The Selection of Couplant for Ulrtrasonic Thickness Measurement or Flaw Detection?

When ultrasonic testing or thickness measurement is carried out, a layer of sound transmitting medium is first applied to the surface of the tested workpiece, which is called a coupling agent, also known as ultrasonic coupling agent, dry powder coupling agent (diluted for use), testing coupling agent, etc. Ultrasonic coupling agent refers to a layer of transparent medium applied between the probe and the surface of the workpiece. The main function of ultrasonic coupling agent is to eliminate the air between the probe and the surface of the measured workpiece, so that ultrasonic waves can effectively penetrate the measured workpiece, ensuring sufficient sound intensity and transmittance on the detection surface to achieve the purpose of detection. In addition, coupling agents with certain fluidity can also reduce friction, lubricate the movement of the probe on the workpiece during direct contact detection, and extend the service life of the probe.

The rougher the surface of the workpiece, the more dependent it is on the action of the coupling agent. Viscous coupling agents are more suitable for such detection surfaces than low viscosity coupling agents, as they can better fill the gaps between interfaces caused by rough surface depressions and protrusions.
But on a smooth surface, high viscosity coupling agents are not necessarily required. Low viscosity coupling agents typically facilitate the easy movement of transducers on smooth or machined surfaces.

The most basic requirement for any non-destructive testing is that the material will not undergo any changes before and after testing. That is to say, the coupling agent used for testing will not damage the tested workpiece in any way.
Unlike some materials commonly used as "coupling agents" (such as high-temperature grease, Vaseline, engine oil, hair gel, etc.), coupling agents specifically designed for ultrasonic testing have been tested to ensure that they are safe for the materials composed of the workpiece.
Some coupling agents have certain rust prevention functions, some coupling agents can be safely used on plastics and glass fibers, while others are approved for use on alloys in aerospace. Therefore, before use, you need to verify whether the coupling agent used can be used for the material of the tested workpiece.

If the area of the workpiece or the area to be tested is relatively large, a coupling agent that can automatically spread can be selected to minimize the time for applying the coupling agent.
In addition, the wetting performance of the coupling agent is also very important. A coupling agent with good wetting properties can ensure that there will be no blind spots in the coupling during large-area ultrasonic testing.

For low-temperature ultrasonic testing, water-based coupling agents are not the best choice. Because as the temperature decreases, they are more prone to freezing. A better choice at low temperatures is to use silicon-based or alcohol based coupling agents.
The coupling agent used for high-temperature ultrasonic testing is specifically designed to ensure normal operation at high temperatures without combustion or drying up. When selecting high-temperature coupling agents, special attention should be paid to the ignition point, which is the self ignition temperature. Never use coupling agents above the self ignition temperature.