How to Choose a Coating Thickness Gauge

Dry film thickness (DFT) is the number most coating inspections live or die by. Too thin and the coating fails early; too thick and you waste paint and risk cracking. A dry film coating thickness gauge is how you prove the applied system meets the specification. The hard part is choosing the right one, because the best gauge for a steel bridge is not the best gauge for an anodised aluminium panel.

Step 1: identify your substrate

This is the first fork in the road.

• Ferrous (steel, iron): magnetic induction gauges measure coating over a magnetic base.
• Non-ferrous (aluminium, stainless, brass, galvanising over a known base): eddy current gauges measure non-conductive coatings over non-magnetic metal.
• Both: dual or "F/N" gauges switch automatically between the two principles, which is the safe choice for a workshop that handles mixed work.

If you only ever inspect painted structural steel, a ferrous-only gauge keeps things simple. If you cannot predict what comes through the door, buy dual.

Step 2: decide separate or integral probe

Integral gauges put the probe in the body of the instrument, which is fast and easy one-handed. Separate (cabled) probes reach into recesses, weld seams and tight geometry that an integral gauge cannot. Inspectors working on complex fabrications usually want a separate probe; a coater checking flat panels is well served by an integral unit.

Step 3: match the standards you report against

If your reports cite a standard, the gauge has to support how that standard expects you to calibrate and record. Common ones in protective coating work include ISO 19840 and SSPC PA 2, which set out how many readings to take and how to handle surface profile. Check that the gauge calibrates against foils or shims you can trace, and that it stores readings if your client expects batch statistics.

Step 4: think about how you record and report

A basic gauge gives you a number on screen. A statistical gauge logs readings, calculates mean, standard deviation and pass or fail against limits, and exports to software for a report. If you produce inspection documentation, on-gauge statistics and data download save hours and remove transcription errors.

A quick decision summary

• Painted structural steel, simple jobs: ferrous integral gauge.
• Mixed substrates in a busy workshop: dual F/N gauge.
• Inspection of complex shapes and welds: separate probe.
• Formal reporting to a standard: a statistical gauge with data logging and traceable calibration foils.

Still not sure?

Tell us the substrate, the coating system and the standard you report against, and we will point you at the right gauge rather than the most expensive one. Ten minutes on a call usually settles it.