Thickness Gauge or Flaw Detector: Which Is Best for Your Application?

What’s the difference between ultrasonic thickness gauges and flaw detectors? This is a commonly asked question when determining the right nondestructive testing (NDT) tool for an application, as the two devices are similar in a few ways.

In this post, we discuss their key similarities, differences, and unique advantages so that you can select the best instrument for your task.

The Similarities Between Ultrasonic Thickness Gauges and Flaw Detectors

Ultrasonic thickness gauges and flaw detectors use the same basic concepts. They both use transducers that generate sound waves, and they both measure the time it takes for that sound to leave a transducer, travel through a material, bounce off a reflector, and return to the transducer.

And both use the basic equation of:

This equation divides by two since the one-way time measurement to the reflector is all that’s required.

Ultrasound—the reflection and transmission of high-frequency sound waves—has long been an important tool for evaluating the integrity of welds, as well as testing structural materials, pipes and tanks, boiler tubes, railroad rails and axles, aerospace composites, and many other industrial applications.

Industrial ultrasonic testing equipment continues to be popular since it

• is cost effective, quick, and reliable;
• usually requires no test piece preparation other than wetting with couplant; and
• has no specific safety hazards or regulatory licensing requirements associated with its use.

Selecting an ultrasonic thickness gauge or flaw detector for an application largely depends on the main goal of the measurement. Before we get into the key differences between these instruments, let’s take a  brief look at their use throughout the years.

History of Ultrasonic Equipment

Ultrasonic test instruments have been used in industrial applications for more than 60 years. Early ultrasonic instruments were released in the 1940s, the majority of which were primarily focused on flaw detection.

The first digital thickness gauges (also known as “D Meters”) were released in the 1960s. However, using a flaw detector to measure thickness was the more popular, preferred method up until the 1980s—when digital thickness gauges offered the capability to display an onboard waveform (A-scan, for example).

The visual display of the waveform on the thickness gauge device gave users more confidence as they  could see what a digital reading corresponded to in real time.

Today, these small, powerful instruments either come standard with waveform or provide it as a software option. Read on to learn more about the current capabilities of thickness gauges and flaw detectors and discover their unique advantages.

What Is an Ultrasonic Thickness Gauge?

An ultrasonic thickness gauge can accurately measure the thickness of a material and locate surface corrosion or erosion from one side of a part. It can measure just about any common engineering material of metals, plastics, composites, fiberglass, ceramics, and glass.

Ultrasonic thickness gauges can measure the thickness of a wide variety of products for manufacturing quality control, such as plastic tubing and parts used in the medical industry. Inspectors also use them to test the structural integrity of critical parts and infrastructure, such as aircraft turbine blades for wear or pipes and tanks for in-service corrosion.

Advantages of an Ultrasonic Thickness Gauge

Here are some advantages of an ultrasonic thickness gauge:

• More consistent and accurate time-of-flight measurement. Straight beam inspection measures the time of flight to a material’s back surface, which corresponds to the total thickness of that  material.
• Compact size and lower cost. Easy to calibrate and operate.
• Specialized features or software for unique application needs, including boiler tube measurements, high temperature, and painted or coated applications.
• Measurement advantages include:
  • Auto zero compensation: enables a more precise zero offset measurement, which  contributes to a more accurate ToF measurement.
  • V-path correction: compensates for the angular sound path of corrosion measurements using a dual element transducer, providing the highest degree of accuracy and repeatability  over a large thickness range. Most flaw detectors do not compensate for this angular sound path.
  • Auto probe recognition: some dual element transducers include an auto recognition pin, enabling the gauge to recognize the transducer; this eases user operation by recalling the optimized setup and receiver gain, and also determines the transducer’s pre-recalled V-path correction.
  • Detection algorithms and digital signal processing (DSP): allows the calibrated accuracy to be maintained when gain or signal amplitude changes.

What Is an Ultrasonic Flaw Detector?

An ultrasonic flaw detector is designed to locate and size discontinuities such as cracks, voids, porosity, and other injurious discontinuities in various materials and welded joints. Flaw detection can potentially be applied to any standard engineering material. Most tests involve steel and other structural metals; however, flaw detectors can also be used on plastics, composites, fiberglass, and ceramics.

A flaw detector also has the ability to reasonably measure material thickness accurately but isn’t designed for precision thickness.

Advantages of an Ultrasonic Flaw Detector

Here are some advantages of an ultrasonic flaw detector:

• Highly versatile:
  • Straight beam inspection measures to a discontinuity or a void to locate the distance to the flaw  from the material’s surface.
  • Angle beam inspection to check the integrity of welds.
  • Ideal for scanning applications (due to a faster update rate of 60 Hz compared to a 25 to 30 Hz update rate of a thickness gauge)
• Includes specialized software for sizing discontinuities:
  • Distance amplitude correction (DAC)
  • Time varied gain (TVG)
  • Distance gain sizing (DGS)

Recommendations for Selecting a Flaw Detector or Thickness Gauge

Determining your ideal instrument comes down to the application and what is being measured. For applications requiring repeatable precision and corrosion thickness measurements only, we recommend a thickness gauge. If you need to detect flaws or voids, check welds, and measure thickness, a flaw detector is the better choice.

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Betsy Kenaston is Marketing Specialist, Portable NDT Instruments, at Olympus. She holds a bachelor of science and master’s degrees from Iowa State University of Science and Technology.

Reprinted courtesy of Olympus.

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Find additional papers on thickness gauges and flaw detectors on the ASNT NDT Online Library.

Information about ASNT’s ultrasonic testing thickness examination is available on ASNT’s website. This exam is part of ASNT’s Industry Sector Qualification-Oil & Gas (ISQ-O&G) program.

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