Metal Finishing Guide Book

2013

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set up eddy currents on the surface. The magnitude of the eddy currents is related to substrate characteristics and the distance,or coating thickness, from the probe. The eddy currents create their own opposing electromagnetic field that can be sensed by the exciting coil or by a second, adjacent coil. Eddy current coating thickness gauges look and operate like electronic magnetic gauges. They are used to measure coating thickness over all metals but steel and iron. They commonly use a constant pressure probe and disFig. 3 Electronic magnetic induction thickness gauges. play results on an LCD. Optionally, they can store measurement results or perform instant analysis of readings and output to a printer or computer for further examination. The typical tolerance is ±1%. Testing is sensitive to surface roughness, curvature, substrate thickness, type of metal substrate, and distance from an edge. The gauge manufacturer\'s instructions should be carefully followed for the most accurate results. Standard methods for the application and performance of this test are available in ASTM B 244 and ASTM D 1400.It is now common for gauges to incorporate both magnetic and eddy current principles into one unit. Some simplify the task of measuring most coatings over any metal by switching automatically from one principle of operation to the other, depending upon the substrate. These combination units are popular with painters and powder coaters. ULTRASONIC The ultrasonic pulse-echo technique is used to nondestructively measure the thickness of various coatings on nonmetal substrates. Applications include paint on plastic, lacquer on wood, and epoxy on concrete. The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating (see Fig. 4). The pulse reflects back from the substrate to the transducer and is converted into a high-frequency electrical signal. The echo wave form is then digitized and analyzed to determine coating thickness. In some circumstances, individual layers in a multilayer system can be measured. Couplant is typically used between the probe and the surface to be measured. Typical tolerance is ±3%. 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 6132. 549

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