You can also find different types of thickness gauges for measuring wall thickness and determine the hardness of walls, metals, plastics and glass. Also, special manual devices can be found, with a test sensor that can operate up to a temperature of 300°C. These devices  are suited for measuring wall thickness and determining the durability of walls with vapors to pressure conduction. So that the sound waves are transmitted correctly, and to guarantee a high level of accuracy, a small amount of coupling gel can be used when placing the sensor head on the surface being measured, then the sound velocity can be adjusted and the device will accurately show the thickness of the surface.