In the field of industrial ultrasonic testing, ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement (gauging) of the local thickness of a solid substrate based on the time taken by the ultrasound wave to return to the surface.
Ultrasonic waves have been observed to travel through metals at a constant speed characteristic to a given alloy with minor variations due to other factors like temperature. Thus, given this information, called celerity, one can calculate the length of the path traversed by the wave using this formula:
To make this process easier on you, they have made a specific tool that performs this measurement for you, called an ultrasonic thickness gauge.
These UTG’s can measure coating thickness over non-metals, including coatings over wood, concrete, plastic, and more.
These devices transmit an ultrasonic sound pulse that travels inside the coating and echoes from any different density interface, such as the substrate. It measures the time it takes for the sound pulse to travel from the probe to the coating/substrate interface and back. By multiplying this travel time by the velocity of sound within the coating, the thickness of the coating is calculated.
While there is almost always a density change at the coating-substrate interface that the UTG’s can detect, density changes between coating layers can also be identified. Advanced models can often detect the interface between layers of different or identical coatings. The initial layer is allowed to cure before applying the following layer fully. Typically, the loudest ultrasonic echo represents the coating-substrate interface, with smaller echoes representing individual layer thicknesses.
In multi-layer mode, the thicknesses associated with the loudest echoes are reported as individual layer thicknesses. For most measurement applications, and particularly when measuring smooth, polymer coatings, this method provides accurate readings of the total coating thickness and individual layers without any setup or adjustment needed.
Max Thick Mode
Devices such as the PosiTector 200 have a Max Thick Mode. When enabled, it searches for the deepest significant echo instead of the loudest echo. This means that any loud echoes near the surface are ignored. This mode is often more effective at reporting the total coating thickness at the substrate interface and eliminates the need to adjust the instrument’s preset Lo Range. Since there will inevitably be some small echoes from within the substrate, only echoes above a preset loudness will be reported.
~The Ultrasonic Coating Thickness Guage measures coating thickness on non-metals, including wood, concrete, plastic, and more. They will give the most accurate readings for most applications while eliminating the need to adjust the Lo Range to ignore incoming surface reflections.