ET13.pdf

Calibration range and the calibration standard

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calibration range and the calibration standard representing the high end of the range to be at least 10 percent International Annealed Copper Standard (IACS) but not to exceed 25 percent. When a calibration curve is established for a general purpose instrument, a sufficient number of calibration standards are necessary to develop a smooth continuous curve over the range of interest. For convenience of transportation and storage, conductivity standards are usually kept relatively small. Reference standards must have sufficient size to prevent edge effects or thickness from having a bearing on conductivity readings. These requirements can be satisfied by requiring length and width to be 25 mm (1 in.) greater than the probe diameter and the thickness greater than 3.5 times the standard depth of penetration at the test instrument frequency. Calibration standards should be flat, have a smooth surface and be free of coatings. Reference standards used for calibrating instruments immediately before measuring conductivity should be accurate within ± 0.5 percent International Annealed Copper Standard of the nominal value. A second set of calibration standards accurate within ± 0.6 MS·m –1 ( ± 0.35 percent of the International Annealed Copper Standard) should be periodically made available for checking the performance of instruments and field calibration standards. Calibration standards should be traceable to the National Institute of Standards and Technology, Gaithersburg, Maryland. Such calibration standards are available from some manufacturers of eddy current conductivity instruments. Many reference standards, particularly those of aluminum alloys, are subject to metallurgical changes if exposed to temperatures of 65 °C (150 °F) or greater. Surfaces of reference standards can also corrode if exposed to moisture or other hostile environments. Damage caused by rough handling can also lead to error in conductivity readings. For these reasons, standards should be transported and stored in dry, clean, protected areas not subject to excessive temperatures. 336 Electromagnetic Testing
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Magnetic Hysteresis 6 The magnetic behavior of a ferromagnetic material is characterized by its hysteresis loop (magnetization curve). In ferromagnetic materials, there is no linear relationship between magnetic field intensity H and flux density B . A very small change in H may produce a large change in B . The relationship is best shown in the magnetization or B , H curve (Fig. 15). As H increases, the flux density increases rapidly up to the knee of the curve. Beyond the knee, a further increase in H causes no useful increase in B and the material is said to be saturated. It is evident from Fig. 15 that the magnetic permeability μ (the ratio of magnetic flux density B to magnetic field intensity H ) up to the saturation magnetization I s is not constant. The magnetic permeability’s variation as a function of B is shown in Fig. 16. The largest value on this curve is the maximum permeability μ max . Another important quantity is the initial
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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