Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

5.9.3 Immersion Methods
Many of the same techniques used in contact testing can be used in
immersion testing. One advantage of immersion testing is that water makes a
very effective coupling agent. A wetting agent is often used with the water to
reduce surface tension and minimize air bubble formation on probes and test
parts.
The main advantages of immersion testing are:
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speed of inspection,
immersion medium provides excellent coupling,
ability to direct the sound at any desired angle, and
the ease of incorporating automatic scanning techniques.
With immersion testing, the time to send the beam through the water is
usually greater than the time to send the beam through the test piece. All
immersion search units are basically straight beam units that are directed to
produce either longitudinal or shear waves in the test material.