Calibration, Sensitivity and Transfer Control

Learning outcomes

By the end of this chapter, the learner should be able to distinguish calibration, standardisation, verification and reference sensitivity; select representative reference reflectors; explain DAC, TCG and DGS concepts and limitations; manage transfer differences and scanning gain; and define calibration validity and restoration actions.

6.1 Four different controls

Calibration establishes a quantitative relationship between instrument indication and a known reference, such as time or amplitude. Standardisation adjusts a system to a stated response using a reference. Verification confirms that required performance remains within tolerance. Reference sensitivity establishes the gain or correction basis used to detect and evaluate responses.

These terms are sometimes used loosely. A procedure should define the required action, tolerance, frequency and consequence of failure rather than relying on the label.

6.2 Reference block selection

A useful reference standard has controlled geometry, material acoustic behaviour, surface condition and reflectors appropriate to the inspection. It should represent the sound paths and response variables that matter. Calibration blocks commonly support time-base, probe index, angle or equipment checks. Reference blocks or representative components establish task-specific sensitivity, transfer or demonstration capability.

A convenient hole is not automatically representative of a planar crack. Artificial reflectors provide repeatability; they do not reproduce every aspect of natural discontinuity response.

6.3 Distance-amplitude correction

A DAC curve joins responses from equivalent reflectors at different sound paths using fixed equipment settings. It accounts for the combined distance effects of beam spread, attenuation and system response under the calibration conditions. Evaluation may compare an indication with the DAC level at the same sound path.

TCG applies electronic gain as a function of time so equivalent reference reflectors appear at a common amplitude. It is a transformed presentation of distance compensation. The uncorrected response, correction limits and saturation risk should remain understood.

DGS or AVG methods compare reflector response using theoretical or empirically derived relationships between distance, gain and equivalent reflector size. Validity depends on probe characterisation, material conditions, sound path and the method's assumptions. Equivalent reflector size is not the physical dimensions of a real discontinuity.

6.4 Transfer correction

Surface condition, curvature, attenuation and coupling may make the test component respond differently from the reference block. Transfer assessment compares representative responses so a documented correction or alternative reference approach can be applied. If the difference is unstable or too large, simply adding gain may not restore detection reliability because signal-to-noise ratio may deteriorate at the same time.

6.5 Scanning and evaluation sensitivity

Scanning gain may be higher than the evaluation reference to improve detection. When an indication is evaluated, the controlled evaluation level is restored or compensation is applied as specified. The relationship must be traceable. Excessive scanning gain can cause saturation, noise masking or broad apparent response length.

6.6 Calibration validity

A procedure should define checks before use, at stated intervals, after equipment change, after interruption, when temperature changes materially and at completion. If a check fails, the examiner must identify the last valid point, quarantine affected data, restore the system and repeat the required examination. "Recalibrated at the end" is not enough if the previous data cannot be shown valid.

Failure modes and decision control

Weak practice commonly includes using a reference block with materially different acoustic behaviour; moving the probe between DAC points instead of maximising each response consistently; changing pulse, filter or frequency settings after calibration; applying transfer gain without considering noise; treating DGS equivalent size as actual flaw size; and failing to define the disposition of data after drift.