Distance-Amplitude Correction (DAC)
DAC is an ultrasonic calibration curve plotted on the A-scan display that shows expected echo amplitude from a reference reflector at varying depths, used to evaluate flaw size while compensating for beam spread and attenuation.
Definition
A Distance-Amplitude Correction (DAC) curve plots the echo amplitude from a chosen reference reflector — typically a side-drilled hole — at a series of metal paths. The curve forms a sloping envelope on the A-scan against which any indication's amplitude is compared.
Technical Context
DAC compensates for the natural amplitude reduction with distance caused by beam divergence and attenuation. Codes typically specify accept/reject thresholds such as 50% DAC, 100% DAC, or 20% DAC.
Related terms
- Distance-Gain-Size (DGS) — DGS is an ultrasonic sizing technique that compares the echo amplitude from an unknown reflector to a family of curves representing equivalent flat-bottom-hole sizes at varying distances, calculated from probe characteristics.
- Time-Corrected Gain (TCG) — TCG is an ultrasonic instrument feature that automatically increases system gain as a function of time-of-flight, flattening the DAC curve so all reference reflectors display at the same amplitude regardless of depth.
- Side-Drilled Hole (SDH) — A side-drilled hole is a cylindrical reference reflector drilled perpendicular to the inspection surface, used as a length-independent angle-beam target for building DAC curves and verifying sensitivity.
More data terms
A-Scan · B-Scan · C-Scan · D-Scan · S-Scan · L-Scan · Gating · Decibel · Indication · False Indication
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
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