This article discusses the inspection/reference standards that are absolutely critical for proper application of ultrasonic inspection systems. Many of the standards and specifications for ultrasonic inspection require the use of standard reference blocks. The article lists the variables that should be considered when selecting standard reference blocks and describes the three types of standard blocks ordinarily used for calibration or reference: area-amplitude blocks, distance-amplitude blocks, and blocks of the type sanctioned by the International Institute of Welding. It reviews the determination of area-amplitude and distance-amplitude curves of a straight-beam pulse-echo ultrasonic inspection system. The article discusses the three principal...