Ultrasonic wall thickness testing on pipes - how does it work?

How does ultrasonic thickness measurement work?

The method relies on the pulse-echo principle: the probe sends a short sound pulse into the material, it travels to the back wall and is reflected there. From the measured transit time and the known sound velocity the gauge calculates the wall thickness, with no access to the far side needed.

Because only one surface has to be reachable, the method is ideal for closed pipes, tanks and pressure vessels in service. A couplant such as gel or oil displaces the air in the gap, otherwise the sound would be fully reflected at the interface.

• Non-destructive testing (NDT) directly on the installed component.
• Single-sided measurement, with no dismantling or drilling.
• Remaining wall thickness measurable under corrosion, erosion and abrasion.
• Result in seconds, recordable with a data logger.

Which probe suits the task?

Probe choice governs accuracy and range of use. For classic corrosion measurement, dual-element (transmit-receive) probes are standard because they cope well with rough, rusted surfaces and reliably capture small remaining wall thicknesses.

High frequencies (10 MHz and above) give fine resolution on thin, smooth walls. Lower frequencies (2 MHz) penetrate thick, attenuating or cast materials better. The probe diameter should match the pipe curvature; on small bores a contoured contact face helps.

How do you calibrate correctly?

Before every measurement series the gauge is calibrated to the material. With a known sound velocity a zero calibration suffices; if the material is uncertain, calibrate over a step wedge or a reference block of known thickness.

• Enter the sound velocity of the material (steel approx. 5920 m/s, aluminium approx. 6320 m/s).
• Perform a zero offset on the reference to compensate for delay line and coupling.
• Two-point calibration over a thin and a thick step for maximum accuracy.
• Verify the result at a spot of known thickness.

How do you monitor corrosion systematically?

Corrosion eats the wall from the inside and often becomes visible only late. You therefore measure at fixed grids and document the values over time. From the decrease you can calculate the corrosion rate and the remaining service life.

Elbows, weld seams and vessel bottoms are especially at risk, where media stand still or swirl. A gauge with minimum capture and scan mode shows the thinnest point along a track and helps you not to miss pitting.

Frequently asked questions