Plastic shims can be used to calibrate

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Plastic shims can be used to calibrate electromagnetic testing instruments for measurement of nonconductive coatings. Shims should should be made of materials that will not change shape (strain or bend) when pressed on with the probe and that will not change in thickness with variations in moisture or temperature. Two or more shims on top of each other should be avoided unless flexibility of thin shims is required for a curved surface. Coated Substrate Reference Standards Each coating specimen used as a reference standard has a coating of a uniform thickness permanently bonded to a substrate material. Calibration reference standards of several known coating thicknesses may be used for a single application. For calibration, the thickness of one reference coating should be as close as possible to the upper limit, one as close as possible the lower limit and another as close as possible to the desired coating thickness. For instruments that measure coatings on magnetic substrates, calibration reference standards should have the same magnetic properties as the coated test specimen. For electromagnetic testing instruments, such as eddy current liftoff gages, the calibration standard should have the same electrical and magnetic properties as the coated test specimens. To determine calibration validity, a reading should be made on a bare specimen identical to the test object in magnetic and electrical properties. If the coating process is changed after previous calibration, the calibration may no longer be valid, especially for magnetic coating gages and eddy current thickness gages, and the initial reading must again be established. In some cases, calibration of instruments with two-pole probes is checked with the poles rotated 0, 90, 180 and 270 degrees. The substrate thickness for testing and calibration should be the same if their thickness can be sensed by the instrument (that is, if their thickness is less than about four or five standard depths of penetration). Often, it is possible to back up the substrates of standard and test specimens with sufficient thicknesses of the same material (to exceed the critical thickness) and to then make readings independent of substrate thickness. If the curvature of the coating is so extreme as to preclude calibration on a flat surface, then the curvature of the coated standard (or of the substrate on which the calibration foil is placed) should have the same contour. Magnetic Thickness Gages Reference standards are used for the calibration of gages that measure the thickness of nonmagnetic coatings on magnetic materials. In such reference standards, for example, the steel substrates may have the magnetic properties of low carbon steel and the nickel coatings may have the magnetic properties of an annealed watts nickel electrodeposit free of cobalt and iron.
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  • Fall '19
  • Nondestructive testing, electromagnetic testing

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