Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 311 of 331

which can either be thoroughly cleaned after each reading and reused or dis- posed of. The notch-type, wet-film thickness gauge is a flat aluminum, plastic, or stainless steel plate with calibrated notches on the edge of each face. The gauge is placed squarely and firmly onto the surface to be measured immedi- ately after the coating application and then removed. The wet-film thickness lies between the highest coated notch and the next uncoated notch. Notched gauge measurements are neither accurate nor sensitive, but they are useful in determining approximate wet-film thickness of coatings on articles where size and shape prohibit the use of more precise methods, such as the lens and eccentric roller gauges (refer to ASTM D 1212). The gauge should be used on smooth surfaces, free from irregularities, and should be used along the length, not the width, of curved surfaces. Using a wet- film gauge on quick-drying coatings (inorganic zinc, vinyls, etc.) will yield in ac- curate measurements. ASTM D 4414 outlines a standard method for measure- ment of wet-film thickness by notch gauges. THICKNESS STANDARDS Coating thickness gauges are calibrated to known thickness standards. There are many sources of thickness standards but care must be taken to ensure they are traceable to a national measurement institute such as NIST. It is also im- portant to verify that the standards are at least four times as accurate as the gauge they will be used to calibrate. A regular check against these standards ver- ifies the gauge is operating properly. When readings do not meet the accuracy spec- ification of the gauge, the gauge must be adjusted or repaired and then cali- brated again. 310

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