Figure 5-55. ASTM Block and Test Part Back Surface Signals.
c. The search unit is now placed on the ASTM block with 1-1/2 inch metal travel distance to the FBH.
The signal from the FBH is maximized, and the gain is adjusted to bring the signal to the DAC curve
level. Transfer is now applied by increasing the gain setting to double the amplitude. On an instrument
with dB gain controls, this is easily accomplished by adding 6 dB to the gain or subtracting 6 dB of
attenuation. On an instrument without dB controls, the gain must be increased to double the amplitude
of a signal on the display. A correct way of doing this is as follows:
(1) Place the search unit on the 3-inch travel distance block and adjust the position for maximum signal
from the FBH. Note the amplitude of the signal. (It should be close to 30% of full screen height.)
Increase the gain until the amplitude of the signal is doubled (to 60% if it was 30% before. The
gain is now set for evaluation of discontinuities in the test part. Any discontinuity signal that
exceeds the DAC curve is cause for rejection.
Doubling the gain by doubling the signal (from 50% of saturation to 100% of
saturation) from the FBH in the 1-1/2 inch metal travel distance ASTM block would
be improper; the 100% of saturation signal is in a possible nonlinear area of the
display. Signals at levels above 90% of saturation shall not be used for applying
d. The gain setting obtained after applying transfer is used for evaluation of discontinuities in the test part.
It is advisable to perform the initial inspection using an even higher gain setting. This provides for
more reliable detection of discontinuities. When discontinuities are found, the gain is reduced to the
level established by the transfer technique. At this gain setting, any discontinuity signal that exceeds
the DAC curve is cause for rejection.