What is a DAC curve used for in UT thickness measurements?

• A. A DAC curve is a distance-amplitude correction curve used to relate echo amplitude to reflector distance and calibrate sensitivity across depths.
• B. A DAC curve is a dynamic area calibration for screen brightness.
• C. A DAC curve maps transducer frequency to material category.
• D. A DAC curve defines the maximum depth to measure.

Answer: (A)

Distance-amplitude correction is about how echo strength changes with depth and why UT testers adjust for it. As a reflector is deeper in the material, the ultrasonic signal loses energy through attenuation and beam spread, so the same material thickness can produce echoes of different amplitudes just because of depth. A DAC curve captures the relationship between depth (distance) and echo amplitude and provides a way to compensate for those depth‑related losses.

In practice, you calibrate the instrument with a known standard to create the curve, then apply it so the gain across depths is adjusted consistently. This lets you interpret the back-wall echo amplitude as a true indication of thickness rather than a depth artifact, ensuring accurate thickness measurements throughout the scanned region.