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

The IIW recommends the use of a contoured wedge whenever the gap
between the wedge and the test surface exceeds 0.5 mm (approximately
0.020 in.). Under this guideline, a contoured wedge should be used whenever
part radius is less than the square of a wedge dimension (length or width)
divided by four:
where
R = radius of test surface
W = width of wedge if testing in axial orientation, length of wedge if testing in
circumferential orientation
Of course switching to a small wedge, if possible within the parameters of
inspection requirements, will improve coupling on curved surfaces. As a
practical matter, contouring should be considered whenever signal strength
diminishes or couplant noise increases to a point where the reliability of an
inspection is impaired.