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by Hallie Gorman

A railcar comes off the tracks, rupturing its tank and spewing toxic chemicals into a river. An oil tanker runs aground, fouling the water and endangering marine life. A truck overturns on a highway, dumping tons of waste.

It may be impossible to eliminate such catastrophes, but their effects can be minimized with proper testing. Gaging the thickness of an oil tanker's hull or a railcar can best be accomplished through ultrasonic thickness gaging.

Thickness gages cater to many different applications, whether you're measuring the thickness of a petrochemical pipe or the paint on a fire hydrant. Anytime you need to measure the thickness of an object and can only reach it from one side, an ultrasonic thickness gage is the answer.

Ultrasonic thickness gages have the ability to measure thickness of materials and some coatings without destroying the material being tested. Using the transmission of high-frequency sound waves to detect imperfections or to measure the thickness of a material allows for less guesswork and leaves the material intact. Most engineered materials, including metals, plastic, ceramics, composites, epoxies and glass, can be measured ultrasonically.

The most commonly used ultrasonic testing technique is pulse echo, which introduces sound into the test object, and reflections from the far side are returned to a receiver. Sound wavelengths travel best through a medium that's thicker than air, so the test piece is either submerged in water, or glycerin or oil is placed between the gage sensor and the material to be measured. Corrosion and precision gages usually operate at frequencies between 2 kHz and 25 MHz. Higher frequencies mean a tighter dispersion of sound waves, resulting in higher accuracy.

History

Ultrasonic thickness gaging has been around for about 60 years. The technology is related to sonar technology developed in the 1940s for use during World War II. But some say thickness measurement dates back even earlier. "Thickness gaging has been around since the beginning of mankind," says Joseph Walker, vice president of Elcometer. "When the pyramids were built, every block was cut to precise standards. Each block was checked to see that it met those standards. Only then was it set in place."

Obviously, thickness gaging has changed significantly since the pyramids were built. Originally--and in some cases today--thickness measuring was done with a caliper. But when making measurements, both sides of the object had to be accessible in order to gage the material's thickness.

Other techniques included tapping the material with a hammer and listening to the resulting sound. This method helped to distinguish a thick spot from a thin spot, but it provided little accuracy.

Formulas were also used. Knowing the original thickness of the material, maintaining it over the years and calculating the wear factors would allow for a reasonable estimate of its thickness.

Most of today's ultrasonic gages are hand-held instruments with digital readouts that display the material's thickness. It once took expensive equipment and highly paid technicians to get the same quality readings that can be done today, and the impact of this technology's improvement isn't lost on those in the industry. "The world has definitely become safer because we can electronically measure things more accurately than we could 50 years ago," says Tim Hasselbeck, president of Staveley Instruments.

Precision vs. corrosion gages

Most commercial ultrasonic thickness gages fall into one of two types: precision or corrosion gages. Determining which is needed depends largely on the material to be tested. A precision gage measures the thickness of curved or flat surfaces, perhaps for measuring a critical flight component in a jet engine or the thickness of a container. Measurement accuracy can reach 0.004 in., and the gage usually operates at frequencies between 5 kHz and 25 MHz. Other applications include plastics, rubber tubing, fiberglass and ceramics.

Corrosion gages measure many types of material that hold or transport corrosive substances. Marine transport, petrochemical pipe and tubing, barges, and railcars are constantly monitored to make sure the hulls don't get too thin from corrosion. Another important difference between a precision gage and corrosion gage, from an engineering standpoint, is the type of transducer used on the gage.

A bit about transducers

Transducers--or probes, as they are more commonly known--generate bursts of sound waves when excited by electrical pulses. Which type of transducer is called for depends on the type of material, its thickness and the accuracy required. There are three types of transducers used in thickness gaging:

Contact transducers--Used when measuring the thickness of an object that's not necessarily a very thin element. These are accurate to about 0.02 in. They are single-element contact transducers normally used for general measurements.

Delay line transducers--Used specifically for measurement, they can measure on flat, concave or convex materials, or materials in confined spaces. Measurements are most often taken in a medium such as water.

Dual-element transducers--Used for monitoring corrosion. They provide better penetration on rough or corroded parts such as pipelines, storage tanks, pressure vessels, hulls and steam lines. Dual-element transducers are very good for finding pitting in material. In addition to measuring, they also look for pitting or corrosion on the inside wall of a pipe. Dual-element transducers use a separate transmitter and receiver bonded to separate delay lines.

