Reference sensitivity and scanning gain

Setting sensitivity is a two step arithmetic job. First, bring the reference reflector - normally a side drilled hole in the calibration block - to the required screen height. If it peaks at A percent of full screen at gain G, the gain that puts it at the target height is G + 20·log10(A_target / A). Twenty times the log of the amplitude ratio, because these are amplitudes and not powers.

Second, correct for the difference between the calibration block and the component. Transfer correction accounts for surface roughness, curvature, coupling and attenuation differences, and is measured, not assumed - typically by comparing two probe back-and-forth signals on the block and on the part. It is added to the reference gain.

Scanning is then carried out above reference level so that small or unfavourably oriented reflectors are not missed while the probe is moving. The addition is fixed by the governing code, not by the operator; ASME Section V Article 4 requires scanning at least 6 dB above the primary reference level. Evaluation is always performed back at reference level.

Record all three numbers - reference, transfer, scanning - separately on the report. A single scanning gain figure with no breakdown cannot be audited, and cannot be reproduced when the scan has to be repeated months later.

ΔG = 20 · log10(A_target / A_measured)
reference gain = measured gain + ΔG
scanning gain = reference gain + transfer correction + code scanning addition