DFT Gauge Reading Dispute on a Coated Bridge Girder
An inspector is checking dry film thickness on a bridge girder coated with a three-coat system: zinc-rich primer, epoxy intermediate, and polyurethane topcoat. The specification requires the total system DFT to be 240-320 micrometres, measured per the referenced DFT standard. The inspector uses a magnetic pull-off gauge and gets readings on the topcoat that average about 360 micrometres, above the maximum. The applicator uses an eddy-current gauge on the same areas and gets about 300 micrometres, within specification. Both gauges are recently calibrated. The inspector must decide whether the coating is out of specification before the girder ships.
1. What are the plausible reasons the two gauges disagree, and how should the inspector resolve it?
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The disagreement is large enough to be meaningful, so the inspector should not simply pick the more favorable number. Plausible causes:
- Gauge type and principle. Magnetic pull-off and eddy-current gauges respond differently to the substrate and to the coating system. On steel, magnetic gauges are generally appropriate; eddy-current gauges are typically used on non-ferrous substrates. Using an eddy-current gauge on steel is a method error unless the gauge is designed for ferrous substrates.
- Calibration vs. verification. "Recently calibrated" is not the same as verified on the actual substrate and at the actual thickness. Each gauge should be checked against certified foils or a known reference near the expected thickness, and zeroed/verified on the actual substrate.
- Probe technique and curvature. Pull-off gauges are sensitive to probe placement, angle, and surface curvature; the girder flanges and web have curvature and edges that bias readings.
- Coating system effects. Multiple conductive/insulating layers can affect some gauges.
- Number and location of readings. The two inspectors may not have measured the same spots.
The inspector should verify both gauges on the actual substrate and with foils near 300 micrometres, standardize the measurement locations, and use the gauge type appropriate to steel per the referenced standard. The resolved readings then govern.
2. If the verified total DFT is genuinely above the maximum, what should the inspector do?
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If verified readings show the total system DFT exceeds the specified maximum, the inspector should:
- Confirm the exceedance is real by re-verifying the gauge and re-measuring a representative number of locations per the standard's sampling plan.
- Determine which coat is over-thick. The total is a sum; the inspector should measure the primer and intermediate individually (or by difference) to see where the excess is. Over-thick primer or intermediate can matter for intercoat adhesion and cure, not just the total.
- Assess acceptability per the specification. Some specifications allow the total to exceed the maximum if individual coats are within their limits and the system meets performance requirements; others are strict. The inspector must follow the specification, not personal judgment.
- Escalate for a disposition. If the specification does not permit the exceedance, the inspector should raise a nonconformance and obtain a written disposition from the owner/engineer (accept, rework, or test). The inspector does not unilaterally accept or reject.
- Consider consequences. Excessive total DFT on a polyurethane topcoat can affect appearance and cure; excessive epoxy can affect adhesion and flexibility. Any disposition should account for these.
The inspector documents the verified readings, the coat-by-coat breakdown, the specification clause, and the disposition.