If the velocity of the test material is not the same as the velocity of the reference standard, which statement is true?

• A. The thickness and distance measurements will be inaccurate
• B. The thickness and distance measurements will be highly accurate
• C. The system will automatically correct for velocity differences
• D. There will be no effect on measurements

Answer: (B)

In ultrasonic testing, thickness and flaw location are calculated from the time it takes for the sound to travel to a reflector and back, using the velocity of sound in the material. If the test material’s velocity is not the same as the velocity used for calibration with the reference standard, the time-to-distance conversion is off. That means the computed thickness and the interpreted depth of a defect will be incorrect unless you adjust the material velocity setting (or re-calibrate) to match the actual material.

The system isn’t assumed to automatically correct for velocity differences in general, and there will be a real effect on measurements. Since the velocity used in the calculation doesn’t match the material, the results won’t be highly accurate.