Ultrasonic Examination – Calibration
Calibration of longitudinal wave UT consists of developing a “distance amplitude curve (DAC)” for the instrument and transducer, using test blocks manufactured from a material similar to the workpiece (i.e., with the same velocity of sound). Calibration for determining the remaining thickness of a corroded component can be accomplished by placing the transducer sequentially on test blocks (or on different locations on a single “step block”) with different known thicknesses, as illustrated in Figure 700-15.
The amplitude of the reflection (echo) received by the transducer from the back surface decreases as the thickness of the test block increases, due to attenuation of the ultrasonic wave propagating through the block. An initial pulse is obtained from the front surface of the test block, which is positioned on the zero thickness line on the oscilloscope, by adjusting the “time delay” (or “sweep”) control on the ultrasonic instrument. The back reflections from the other known thicknesses are then shifted as necessary to coincide with the proper thickness lines on the oscilloscope display, by adjusting the material velocity control on the ultrasonic instrument.
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