Measuring DFT on a Curved Surface
An inspector is verifying the dry film thickness (DFT) of a polyurethane topcoat applied to a large curved steel structure. The specified DFT is 50-75 μm. The inspector uses a magnetic pull-off gauge and takes 10 readings at various points on the curved surface. The readings are: 45, 55, 60, 65, 70, 80, 50, 55, 65, 75 μm. The inspector notes that the gauge is calibrated with a certified shim before use. The structure's curvature makes it difficult to ensure the gauge is perpendicular to the surface at all times. The inspector is unsure whether the readings are reliable and whether the coating meets the specification. The specification requires that no single reading be below 80% of the specified minimum DFT, and the average of all readings must be within the specified range.
1. Evaluate whether the DFT readings meet the specification.
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First, check each reading against the minimum requirement: 80% of 50 μm = 40 μm. The lowest reading is 45 μm, which is above 40 μm, so no single reading is below the minimum allowed.
Next, calculate the average: (45+55+60+65+70+80+50+55+65+75)/10 = 620/10 = 62 μm. The average is within the specified range of 50-75 μm.
Therefore, based on these readings, the DFT meets the specification. However, the inspector should consider the reliability of the readings due to the curvature. The gauge must be perpendicular to the surface; otherwise, readings may be erroneously high or low. The inspector should take additional readings with careful technique, possibly using a gauge with a curved surface adapter, to confirm. Also, the specification may require a certain number of readings per area; 10 readings may not be sufficient for a large structure. The inspector should follow the specification's sampling plan.
2. What steps should the inspector take to ensure accurate DFT measurements on the curved surface?
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To ensure accurate measurements on a curved surface:
- Use a gauge with a curved surface adapter or a gauge designed for curved surfaces, which helps maintain perpendicularity.
- Ensure the gauge is held perpendicular to the surface at the measurement point; this may require practice and careful positioning.
- Take multiple readings at each location and average them to reduce the effect of random errors.
- Calibrate the gauge on a flat surface of the same material and with shims; if the gauge is calibrated on a flat surface but used on a curved one, verify with a shim on a curved surface if possible.
- Mark the measurement locations and record readings systematically.
- Compare with destructive reference if doubts persist, e.g., use a Tooke gauge or destructive DFT measurement on a sample coupon.
Additionally, the inspector should follow the specification's requirements for sampling frequency and acceptance criteria. If the gauge readings are questionable, the inspector may need to use a different method or consult the manufacturer.