How Does an Ultrasonic Thickness Gauge Work?

Ultrasonic thickness measurement plays an important role in most industrial thickness measurements, especially for pipelines. To better understand how this technology works and how it can be applied, Quốc Huy introduces the following overview of ultrasonic thickness gauges from Cygnus Instruments.

How does ultrasonic thickness measurement work?

Ultrasonic thickness measurement is one of the basic measurement methods in a group of analytical techniques known as non-destructive testing (NDT).

An ultrasonic thickness gauge (UTG) allows an engineer to check a material's thickness from just one side, instead of drilling a hole through the material and using calipers.

To operate the instrument, a simple measurement is taken on the material being inspected. Ultrasound is then transmitted into the material through a probe, with the help of ultrasonic gel. The gauge measures how long the ultrasonic wave takes to travel through the material and return. The thickness can then be calculated and reported.

What are typical applications for ultrasonic thickness testing in plant maintenance?

Ultrasonic thickness gauges can measure any metal (or coated material), so the range of applications in plant maintenance is broad. However, the most common application for an ultrasonic thickness gauge (UTG) is pipeline inspection.

Pipework can suffer from many forms of corrosion. Galvanic corrosion, scaling, microbiologically influenced corrosion (MIC), and chemical corrosion can all cause pipe walls to thin. Unlike some other assets in plant maintenance, pipework is rarely taken out of service. Erosion caused by flowing fluids (and gas bubbles), for example, is a straightforward and persistent cause of wall-thickness loss. If it is ignored or overlooked, a pipe failure may eventually cause a costly shutdown—or, worse, an industrial accident.

Using an ultrasonic thickness gauge as part of routine, planned plant maintenance—or for spot checks when needed—helps ensure that corrosion in pipework is monitored and included in a predictive maintenance program. 

Cygnus thickness gauges can record and report findings. These reports can be exported (by cable, SD card, or Bluetooth) and uploaded to a spreadsheet, where they can be analyzed and incorporated into a broader maintenance program.

Can ultrasonic thickness gauges be used when the materials being inspected have a coating?

With most ultrasonic thickness gauges, measuring through a coating simply and quickly is not possible. Cygnus Instruments, however, offers an ultrasonic technique called multiple echo. This technique is highly accurate and easy to use, and it ignores the coating so that only the substrate is measured.

What about hazardous areas—is testing equipment safe to use there? 

Ultrasonic thickness gauges are not usually used in hazardous environments. However, Cygnus Instruments manufactures the only ultrasonic thickness gauge in the world that can be used in a Zone 0 hazardous area. 

The Cygnus Intrinsically Safe thickness gauge has been a trusted product in the inspection industry for nearly two decades, thanks to its ATEX certification for Group I and Group II. 

What should you consider when choosing an ultrasonic thickness gauge? 

An ultrasonic thickness gauge should be easy to use. Some gauges have very advanced features, but that complexity is often of little value to engineers who need to take fast, accurate measurements.

Along with ease of use, accuracy is essential, and the multiple-echo technique is especially useful. In other measurement modes, viewing and recording the ultrasonic waveform (often called an A-scan) and support functions such as MSI (Measurement Stability Indicator) can also help achieve the most accurate results possible.

Finally, durability is another important factor. As with any product, not all ultrasonic thickness gauges are built alike. An instrument that has passed official drop tests and carries an IP rating can help extend the gauge's service life and ensure it continues to operate accurately.