Ultrasonic Thickness Gauge measurement why must use coupling agent?

Why is it necessary to use a couplant when measuring an ultrasonic Thickness Gauge?
　　Ultrasonic waves will rapidly attenuate when they encounter air. In order to discharge the air between the ultrasonic probe and the workpiece, ultrasonic coupling agent is used to remove it. Usually, general machine oil or other non-corrosive liquids can be used to measure the smooth surface of the workpiece in the factory, and thicker butter can be used for the rough surface. After the measurement, the surface of the probe and the surface of the standard block must be wiped off with coupling agent.

Ultrasonic Thickness Gauges can detect thickness by resonance method, interference method, pulse echo method, etc. Now the pulse echo method is mainly used to detect the working principle of thickness.

The ultrasonic probe is in contact with the surface of the measured object. The main controller controls the transmitting circuit so that the ultrasonic wave emitted by the probe reaches the bottom surface of the measured object and is reflected back. The pulse signal is received by the probe, amplified by the amplifier and added to the vertical deflection plate of the oscilloscope. The mark generator outputs the time mark pulse signal, which is applied to the vertical deflection plate at the same time. The scanning voltage is applied to the horizontal deflection plate. Therefore, the time interval t between transmitting and receiving ultrasonic waves can be directly read on the oscilloscope. The thickness h of the object to be measured is: h=ct/2, where c is the propagation speed of ultrasonic waves.

Why use high temperature couplant for measuring high temperature pipelines?

Generally, when the couplant contacts a high-temperature object, it will evaporate or dry out instantly, so that it is difficult for the ultrasonic wave to reach the workpiece under test. The high-temperature couplant means that its object properties will not change under a certain high-temperature state, so that it will be able to measure high temperature smoothly. object.