Metal Finishing Guide Book


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or hard-to-reach surfaces. A triple indicator ensures accurate measurements when the gauge is pointed down, up, or horizontally with a tolerance of ±10%. Roll-back dial models are the most common form of magnetic pull-off gauge (see Fig. 2). A magnet is attached to one end of a pivoting balanced arm. Fig. 2 Roll-back dial magnetic pull-off thickness gauge This assembly is connected to a calibrated hairspring. By rotating the dial with a finger, the spring increases the force on the magnet and pulls it from the surface. These gauges are easy to use and have a balanced arm, which allows them to work in any position, independent of gravity. They are safe inexplosive environments and are commonly used by painting contractors and small powder coating operations. Typical tolerance is ±5%. Magnetic and Electromagnetic Induction These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it is brought near steel. 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. 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, ferro magnetic 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. Electronic magnetic gauges come in all shapes and sizes (see Fig.3). They commonly use a constant pressure probe to remove operator influence and quickly display results on a liquid crystal display (LCD). Optionally, they can store measurement results, perform instant analysis of readings on their displays, and output results to a printer or computer for further examination. Typical tolerance is ±1%. The manufacturer's instructions should be carefully followed for most accurate results. Standard methods for the application and performance of this test are available in ASTM D 1400, ISO 2360, and ISO 2808. EDDY CURRENT This technique is used to nondestructively measure the thickness of nonconductive coatings on nonferrous metal substrates. Paint on aluminum and acrylic on copper are typical examples. Eddy current inspection is based on the principles of electromagnetic induction and, therefore, has many similarities to the electromagnetic induction test method. 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 548

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