Search Unit Shape and Size.
The variety of sizes and configurations of search units that can be used is almost endless. Faces can be round or
rectangular. Search units 1/8-inch diameter and smaller have been used. For field inspection, the maximum search
unit size will generally not be over 3/4-inch diameter.
Dual Search Units.
Dual search units are used primarily in applications where good near-surface resolution is required. Ultrasonic
thickness measurement instruments as discussed in Section 5.5, commonly use dual search units. Figure 5-31 depicts
operation of a typical dual search unit. The spaces under the transducer elements are usually filled with plastic
material that serves as a delay line. Thus, the initial pulse will not interfere with any echoes from near surface effect on
the sound beam. Dual search units are also used in angle beam inspection. Figure 5-32 shows two types of angle beam
dual search units.
Figure 5-31. Dual Search Unit Operation.
Figure 5-32. Angle Beam Dual Search Units.