a weld for cracks requires angled beam probes, while monitoring corrosion on a flat plate may call for dual-element straight beam probes. The type of probe also dictates compatibility with instruments, such as basic UT flaw detectors versus phased array (PAUT) systems. Choosing wisely means better data and fewer re-tests.

How NDT Probes Are Categorized

Ultrasonic probes can be categorized in several ways:

• By structure: straight beam, angle beam, dual-element, immersion, phased array, TOFD
• By use: contact, delay line, or immersion coupling
• By purpose: flaw detection, wall thickness gauging, weld inspection

Probe Type	Key Use	Typical Frequency Range
Straight Beam	Internal flaws in plates	2–10 MHz
Angle Beam	Weld inspection	2–7.5 MHz
Dual-Element	Corrosion/thickness	1–10 MHz
Immersion	Lab-based scanning	5–20 MHz
Phased Array	Multi-angle flaw sizing	1–16 MHz
TOFD	Crack sizing in welds	5–15 MHz

Straight Beam Contact Probes

Straight beam probes are one of the most fundamental tools in ultrasonic testing. They utilize a single piezoelectric crystal that emits longitudinal waves perpendicularly into the material. This simple design allows them to identify internal flaws such as inclusions, porosity, or laminar defects by analyzing the returned echoes.

Typical Frequency Range: Common frequencies range from 2 MHz to 10 MHz—with lower frequencies used for deeper penetration and higher frequencies offering better near-surface resolution.

Straight beam probes are ideal for:

1. Wall thickness measurement in pipes and plates
2. Internal defect detection in forged and rolled materials
3. Weld inspection for vertical or centrally located flaws

Best Practice Tips:

1. Use high-frequency probes (5–10 MHz) for surface-near flaw resolution
2. Choose low-frequency probes (2–3.5 MHz) for inspecting thicker sections
3. Ensure proper coupling and compatibility with UT instruments like those from Olympus or GE

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Angle Beam Probes for Weld Inspection

Angle beam probes, often paired with wedges, generate shear waves at specific incident angles (typically 45°, 60°, or 70°). They are essential for weld inspections, particularly for detecting lack of fusion or cracks parallel to the weld axis. Common in pipeline and structural steel applications, these probes enhance detectability of angled flaws.

Typical Angle Options: Most common refracted angles in steel are 45°, 60°, and 70°, selected based on weld thickness and joint configuration.

Angle beam probes are ideal for:

1. Detecting sidewall and root flaws in welded joints
   
2. Scanning across weld volumes for angled or offset defects
   
3. Inspecting complex geometries where straight beam access is limited
   

Best Practice Tips:

1. Select the appropriate angle based on weld geometry (e.g., 60° for most standard welds)
   
2. Calibrate using reference blocks with known notches to verify detection capability
   
3. Maintain consistent coupling and scan coverage, especially over uneven weld caps

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Dual-Element Probes for Thickness and Corrosion

A dual-element probe uses two crystals—one to transmit and one to receive—positioned at a slight angle to converge at a focal point within the material. This configuration reduces surface noise and improves detection on rough or curved surfaces. They’re ideal for corrosion mapping, wall thinning, and rough-coated components. Frequently used in oil & gas pipelines.

Typical Frequency Range: Most dual-element probes operate between 2 MHz and 10 MHz, depending on the material thickness and required resolution.

Dual-element probes are ideal for:

1. Corrosion mapping in pipelines, tanks, and vessels
   
2. Thickness measurement on rough or curved surfaces
   
3. Detection of mid-wall pitting or localized thinning

Best Practice Tips:

1. Use lower frequencies (e.g., 2.25 MHz) for thick or heavily corroded materials
   
2. Apply adequate couplant to improve surface contact and signal clarity
   
3. Ensure proper alignment of the probe to maintain focus at the desired depth

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Delay Line and Immersion Probes

Delay Line Probes

Delay line probes are ultrasonic transducers equipped with a replaceable delay line material placed between the piezoelectric element and the test surface. This delay layer helps isolate the initial pulse from near-surface echoes, improving resolution for shallow flaw detection and reducing dead zone effects. They are especially useful when inspecting thin materials or detecting surface-adjacent defects.

The delay line can also help protect the crystal from wear during frequent contact, and its shape can be customized to fit curved or hard-to-reach surfaces.

Typical Frequency Range: These probes often operate at higher frequencies, commonly between 5 MHz and 20 MHz, to enhance resolution for small or shallow defects.

