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Dry film thickness (DFT) or coating thickness is arguably the single most important measurement made during the application and inspection of protective coatings. Coatings are designed to perform their intended function when applied within the DFT range as specified by the manufacturer. Correct paint thickness ensures optimum product performance. Even the most basic specification will require DFT to be measured.
Magnetic film gages (paint depth gauges) are used to nondestructively measure the thickness of a nonmagnetic coating on ferrous substrates. Most coatings on steel and iron are measured this way. Magnetic paint mil gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction.
Magnetic pull-off gages use a permanent magnet, a calibrated spring, and a graduated scale. The attraction between the magnet and magnetic steel pulls the two together.
Magnetic induction instruments use a permanent magnet as the source of the magnetic field. A Hall-effect generator or magneto-resistor is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic induction instruments use an alternating magnetic field. A soft, ferromagnetic rod wound with a coil of fine wire is used to produce a magnetic field. A second coil of wire is used to detect changes in magnetic flux.
These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it nears a steel surface. The magnitude of the flux density at the probe surface is directly related to the distance from the steel substrate. By measuring flux density, the paint thickness can be determined.
Eddy current techniques are used to nondestructively measure the thickness of nonconductive coatings on nonferrous metal substrates. A coil of fine wire conducting a high frequency alternating current (above 1 MHz) is used to set up an alternating magnetic field at the surface of the instrument's probe. When the probe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on the surface. The substrate characteristics and the distance of the probe from the substrate (the thickness) affect the magnitude of the eddy currents. The eddy currents create their own opposing electromagnetic field that can be sensed by the exciting coil or a second, adjacent coil.
The ultrasonic pulse-echo technique is used to measure the thickness of paint and other coatings on non-metal substrates (plastic, wood, concrete, and composites) without damaging the coating.
Thickness measurements of powder can be taken before and after curing. Substrate type, thickness range, part shape, and economics determine the best method to be employed.
For uncured applied powders, height measurement can be performed with DeFelsko Powder Combs. This technique is destructive and may require recoating the part. Coating powders generally diminish in thickness during the curing process, so these procedures require a reduction factor be determined to predict cured film thickness.
Non-contact ultrasonic instruments, such as the PosiTest PC Powder Checker also measure uncured powder but do so without touching the surface. Instead of measuring powder height, they automatically display a predicted cured thickness result. Accurate measurement at the time of application eliminates over usage, improves quality, controls set-up, reduces rework & waste — saving money.
For after-cure measurement, a variety of handheld paint meters are available. These non-destructive instruments employ either magnetic, eddy current, or ultrasonic principles depending on the substrate.