Quick answer: The word “stainless” does not determine the measuring mode. For a nonmagnetic coating on a suitable magnetic substrate, evaluate magnetic measurement; for a nonconductive coating on a suitable nonmagnetic conductive substrate, evaluate eddy current. Weak or inconsistent magnetism needs a validated solution. Do not accept automatic Fe/NFe identification as proof of accuracy.
What must be known before choosing a gauge?
Record the alloy or material specification, coating system, expected thickness, part shape and accessible measurement zones. Separate the question “does this gauge produce a number?” from “does the number represent this coating?” A finish containing conductive or magnetic material may require a different method. A material certificate helps identify the substrate, but verification still needs to represent the finished part.
How should magnetic response be checked?
Use an uncoated sample representing the same batch and forming history. A simple magnet test is a screening step rather than a quantified measurement validation. Check several zones instead of relying on one contact. If the response changes across the part, document where it changes and test the gauge separately in those zones. A generic steel zero plate is not automatically a substitute for the actual stainless substrate.
Use the method whose assumptions the sample meets
ISO 2178 describes magnetic measurement on magnetizable base metals. ISO 2360 describes eddy-current measurement of nonconductive coatings on nonmagnetic conductive base metals. These scopes explain the two branches of the decision. Partially magnetic stainless can disturb ordinary measurements; use equipment and settings specifically validated for that situation, or agree an alternative method with the inspection authority. A dual-mode label alone does not establish suitability.
Build a repeatable verification sequence
First check the instrument on its recommended reference. Then evaluate a representative bare sample and, where the instrument procedure permits, known-thickness shims or coated references over the working range. Lift and replace the probe for repeated contacts. Compare results with the reference values and instrument tolerance stated in its documentation. Repeat the assessment for a different geometry or magnetic response; retain failed checks as well as successful ones.
Control geometry during the actual inspection
Keep the probe normal to the surface and use consistent contact. Follow the probe limits for substrate thickness, curvature, edge distance and minimum area. Plan separate zones for flat panels, bends and narrow features when necessary. Confirm the coating is ready for contact measurement. Apply the sampling plan and acceptance limits specified for the job; this article does not prescribe a universal number of readings or an acceptable coating thickness.
What should a supplier demonstration include?
Ask for results on your actual alloy, finish and geometry, including raw repeated readings, reference details, instrument settings and stated limitations. Request a sample test before committing to an instrument for weakly magnetic stainless. In the inspection record, retain the selected method, gauge and probe IDs, checks, measurement locations and contract rule. If no validated method is available, report the limitation and seek a suitable alternative instead of averaging unstable numbers.
Related field guides
- Comparing steel and aluminum substrates
- Twelve checks before accepting coating readings
- Recording the measurement basis