Paints, linings, powder coats and pipeline coatings protect metal only if three things are right: the surface was prepared to the right profile, the film was applied at the specified thickness, and the film is continuous, with no pinholes. Each has its own instrument. Roughness meters check the substrate or finish, thickness gauges check the film, and holiday detectors check continuity.
Coating Thickness: Magnetic Induction (F) and Eddy Current (NF)
Handheld dry-film thickness gauges use one of two principles, depending on the substrate:
| Probe type | Principle | Measures | Examples |
|---|---|---|---|
| F (ferrous) | Magnetic induction | Non-magnetic coatings on steel or iron | Paint, powder, enamel, zinc galvanising, chrome |
| NF (non-ferrous) | Eddy current | Non-conductive coatings on non-magnetic metals | Anodising, paint or powder on aluminium, brass, austenitic stainless |
A dual F/NF gauge detects the substrate and selects the method automatically. The magnetic method is standardised in ISO 2178 and the eddy current method in ISO 2360.
MEWOI's coating thickness gauges cover F-only (DM9820, DM9821, ST320), NF-only (DM9823) and combined F/NF (DM9822, ST360, MEWOI870) types. For the DM982x series the supplier gives a 0–1000 µm range, accuracy of ±1–3% of reading or ±2.5 µm, minimum convex radii of 1.5 mm (F) and 3 mm (NF), and a minimum measuring area of 6 mm. The ST360 extends to 10,000 µm with an optional probe.
For reliable readings: zero the gauge on uncoated substrate of the same material, geometry and surface profile; verify it with certified foils close to the expected thickness; keep away from edges and small radii; and take enough readings to describe the area rather than relying on one spot.
Coating Continuity: Holiday (Porosity) Detectors
A "holiday" is a pinhole, void or thin spot that exposes the substrate. On pipelines and tank linings, a single holiday can concentrate corrosion. A high-voltage holiday detector applies a DC voltage between a brush or spring electrode on the coating surface and the earthed substrate. Where the electrode passes over a holiday, the gap breaks down, current flows and the detector alarms, often with a visible spark at the flaw.
The test voltage must be high enough to break down air across the coating thickness at a holiday, yet below the coating's own dielectric strength. Test standards such as NACE SP0274 for pipeline coatings and ASTM G62 give the methods for calculating it from thickness. Low-voltage wet-sponge testing is used for thin films.
MEWOI's holiday detectors come in ranges matched to coating thickness: A covers 0.03–3.5 mm at 0.5–15 kV, B covers 3.5–10 mm at 15–35 kV, and C covers 0.03–10 mm at 0.5–35 kV. They range from battery-powered analogue units (TL4, E2) to digital-control P78/P96 models and mains-powered "U" versions for continuous production-line inspection.
Surface Roughness: Ra, Rz and Friends
Roughness affects coating adhesion, sealing, friction, fatigue and appearance. A stylus roughness meter draws a diamond tip across the surface, filters the profile to separate roughness from waviness, and calculates parameters over a set sampling length:
- Ra — arithmetic mean deviation of the profile; the most widely specified value.
- Rz — mean peak-to-valley height over the sampling lengths; more sensitive to isolated peaks and scratches.
- Rq, Rt, Rsk, Rmr and others — describe the shape of the profile more fully.
MEWOI's roughness meters range from simple Ra/Rz testers (TRT7100) to instruments such as the SR300, which reports 19 parameters with selectable cut-off lengths of 0.25, 0.8 and 2.5 mm and RC, PC-RC, Gaussian and D-P filters. The supplier lists compatibility with ISO 4287, DIN, JIS and ANSI parameter definitions.
Blast-cleaned steel for heavy-duty coatings is usually specified by surface profile height, assessed with comparators, replica tape or profile gauges. Stylus Ra and Rz are mainly used for machined and finished surfaces, so check which measure a coating specification actually calls for.
Frequently Asked Questions
Can one thickness gauge measure zinc on steel? Yes. Zinc is non-magnetic, so an F-type (magnetic induction) gauge measures galvanising on steel.
Can a holiday test damage the coating? It can if the voltage is set too high, which is why the voltage is calculated from coating type and thickness rather than set to maximum.