DFT (Dry Film Thickness) coating thickness gauges are portable instruments dedicated to the non-destructive measurement of the thickness of paints, polymer coatings, enamels, anodising and surface treatments on metal substrates. They are used intensively in industrial painting, in the corrosion protection of metal structures, and in the quality control of automotive, marine, railway and architectural coatings. They operate on magnetic principles (for ferrous substrates) or eddy current (for non-ferrous substrates such as aluminium, brass, copper and non-magnetic stainless steel), with typical accuracies of 1-3 μm on thicknesses from 0 to 1500 μm or more.
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Measuring principles
The magnetic principle (for iron and steel) can be implemented as magnetic pull-off (with a permanent magnet whose force of attraction varies with the thickness of the non-magnetic coating in between) or as magnetic induction (with a high-frequency coil generating a magnetic flux whose variation, as a function of the distance from the substrate, is detected by a Hall sensor). The eddy current principle (for conductive non-ferrous substrates) uses a high-frequency coil that induces eddy currents in the substrate; the impedance of the coil depends on the distance from the substrate, that is, on the thickness of the insulating coating.
Combo instruments
The most widely used models are of the combo (dual) type: they integrate both the magnetic and eddy current principles in a single probe, automatically recognising the substrate and selecting the correct principle. This flexibility is valued in laboratories and on sites where mixed substrates are worked on. Probes are available in various sizes, from micro-probes for small or curved surfaces to standard probes for flat surfaces. Digital displays integrate statistics (mean, max, min, standard deviation), acceptance thresholds (PASS/FAIL), Bluetooth and USB data loggers, and software for managing extended inspection campaigns on large surfaces.
Applications
Applications include quality control of the industrial painting of metal fabrications, tanks, bridges, ships, cranes, gas pipelines, pipelines, offshore platforms, automotive bodies, metal window and door frames, anodised aluminium profiles, fencing, lamp posts and street furniture. DFT measurement is generally an integral part of the quality plan in corrosion protection specifications and follows standardised sampling procedures (for example ISO 19840 for the statistics of measurements on large surfaces, which lays down minimum numbers of readings per area and acceptance criteria).
Reference standards
The main standards are ISO 2178 (magnetic method), ISO 2360 (eddy current method), ISO 19840 (statistical procedure for DFT measurements on metal structures) and ASTM D7091 (equivalent US combo standard). Instruments are calibrated with non-magnetic shims (for the magnetic principle) or non-conductive shims (for eddy current) of certified thickness, on an appropriate reference substrate, in accordance with ISO 17025 procedures. Periodic checks with calibration standards are required before every measurement campaign on critical structures.
Selection criteria
The choice depends on the type of substrate (ferrous, non-ferrous, mixed), the typical thickness range of the coatings, the precision required, the size of the surfaces (miniaturised probes for small surfaces), data logging and reporting requirements, and portability for use on site (IP protection rating, battery life, robustness). For professional corrosion protection inspections, instruments with ISO 17025 traceable calibration and software compliant with ISO 19840 procedures for the automatic generation of conformity reports are recommended.
The integration of wireless probes with mobile applications for georeferencing measurements is the most recent development: each reading is associated with GPS coordinates and photographs of the measuring point, generating campaign documentation that stands up to dispute.
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