Ultrasonic probes

How to choose an ultrasonic probe

The probe turns an electrical pulse into an elastic wave and receives the echoes coming back from inside the material. It is the probe, not the instrument, that sets the sensitivity, the resolution and how deep the beam reaches at all. A poorly matched probe can miss a discontinuity that another one shows as a clear echo.

The choice comes down to four things: beam angle, frequency, the size and number of elements, and how the probe couples to the surface. Welded joints are inspected with angle beam probes, the angle chosen so the beam crosses the fusion zone. Thickness measurement and lamination detection call for normal and dual element (TR) probes, which have a small dead zone just below the surface.

Frequency: resolution against penetration

Frequency is a trade-off between how fine a discontinuity you can see and how deep the beam reaches. The higher it goes, the shorter the wavelength and the better the resolution — and the faster the wave attenuates.

How a phased array probe is built

An array probe holds many independently driven elements, typically 16, 32, 64 or 128. The distance between their centres — the pitch — decides whether grating lobes appear at large steering angles; in practice the pitch is kept below half the wavelength, which at 5 MHz means 0.5–0.6 mm.

The active aperture is the element count times the pitch, and it governs the depth at which the beam can be focused. The passive aperture, the element height, is usually 8–10 mm and sets the beam thickness in the plane that is not steered. For TFM and PCI, 64-element probes at 5 or 7.5 MHz with a small pitch are the choice — they give the full range of diffraction angles and the best image reconstruction; from our range Phased array probes and Phased array probes.

Types of probes we supply

• Conventional ultrasonic probes — normal, angle beam, TR and TRL for routine inspection of welds, plates, forgings and castings
• Phased array probes (PAUT) — multi-element probes with electronic beam steering, for sector scanning and recorded images
• Matrix array probes for 3D TFM — a full transducer matrix for 3D TFM imaging
• TOFD probes — matched pairs for time-of-flight diffraction on thick-walled joints
• Immersion probes — for testing in water and automated stations
• High temperature probes — for installations inspected without cooling them down
• High frequency probes — for thin walls and fine discontinuities
• Special ultrasonic probes — for geometries and materials a standard probe cannot handle

Wedge, cable and reference block — what goes with the probe

An angle beam probe works with a wedge, and it is the wedge that sets the angle in the material, so both are chosen together — see Wedges and delay lines. Add Cables and connectors for flaw detectors and a reference block on which the beam exit point and the actual angle are verified before the inspection. Without that last step the result cannot be referred to a standard.