In TFIFRM thickness testing, what is the lowest allowable mil thickness for an individual thickness determination on a column specified at 65 mils?

• A. 50 mils
• B. 60 mils
• C. 58 mils
• D. 52 mils

Answer: (D)

Thickness testing uses a tolerance band around the specified thickness to account for application variation and measurement uncertainty. For a column specified at 65 mils, the lower limit is 20% below the specified value, so 65 × 0.80 = 52 mils. Therefore the lowest allowable thickness for an individual reading is 52 mils; anything thinner would fail to meet the minimum requirement. Readings of 58 or 60 mils are thicker than the minimum and would still satisfy the minimum, but they’re not the lowest allowable value, while a reading of 50 mils would fall below the tolerance and would be unacceptable. In practice, multiple readings may be taken and evaluated to ensure the overall thickness meets the specification.