Why use coupling agent when using ultrasonic flaw detector? What are the commonly used coupling agents?

This is because there is an air barrier between the ultrasonic probe and human skin, and the coupling agent is mainly used to make the ultrasonic probe contact the skin more closely. Because the frequency of medical ultrasound is 2.5-5M, it cannot be conducted in the air. If there is air in the probe and skin, the ultrasonic wave will return as soon as it meets the air, and will not enter the human body, so it will not play a role in inspection. The air between the probe and the skin can be eliminated by first coating a layer of coupling agent on the skin. Su Wen brand ultrasonic coupler is a good one.