Delay line probes are ideal for:

1. Detecting near-surface flaws in thin materials
   
2. Measuring thickness in thin-wall components
   
3. Inspecting small or localized areas with challenging access
   

Best Practice Tips:

1. Choose higher frequencies (e.g., 10–20 MHz) for better near-surface resolution
   
2. Select delay line materials suitable for the inspection temperature and surface conditions
   
3. Replace worn or damaged delay lines regularly to maintain signal clarity
   

Immersion Probes

Immersion probes are designed for ultrasonic testing in which both the probe and the test piece are submerged in a liquid, typically water, which acts as a coupling medium. This setup allows for precise and consistent acoustic transmission without direct contact with the part, making these probes ideal for automated or high-resolution scanning applications.

Immersion probes are commonly used in high-precision inspections where consistent coupling and probe positioning are critical, such as in aerospace components or fine-grain material evaluations.

Typical Frequency Range: Frequencies usually range from 2 MHz to 25 MHz, with higher frequencies offering superior resolution for small or shallow defects.

Immersion probes are ideal for:

1. Automated or mechanized scanning systems
   
2. High-resolution flaw detection in aerospace and composite materials
   
3. Complex or delicate parts where direct contact is undesirable

Best Practice Tips:

1. Use focused probes (e.g., spherically or cylindrically focused) to improve flaw detectability
   
2. Ensure water clarity and degassing to reduce signal interference
   
3. Maintain precise probe positioning using scanners or manipulators for consistent results

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Phased Array Probes for Complex Geometry

Phased array transducers consist of multiple small elements arranged in a linear or matrix configuration. By controlling the timing of each element, you can steer and focus the beam electronically. This allows for multi-angle scans without moving the probe, making them powerful for weld inspection, complex geometry scanning, and imaging (B-scan/C-scan).

Phased array probes are ideal for:

1. Weld inspection on curved or angled surfaces
   
2. Scanning components with varying thickness
   
3. Reducing blind zones in difficult-to-reach areas

Best Practice Tips:

1. Use sectorial scanning to cover a wide range of angles in one pass
   
2. Optimize focal laws based on part geometry and expected flaw types
   
3. Pair with advanced UT instruments for real-time imaging and data analysis

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TOFD Probes for Accurate Crack Sizing

Time-of-Flight Diffraction (TOFD) probes use a transmitter-receiver pair placed on either side of a weld. They measure diffracted signals from flaw tips to size cracks with high precision. TOFD is commonly used in critical applications like power generation or pressure vessel inspection. Often combined with PAUT for comprehensive weld assessment.

TOFD probes are ideal for:

1. Accurate sizing of cracks and planar defects
   
2. Weld inspection in critical components
   
3. Detecting and monitoring flaw growth over time

Best Practice Tips:

1. Use in combination with phased array for comprehensive weld evaluation
   
2. Ensure precise probe alignment and consistent spacing
   
3. Apply appropriate wedge angles and frequencies based on material thickness

See TOFD Solutions →

How to Select the Right Probe

Choosing the right ultrasonic probe isn’t just about matching model numbers—it’s about understanding how your inspection context shapes the technical requirements. Before you decide, ask yourself a few key questions grounded in real-world inspection goals.

What type of flaw are you targeting?

If your goal is to detect internal volumetric defects—such as voids or inclusions—straight beam probes may be appropriate. For surface-breaking or angled flaws in welds, an angle beam or TOFD probe provides better coverage. If corrosion or wall thinning is the concern, dual-element probes offer the necessary near-surface sensitivity.

What material are you working with?

Material properties like grain structure, density, and acoustic impedance directly impact wave behavior. Steel and aluminum are compatible with a wide range of probes, but composites or coarse-grained metals may require lower frequencies or specialized probe configurations—such as phased arrays with customized settings.

What is your access condition?

Can you reach both sides of the component, or are you limited to single-side access? Will you be performing field inspections or working in a controlled lab environment? For tight-access or one-sided inspections, a compact phased array or angle beam probe may provide more flexible coverage. If immersion is possible, high-resolution scanning with immersion probes could yield more detailed results.

How precise does your measurement need to be?

For basic go/no-go testing, a conventional contact probe may suffice. But if you’re tasked with sizing a weld flaw or mapping corrosion in critical infrastructure, you’ll need probes capable of high-resolution imaging—often phased array or TOFD—paired with appropriate instrumentation and data recording.

By aligning probe selection with inspection objectives, material characteristics, and field conditions, you ensure better signal reliability, more accurate flaw interpretation, and smoother integration with your overall NDT workflow.

Need guidance? Contact our NDT engineers →

Conclusion

Each probe type serves a specific purpose—from weld flaw detection to corrosion mapping—and understanding their strengths helps you make informed, application-driven choices.

For tailored recommendations or to explore compatible solutions for your inspection setup, browse our full selection of probes at NDT-KITS or reach out to our technical team for support.