Thinner coatings will have stronger magnetic attraction while thicker films will have comparatively less magnetic attraction. Coating thickness is determined by measuring this pull-off force. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. ![]() The attraction between the magnet and magnetic steel pulls the two together. Magnetic pull-off gages use a permanent magnet, a calibrated spring, and a graduated scale. Magnetic gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction. Most coatings on steel and iron are measured this way. ![]() Magnetic film gages are used to non-destructively measure the thickness of a nonmagnetic coating on ferrous substrates. The PosiTector RTR can also display a height value (H) that is comparable to what conventional analog spring micrometers would display after the 50.8 μm / 2 mils of incompressible polyester film has been subtracted. The advantages are a reduction in measurement uncertainty, inspector workload, likelihood of error, and the number of replicas needed by inspectors to assure accuracy. There is no need to average two or more replicas from different grades of tape AND there is no need to subtract the 50.8 μm / 2 mils of incompressible polyester film. With a single measurement, the PosiTector RTR produces a more accurate peak-to-valley height measurement HL from Coarse or X-Coarse tapes that has been adjusted for their non-linearity. Measurements with conventional micrometers require a complicated and time consuming procedure of averaging one reading using Coarse grade and one reading using X-Coarse grade to achieve reasonable accuracy. Inside the primary range, Coarse and X-Coarse tape share a 38 - 64 μm (1.5 - 2.5 mils) "overlap" region. That is why two other grades, Coarse Minus ( 115 µm / 4.5 mils), are used to check and, if necessary, adjust measurements at the upper and lower ends of the primary range. Two grades of Testex™ Press-O-Film™ replica tape, “Coarse” and "X-Coarse", are available to span the primary range of surface profiles for the coatings and linings industry –– 20 to 115 µm / 0.8 to 4.5 mils.Īn unfortunate characteristic of replica tape is that conventional spring micrometer measurements are most accurate near the middle of each grade's range and least accurate at the outer ends of each grade's range.
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