Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 308 of 331

rents create their own op- posing electromagnetic field that can be sensed by the ex- citing coil or by a second, ad- jacent coil. Fig. 3 Electronic magnetic induction thickness gauges. curvature, substrate thickness, type of metal sub- strate, and distance from an edge. The gauge manufacturer\'s in- structions should be carefully followed for the most accurate results. Stan- dard 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 incorpo- rate both magnetic and eddy current principles into one unit. Some sim- plify the task of measuring most coat- ings over any metal by switching au- tomatically from one principle of op- eration 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 in- clude paint on plastic, lacquer on wood, and epoxy on concrete. The probe of the instrument con- Eddy current coating thickness gauges look and operate like electronic mag- netic gauges. They are used to measure coating thickness over all metals but steel and iron. They commonly use a constant pressure probe and display results on an LCD. Optionally, they can store measurement results or per- form instant analysis of read- ings and output to a printer or computer for further ex- amination. The typical tol- erance is ±1%.Testing is sen- sitive to surface roughness, 307

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