61:08:09:29Ultrasonic examination for thickness

determination. Ultrasonic examination for thickness determination may be made using the pulse-echo technique. Ultrasonic examination provides indications of surface and subsurface discontinuities. If an ultrasonic thickness reading is below a desired value, the area around that reading must be examined as follows: Using the low reading point as the center, superimpose an 8-inch point as the center, superimpose an 8-inch square on the area, and take 20 readings spaced on 2-inch centers on the perimeter and within the square. The average of these 20 readings is used as the minimum boiler thickness in the 8-inch square area.

Source: 23 SDR 139, effective March 5, 1997.