A critical part of any ASTM E428 or E127 Flat-bottom Hole (FBH) Block Set is the plot / graph of the ultrasonic responses from the FBHs. The graph shows the relationship between metal travel distance (MTD) and signal amplitude (for distance amplitude sets,) or hole diameter and signal amplitude (for area amplitude sets.) Any good set of blocks should show a linear, “non-erratic” relationship from block to block. This ensures they will perform properly when used to set inspection sensitivity.

For decades,
Source: NDT