T.O. 33B-1-1
1-54
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.
FBH Number
(size in 64ths of an inch)
Difference (dBs)
From previous FBH size
2
3
4
5
6
7
8
7
5
4
3
3
2
b. Hole bottoms are checked for flatness and hole orientation is checked for perpendicularity to the block
surface.
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.
