Introduction to Thickness Gages

For more than fifty years, ultrasonic thickness gages have been used by quality control professionals to measure the thickness of a wide variety of products in manufacturing, by inspectors testing critical parts like aircraft turbine blades for wear, and by maintenance crews to check pipes and tanks for in-service corrosion. In recent years, as hardware and software development has advanced, ultrasonic gaging has steadily become a more powerful, reliable, and user-friendly tool. This tutorial describes how they work and how they can be used.

Ultrasonic thickness gages are frequently employed in situations where an inspector has access to only one side of a part whose thickness must be determined, such as the case of a pipe or tube, or where simple mechanical measurement is impossible or impractical for other reasons such as part size or access limitations. The fact that thickness measurements can quickly and easily be made from one side, with no need to cut parts, is one of the major advantages of this technology.

Virtually any common engineering material can be measured ultrasonically. Ultrasonic thickness gages can be set up for metals, plastics, composites, fiberglass, ceramics, and glass. On-line or in-process measurement of extruded plastics and rolled metal is often possible, as is measurement of individual layers or coatings in multilayer fabrications. Liquid levels and biological samples can also be measured. Ultrasonic gaging is always completely nondestructive, with no cutting or sectioning required. The range of measurement can be as large as 0.08 mm (0.003") to 635 mm (25") depending on material and transducer selection. (Materials that are generally not suitable for measurement with conventional ultrasonic gages include wood, concrete, paper, and foam products.)

All ultrasonic thickness gages work by very precisely measuring how long it takes for a sound pulse that has been generated by a probe called an ultrasonic transducer to travel through a test piece. Because sound waves reflect from material boundaries, timing the echo from the far side of the test piece can be used to gage its thickness, in the same way that radar or sonar measure distance. Resolution can be as fine as 0.001 mm or 0.0001".

Most applications use small, handheld transducers, but some complex geometries as well as on-line testing require noncontact transducers that focus a sound beam through a water column or bath. Measurements are essentially instantaneous and can be recorded on internal instrument data loggers for documentation or analysis.

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