Quick answer: Check the measurement basis before judging the coating. A valid inspection needs a suitable method, verified instrument, controlled contact, agreed sampling and traceable records. If a check fails, retain the readings with their status, investigate the cause and repeat under a documented procedure. Do not make unreliable data look reliable by averaging it.
Use the list at setup and after a change
Run the checks before the first inspection zone, after changing a probe or measurement setting, and whenever the readings become implausible. The list supports your instrument manual and project procedure; it does not replace them. Record the actual verification result rather than ticking every box by habit.
Keep method requirements and acceptance separate
ASTM D7091 addresses measurement practice, including calibration, verification and adjustment for the methods in its scope. The project specification supplies acceptance limits and sampling rules. Neither a manufacturer range nor a gauge accuracy statement is a coating acceptance limit. Select the cited procedure before deciding how many readings to take.
What to do when the checks fail
Mark the affected zone and retain the original values. Check the reference and contact conditions, document any adjustment and remeasure the same identifiable location. If the method is unsuitable or the geometry falls outside probe limits, seek a validated alternative. Keep original and repeat results connected in the report so a reviewer can reconstruct the decision.
| # | Check | Action before accepting the result |
|---|---|---|
| 1 | Correct substrate and mode | Identify the actual substrate and verify the selected method on a representative sample; automatic mode is a starting point. |
| 2 | Coating properties | Confirm whether the target layer is nonmagnetic or nonconductive as required by the method; identify stacked layers. |
| 3 | Coating condition | Confirm the surface is suitable for contact and the cure state matches the specified inspection procedure. |
| 4 | Gauge and probe limits | Check the expected thickness and all applicable probe limits; an over-range indication is not an acceptable measurement. |
| 5 | Reference check | Use the recommended reference and compare with documented tolerances; keep reference ID and result. |
| 6 | Representative substrate setup | Follow the relevant procedure for bare substrate, roughness and permitted adjustment; do not automatically zero away a correction. |
| 7 | Curves and edges | Respect curvature, minimum area and edge distance limits; validate special zones separately. |
| 8 | Contact and repeatability | Hold the probe normally, lift between contacts and investigate unstable repeats instead of selecting a favorable value. |
| 9 | Sampling plan | Use the specified point, spot and area scheme; do not apply one universal reading count to every project. |
| 10 | Shims and surface state | When using a shim on a bare substrate, keep it clean, flat and correctly placed. Other uses require an explicitly validated procedure. |
| 11 | Instrument traceability | Record gauge/probe identification, calibration status and the verification performed for this job. |
| 12 | Raw data and decision | Keep individual values, units, locations, excluded values with reasons, calculation and the cited acceptance rule. |
Related field guides
- Selecting a method for stainless steel
- Measuring powder on steel and aluminum
- Creating a traceable inspection report
Technical sources
- ASTM D7091: measurement, calibration and verification scope
- ISO 2178: magnetic-method scope
- ISO 2360: eddy-current-method scope
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