Search units are often fabricated with removable plastic or rubber wear faces. These faces improve coupling on rough
surfaces and prevent wear of the search unit face. However, the flexible wear faces reduce the amount of power
available from the search unit.
A search unit may have a solid or a fluid delay line.
Solid Delay Line.
A solid delay line may be an integral part of the search unit or may be removable. An integral delay line is bonded to
the transducer element. A removable delay line requires a couplant between it and the transducer face. Various
lengths of removable delay lines can be interchanged and can be replaced when worn. Delay lines improve near-
Fluid Delay Line.
Some search units are equipped with water delay columns. An example is shown in Figure 5-33. The water column
also permits the use of focused search units. The delay line can either have an open bottom requiring a rapid flow of
water to maintain coupling, or it can be equipped with a thin membrane at the bottom. The form is common in large
automated scanning systems. The membrane is usually punctured in the middle to provide a slow flow of water for
coupling. Water delay lines with flowing water are also called "bubblers" or "squirters". A variety of sizes are used.
Figure 5-33. Water Delay Column Search Unit.
Wheel Search Units.
A wheel search unit consists of a flexible tire filled with liquid and containing one or more transducer elements. As
shown in Figure 5-34, sound is transmitted through the liquid, the tire, and to the part through a thin couplant film
between the tire and the part. Wheel search units can be made to transmit the sound beam in several different
directions and modes. Wheel search units are not normally used for field NDI.
Figure 5-34. Wheel Search Unit.