In immersion ultrasonic testing, water is used as the coupling medium.

• A. False
• B. True
• C. Only for certain metals
• D. Not required

Answer: (B)

Water as the coupling medium is essential in immersion ultrasonic testing because it transmits ultrasonic energy between the transducer and the test piece very efficiently. Water provides a much smaller impedance mismatch than air, so far more of the sound energy enters the material rather than being reflected at the surface. This enables strong, clear signals and good penetration into the test part.

Using water also offers practical advantages: it creates a uniform, controllable gap between the probe and the surface, fills gaps and irregularities so there are no air pockets, and allows easy positioning and focusing to inspect complex geometries. This versatility is why immersion testing commonly uses water across a wide range of materials, not just one type.

If air were used, most of the energy would reflect at the interface, yielding a weak or unusable signal. Not having any coupling medium would leave air gaps and also degrade transmission. And immersion isn’t limited to certain metals; the technique is applicable to many materials, with water chosen for its favorable acoustic properties. Thus, water is the correct coupling medium in immersion ultrasonic testing.