ET11.pdf

These reference standards are often used to measure

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These reference standards are often used to measure the thickness of paint and other organic coatings on steel, as well as galvanized zinc and other nonmagnetic metallic coatings. The number of different thicknesses required for these calibrations depends on the type of gage and the coating thicknesses to be measured. Magnetic thickness gages may also be used to estimate the magnetic properties of austenitic stainless steel weld metal. Because the magnetic properties of the weld metal are closely related to the ferrite content of the weld, magnetic thickness gages are used to estimate the weld’s ferrite content. Sorting Reference Standards 8 When sorting with the absolute encircling coil technique, a known acceptable calibration standard and a known unacceptable standard are required. When using the comparative encircling coil technique, usually two known acceptable specimens of the test object and one known unacceptable specimen are required. For a three-way sort, it is best to have three calibration reference standards, including either two for the high and low limits of acceptability for one group or one each for two unacceptable groups. The third reference standard represents the acceptable lot of material. Calibration and Standardization Electromagnetic testing is used to sort objects by comparing them to standard specimens. Empirical data and physical tests on samples representing properties to be separated determine the validity of the sorting. The calibration and standardization procedure is based on the properties of the sample requiring separation. The sorting may require more than one test operation. 274 Electromagnetic Testing
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When using the absolute encircling coil technique, the known acceptable calibration standard is inserted in a fixed position in the coil and the test instrument is adjusted to achieve an on-scale meter reading, an oscilloscope reading or both. The acceptable standard is replaced in the exact position with a known unacceptable standard and the sensitivity of the instrument is adjusted to maximize the indicated difference reading without exceeding 90 percent of the available scale range. When using the comparative encircling coil technique, a reference standard is selected (usually one that falls within the acceptable limits of the specimens being tested) and placed in the reference coil. This coil and the reference standard are placed in a location where they will not be accidentally disturbed during the sorting operation. When used with a two-way mix, two calibrated reference standards are chosen: one represents the acceptable group and the other represents the unacceptable group. The acceptable calibration standard is placed at a fixed position in the test coil coinciding with the position of the reference standard in the reference coil.
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  • Fall '19
  • Nondestructive testing, electromagnetic testing

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