Ultrasonic Thickness Gauging

Ultrasonic thickness gauging, a nondestructive method of measuring thickness from one side of material, is widely used. It’s fast, reliable, versatile, and requires only one side access, unlike micrometers or calipers. In the late 1940s, the first commercial ultrasonic meters were created using sonar principles. In the 1970s, small, portable instruments that could be used for many test applications were common. The microprocessor technology’s later advances led to improved performance in the modern, compact and easy-to-use mini-instruments.

What can be measured?

Ultrasonically measuring almost any engineering material is possible. You can set up ultrasonic thickness gages for metals and plastics, as well as composites, glass, fiberglass, ceramics, or other materials. Extruded plastics can be measured online or in-process. It is also possible to measure individual layers and coatings in multilayer constructions. It is also possible to measure liquid levels and biological samples. Ultrasonic gauging does not require any cutting or sectioning.

Because of their poor transmission of high-frequency sound waves, wood, concrete, and foam are not suitable for ultrasonic gauging.

How ultrasonic thickness gauges work

A wide frequency range can generate sound energy. The frequency range of audible sound is relatively low, with a maximum limit of twenty thousand cycles per second (20 Kilohertz). Higher frequencies will produce a higher pitch. Ultrasound is sound energy that resonates at higher frequencies than the human hearing limit. Ultrasonic testing is mostly performed between 500 KHz to 20 MHz. However, some instruments can go as low as 50 KHz or as low as 100 MHz. Sound energy, regardless of frequency, is a series of mechanical vibrations that travel through a medium like steel or air according to wave physics’ basic laws.

Ultrasonic thickness gauges work by measuring the time it takes for a sound pulse generated by a small probe called a ultrasonic transducer, to travel through a test part and reflect back from the far wall or inside. This measurement is usually made in a “pulse/echo mode” because sound waves reflect off boundaries between dissimilar materials.

Methods of measurement

There are three ways to measure the time interval during which the sound waves travel through the test object. Mode 1 is the most popular approach. It simply measures the time between the excitation pulse which generates the soundwave and the first echo. A small zero offset value is added to compensate for transducer, fixed instrument and cable delays. Mode 2 measures the time between the echo that was returned to the test piece’s surface and the first backwall echo. Mode 3 measures the time between two backwall echos.

Types of gage

There are two main types of commercial ultrasonic thickness gauges: precision gages or corrosion gages. Ultrasonic gauging’s most important use is to measure the remaining wall thickness of pipes, tanks, structural parts and pressure vessels that are susceptible to internal corrosion. This can’t be seen outside. This type of measurement is done by corrosion gages. They use signal processing techniques that are optimized to detect the minimum thickness in a rough or corroded test part.

Types of transducers

Contact transducers – These are, as the name suggests, used in direct contact to the test piece. Contact transducers are the easiest to use and are the best choice for thickness gauging applications that are not corrosion.

Delay Line Transducers: A cylinder made of plastic, epoxy or fused silica is used to create a delay between the active element (or the test piece) and the transducer. They are used for thin material measurements where it is necessary to separate backwall echoes from excitation pulse recovery. The delay line can be used to protect the heat-sensitive transducer from direct contact with hot test items. Delay lines can also be contoured or shaped to enhance sound coupling in tight spaces or sharply curved areas.

The material being measured, thickness range, temperature, precision requirements and any other special conditions will all affect the choice of transducer and gage. Olympus NDT has detailed information for specific applications. Below are some of the most important factors to consider.

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