What is the key difference between contact and immersion ultrasonic testing?

The key difference between contact and immersion ultrasonic testing lies in how the ultrasonic transducer interfaces with the test material. In contact testing, the ultrasonic transducer is placed directly against the surface of the material being tested. This direct contact allows the transducer to send ultrasonic waves into the material while requiring a coupling medium, like gel or oil, to facilitate the transfer of sound waves from the transducer into the material.

In contrast, immersion testing involves submerging both the part being tested and the transducer in a tank of water or another coupling medium. This method allows ultrasonic waves to travel through the water and into the material, which can be particularly advantageous for detecting flaws and obtaining high-quality signals due to reduced noise from surface variations.

The other options do not correctly represent the fundamental distinctions between the two testing methods. For example, immersion testing is indeed used on metals and other materials, debunking the notion of option C. Furthermore, contact testing is quite capable of detecting internal flaws, provided the conditions for good coupling are met, which challenges option D. Finally, option A incorrectly states that contact testing uses water, which it does not inherently require.