Once data are gathered, the information can be transmitted and stored. Thickness gages can be configured with or without data logging capability. In addition, they can store and replay data through software packages that allow review of information on computer displays. Usually, about 10,000 thickness readings can be stored, and there are several software programs available that allow you to capture thickness readings and store the data.

Environmental conditions

Sometimes gages must be used in explosive atmospheres, which means the packaging of a gage becomes very important. Additionally, in harsh environments, a gage's ability to withstand extreme temperatures is critical. The different types of packaging range from general-purpose to very application- and environment-specific.

Temperature changes in the materials may also require inspectors to closely monitor changes in material velocities and equipment calibrations. Each material has its own rate of velocity for sound. When the temperature of that material changes, the rate at which the sound wavelengths resonate also changes. Standard velocities are known for specific materials, but each piece must be tested on an individual basis to ensure more accurate readings. Steel, for example, can minutely change after heat-treatment or coating, which can have an effect great enough to affect accuracy due to velocity change. Common variations in the manufacturing process from batch to batch can affect cure characteristics in coatings. This can cause changes in their ultrasonic properties, calling for precise measurement of specific materials on specific substrates.

Thickness gages have digital readouts that range from simple one-button operations to sophisticated readouts that provide a visual representation of the sound path and eliminate false indicators. But in critical inspections--often for safety concerns--there's no room for a false reading. Such circumstances call for gages equipped with A-scans. These gages can "see" the sound as it goes through the part, which means a user can watch the sound path as the sound enters the part and bounces back.

New developments

As manufactured parts become more complex, thickness gages must evolve to keep up.

"How powerful, sophisticated and versatile can you make an instrument and still keep it simple to operate?" contemplates Hasselbeck. "Performance and simplicity of operation are big issues when it comes to gages. We're obviously trying to make technology simpler while maintaining powerful performance."

This evolution can be seen in the new products thickness gage suppliers are developing. Here's what a few suppliers have in the works:

Stavely Instruments has been working on sensor recognition for its instruments, i.e., linking the sensor with the instrument so there's an enhanced communication line. "We now make an instrument that can recognize the transducer you plug into it," says Hasselbeck. Sensor-instrument recognition simplifies the instrument use because it automatically knows which test is about to be performed and sets itself up for that type of thickness measurement. Certain controls can be automatically shut off when the full versatility of the instrument isn't needed.

"We're also making more serviceable products," he continues. "We're designing instruments with a unique modularity, so downtime is kept to a minimum."

Defelsko has used ultrasonics to measure the thickness of coatings over nonmetals.

"Prior to reliable ultrasonic coating thickness measurement, parts were often destructively tested by cutting them and examining them under a microscope," recalls David Beamish, general manager of Defelsko Corp. "Alternatively, some manufacturers were able to substitute a metal test panel in the coating process that could be measured with a conventional magnetic coating thickness gage."

Elcometer has found a way to test coating powder or dry paint and use that test to calculate the cured thickness of the powder before the part enters the curing process. The company has developed a thickness tester that works without relying on a medium. "This is an expensive gage, but it's the only thing in the world that will do what it does," says Walker.

BETA LaserMike has put ultrasonic thickness gaging to use in the telecommunications industry. It provides online measurement solutions for applications with ultrasonic measurement of wall thickness and concentricity of materials. Because of a drop in the bandwidth market, BETA LaserMike has had to look elsewhere.

"That's the beauty of the ultrasonic product," says Tom Riley, marketing communications manager. "Rather than using things like a laser scanner or other technologies that were used to measure thickness and diameter in the past, we're able to use ultrasonic technologies and broaden our applications into plastics."

To learn more…

This article is only a basic overview of nondestructive ultrasonic thickness gaging, its history and new developments. From the pyramids of Egypt to measuring bandwidth in telecommunications, thickness gaging has progressed and diversified. One of the most visible types of thickness gaging focuses on minimizing the effects of accidents and promoting safety by measuring corrosion in the petrochemical industry. But this is just one area of application for ultrasonic testing. For more information on the subject, visit the e-Journal of Nondestructive Testing at www.ndt.net or the American Society for Nondestructive Testing at www.asnt.org.

About the author

Hallie Gorman is an editorial assistant at Quality Digest. Letters to the editor regarding this article can be sent to letters@qualitydigest.com.