Why Dry Film Thickness Measurement is Important?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 thickness ensures optimum product performance. Even the most basic specification will require DFT to be measured
What are Magnetic Film Thickness Gages?Magnetic film gages 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 gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction.
Magnetic Pull-off (Type 1 - Pull Off Coating Thickness Gages)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.
In Type 1 pull-off (PosiTest or PosiPen) gages, a permanent magnet is brought into direct contact with the coated surface. The force necessary to pull the magnet from the surface is measured and interpreted as the coating thickness value on a scale or display on the gage. The magnetic force holding the magnet to the surface varies inversely as a non-linear function of the distance between magnet and steel, i.e., the thickness of the dry coating. Less force is required to remove the magnet from a thick coating.
Magnetic and Electromagnetic Induction (Type 2 - Electronic Coating Thickness Gages)
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 coating thickness can be determined.
A Type 2 electronic gage (PosiTector 6000/PosiTest DFT) uses electronic circuitry to convert a reference signal into coating thickness. Electronic ferrous gages operate on two different magnetic principles. Some use a permanent magnet that when brought near steel, increases the magnetic flux density at the pole face of the magnet. Coating thickness is determined by measuring this change in flux density, which varies inversely to the distance between the magnet and the steel substrate. Hall elements and magnet resistance elements positioned at the pole face are the most common ways this change in magnetic flux density is measured. However, the response of these elements is temperature dependent, so temperature compensation is required.
Other ferrous electronic gages operate on the principle of electromagnetic induction. A coil containing a soft iron rod is energized with an AC current thereby producing a changing magnetic field at the probe. As with a permanent magnet, the magnetic flux density within the rod increases when the probe is brought near the steel substrate. This change is detected by a second coil. The output of the second coil is related to the coating thickness. These gages also need temperature compensation due to the temperature dependence of the coil parameters.
What are Eddy Current Film Thickness Gages? (Type 2 - Electronic Coating Thickness Gages)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 surfaceof the instrument's probe. When the probe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on thesurface. The substrate characteristics and the distance of the probe from the substrate (the coating thickness) affect the magnitude of the eddycurrents. The eddy currents create their own opposing electromagnetic field that can be sensed by the exciting coil or asecond, adjacent coil.
