Figure 1-10. ASTM Reference Blocks.
a. When applicable, Table 1-6 may be used as an aid if the required flat-bottom hole, (FBH), reference
standard is not available. The second column lists relative amplitudes of echoes from successive sizes
of FBHs. For example the signal from a #5 FBH is 4 dB larger than the signal from a #4 FBH, so the
instrument gain would have to be decreased by 4 dB if a #4 FBH were available but a #5 FBH was the
required standard. If a #5 FBH were available but a #2 FBH were required, the instrument gain would
have to be increased 16 dB (7+5+4) for the inspection.
Table 1-6. Relative Signal Response from FBHs in ASTM Blocks.
(size in 64ths of an inch)
From previous FBH size
b. Hole bottoms are checked for flatness and hole orientation is checked for perpendicularity to the block
c. When FBH size is plotted versus respective ultrasonic echo amplitude for a given equipment set-up, a
straight line ± 2 dB should pass through the plotted points.