Measuring Dry Film Thickness on Fins
Key facts
| Method | Standard | Use on coils |
|---|---|---|
| Eddy-current gauge | ISO 2360 | Witness panels and flat areas; unreliable on thin fins |
| Cross-section microscopy | ISO 1463 | Destructive; most accurate on actual fins |
| Film weight (gravimetric) | — | g/m², from weighing panels before and after coating |
| Witness panels | — | Coated and cured with the batch, measured by gauge |
Why fins are hard to measure
Eddy-current gauges for coatings on aluminium and copper assume a substrate thicker than a critical minimum and a flat surface. Fins are around a tenth of a millimetre thick, curved, louvered and closely spaced, so the probe cannot sit flat and the thin metal distorts the reading.
How it is done in practice
- Witness panels of the same metal are coated and cured with each batch and measured by gauge.
- Flat areas of the coil — tube sheets, end plates — are measured by gauge.
- Microscopy of a fin cut from a sample coil shows the actual film, including at the fin root and edges.
- Film weight is measured by weighing panels before and after coating, which is how PG-W is specified (5–10 g/m² per side).
Where to measure
Fin faces, fin edges and fin roots behave differently: films are thinner on edges and thicker at roots. A thickness record should say where and how each reading was taken.
Reading a batch record
Look for the method, the location of readings, the number of readings and the range, not just an average. PG batch records report thickness with the method and location.
Related
Can I check film thickness on arrival with a handheld gauge?
On flat areas such as tube sheets and end plates, roughly. On fins the reading is unreliable. Compare with the batch record rather than relying on fin readings.
How do g/m² and μm relate?
Film thickness in μm equals film weight in g/m² divided by the dry film density in g/cm³. At a density of 1.0 g/cm³, 1 g/m² is 1 μm.
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