T.O. 33B-1-15-305.2.2.3 SearchUnitShapeandSize.The variety of sizes and configurations of search units that can be used is almost endless. Faces can be round orrectangular. Search units’ 1/8-inch diameter and smaller have been used. For field inspection, the maximum searchunit size will generally not be over 3/4-inch diameter.5.2.2.4 DualSearchUnits.Dual search units are used primarily in applications where good near-surface resolution is required. Ultrasonicthickness measurement instruments as discussed in Section 5.5, commonly use dual search units. Figure 5-31 depictsoperation of a typical dual search unit. The spaces under the transducer elements are usually filled with plasticmaterial that serves as a delay line. Thus, the initial pulse will not interfere with any echoes from near surface effect onthe sound beam. Dual search units are also used in angle beam inspection. Figure 5-32 shows two types of angle beamdual search units.Figure 5-31. Dual Search Unit Operation.Figure 5-32. Angle Beam Dual Search Units.
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