What is the range of thickness that ultrasound may be used on?

• A. 2 mm - 50 mm
• B. 5 mm - 100 mm
• C. 8 mm - 200 mm (5/16" - 8")
• D. 10 mm - 300 mm

Answer: (C)

Ultrasound used for thickness or flaw detection relies on a clear back-wall echo returning after a known path through the material. For typical weld inspections in structural steel, a practical and reliable thickness range is about 8 mm to 200 mm (roughly 5/16 inch to 8 inches). Thin sections under about 8 mm challenge the measurement because near-surface reflections and surface roughness can obscure the back-wall signal, making accurate thickness judgment difficult. Very thick sections over about 200 mm suffer more attenuation and beam spreading, which weakens the returning echo and can require different techniques or lower frequencies that aren’t standard for everyday field UT on welds. Within this window, standard UT setups provide the best balance of resolution, signal strength, and practical probe options for thickness measurements and flaw detection.