In straight-beam thickness measurement, how is thickness calculated?

• A. Thickness = velocity × time / 2
• B. Thickness = velocity × time ÷ 4
• C. Thickness = 2 × velocity ÷ time
• D. Thickness = time ÷ velocity

Answer: (A)

In straight-beam thickness measurement, the important idea is that the ultrasonic pulse travels to the far surface and back to the transducer, so the thickness is the one-way distance the wave covers. The instrument uses the round-trip travel time and the material’s velocity to get that one-way distance, which gives thickness = velocity × round-trip time ÷ 2.

For example, with a material velocity of 5 mm/μs and a measured round-trip time of 40 μs, the thickness would be 5 × 40 ÷ 2 = 100 mm. The factor of 2 accounts for the up-and-back path. The other forms would mix up the two-way travel time or the relationship between time and distance, leading to incorrect thickness values.