ET13.pdf

The temperature at which the conductivity of the

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the temperature at which the conductivity of the reference standard was originally established and the temperature at which instrument calibration and conductivity measurements are performed. To prevent errors from differences in temperature between reference standards and test object, the instrument and 335 Electromagnetic Techniques for Material Identification F IGURE 13. Decrease of measured conductivity with decreasing radius for Unified Numbering System A92024 wrought aluminum alloy, temper 42. Conductivity, MS·m –1 , (percentage of International Annealed Copper Standard) 17 (30) 15 (25) 12 (20) 9 (15) 6 (10) 3 (5) Convex 25 50 75 100 (1) (2) (3) (4) Radius of curvature, mm (in.) Concave F IGURE 14. Effects of liftoff on conductivity measurements. Conductivity meter reading (percent International Annealed Copper Standard) 40 39 38 37 36 35 34 33 32 31 30 29 28 0.07 mm (0.003 in.) liftoff adjustment No liftoff adjustment 0 0.05 0.10 0.15 0.20 0.25 (2) (4) (6) (8) (10) Liftoff, mm (10 –3 in.)
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reference standards should be allowed to stabilize at the test object temperature before calibration and conductivity measurements are performed. In no instance should measurements be taken if the temperatures of the test object and the reference standard differ by more than 5 °C (10 °F). Even though reference standards and test objects are at the same temperature, error in determining conductivity occurs when the measuring temperature differs from the temperature at which the conductivity of a reference standard was originally established. The magnitude of the error becomes larger as this difference in temperature increases. Two other factors also contribute to errors caused by temperature differences: (1) increased difference in conductivity between the upper and lower reference standards and (2) differences in the temperature coefficient of electrical resistivity between reference standards and the test object. These two sources of error can be reduced by decreasing the range between the conductivity standards and using standards of the same or about the same temperature coefficient of electrical resistivity as the test object. Because all aluminum alloys have about the same rate of change of electrical resistivity with temperature change, aluminum conductivity reference standards are preferred for aluminum alloys. Conductivity measurements should not be performed under conditions where the relative humidity exceeds 85 percent. Conductivity Reference Standards For calibration of eddy current conductivity requirements, the number of reference standards needed may vary depending on the test purpose and the accuracy required. The conductivity range of the reference standards used for calibration (calibration blocks) must be within the range of the instrument and must cover the range of conductivity values to be measured. Preferably the calibration reference standards should have the same change in resistivity with temperature as the test objects. It is desirable for the difference in conductivity between the calibration standard representing the low end of the
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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