Which instrument is commonly used to measure root opening at the root of a butt or groove weld?

• A. Feeler gauge
• B. Ruler
• C. Tape measure
• D. Micrometer

Answer: (A)

Measuring small, precise gaps at the joint root requires an instrument that can fit into the narrow opening and give a reliable thickness reading. A feeler gauge does this best: it’s a set of blades of known thickness that you slip between the root faces until you find the largest blade that fits without forcing. The thickness of that blade gives the root opening in thousandths of an inch (or hundredths of a millimeter). This makes it ideal for confirming root openings on butt or groove welds.

Rulers or tape measures aren’t precise enough for tiny gaps, and a micrometer measures the thickness of a single part rather than the clearance between two mating faces in a confined root area.