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.

Worked example

Measured amplitude of the reference reflector	55 %
Gain at which it was measured	42 dB
Required screen height	80 %
Transfer correction	4 dB
Scanning gain rule	asme_v_art4
Maximum usable instrument gain	80 dB
Correction to the required screen height	3.25 dB
Primary reference gain	45.25 dB
Code scanning gain addition	6 dB
Evaluation gain (reference plus transfer)	49.25 dB
Scanning gain	55.25 dB
Headroom at scanning gain	24.75 dB

The hole peaks at 55 percent at 42 dB; raising it to 80 percent needs 20 × log10(80/55) = 20 × 0.16256 = 3.25 dB, so the reference gain is 45.25 dB. Adding 4 dB of transfer correction gives an evaluation gain of 49.25 dB and, with the 6 dB ASME scanning addition, a scanning gain of 55.25 dB, leaving 24.75 dB of headroom.

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