Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 307 of 331

Fig. 2 Roll-back dial magnetic pull-off thickness gauge 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 bal- anced arm. 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 pow- der coating operations. Typical tolerance is ±5%. Magnetic and Electromagnetic Induction These electronic instruments measure the change in magnetic flux density at the sur- face of a magnetic probe as it is brought near steel. The magnitude of the flux den- sity 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 in- struments 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 com- monly use a constant pressure probe to remove operator influence and quick- ly 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 tol- erance is ±1%. The manufacturer's instructions should be carefully followed for most accu- rate 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 noncon- ductive coatings on nonferrous metal substrates. Paint on aluminum and acrylic on copper are typical examples. Eddy current inspection is based on the princi- ples of electromagnetic induction and, therefore, has many similarities to the elec- tromagnetic induction test method. A coil of fine wire conducting a high-fre- quency alternating current (above 1 MHz) is used to set up an alternating mag- netic 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 magnitude of the eddy currents is related to substrate char- acteristics and the distance,or coating thickness, from the probe. The eddy cur- 306

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