9.2.2.1 Back-Reflection Technique (Back-Reflection Calibration
Applicable to Forgings with Parallel Entry and Back
Surfaces) —With the attenuator set at an appropriate level, for
example 5 to 1 or 14 dB, adjust the instrument controls to
obtain a back reflection approximately 75 % of the full-screen
height from the opposite side of the forging. Scan the forging
at the maximum amplification setting of the attenuator (attenuator
set at 1 to 1). Carry out the evaluation of discontinuities
with the gain control set at the reference level. Recalibration is
required for significant changes in section thickness or diameter.
评述:5 to 1是14dB;下面的 1 to 1 就是0dB,只是ASME没在进一步像上文说明,实际上就是不需要提高灵敏度。