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MEASUREMENT AND INSTRUMENTATION

TABLE 4

Resistance versus Temperature for Various Metals

Resistivity, Metal g · cm Alumel∗ 28.1 Copper 1.56 Iron 8.57 Nickel 6.38 Platinum 9.83 Silver 1.50 ∗

85

Relative Resistance Rt/R0 at 0◦C − 200

− 100

0.117 0.557

0.177 0.599 0.176 0.596

0 1.000 1.000 1.000 1.000 1.000 1.000

100 1.239 1.431 1.650 1.663 1.392 1.408

200 1.428 0.862 2.464 2.501 1.773 1.827

300 1.537 2.299 3.485 3.611 2.142 2.256

400 1.637 2.747 4.716 4.847 2.499 2.698

500 1.726 3.210 6.162 5.398 3.178 3.616

600 1.814 3.695 7.839 5.882 3.178 3.616

700 1.899 4.208 9.790 6.327 3.500 4.094

800 1.982 4.752 12.009 6.751 3.810 5.586

900 2.066 5.334 12.790 7.156 4.109 5.091

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Certain suitably chosen and prepared materials that vary in resistance in a well-defined and calibrated manner with temperature became readily available around 1925, prompting the use of resistance thermometers as primary sensors for industrial applications where reproducibility and stability are of critical importance. Platinum resistance thermometers became the international standard for temperature measurements between the triple point of hydrogen at 13.81 K and the freezing point of antimony at 730.75◦C. Since the 1970s RTDs have made very serious inroads on the thermocouple for very broad usage in industry—for practical industrial use, not just for applications requiring exceptional accuracy. The advantages and limitations of RTDs as compared with thermocouples in this present time span are presented later in this article. Principles of Resistance Thermometry For pure metals, the characteristic relationship that governs resistance thermometry is given by Rt = R0 (1 + at + bt 2 + ct 3 + · · ·)where

R0 = resistance at

reference temperature (usually at ice point, 0◦C), Rt = resistance at temperature t, a = temperature coefficient of resistance, / (◦C) b, c = coefficients calculated on the basis of two or more known resistance-temperature (calibration) points For alloys and semiconductors, the relationship follows a unique equation dependent on the specific material involved. Whereas most elements constructed from metal conductors generally display positive temperature coefficients, with an increase in temperature resulting in increased resistance, most semiconductors display a characteristic negative temperature coefficient of resistance. Only a few pure metals have a characteristic relationship suitable for the fabrication of sensing elements used in resistance thermometers. The metal must have an extremely stable resistancetemperature relationship so that neither the absolute value of the resistance R0 nor the coefficients a and b drift with repeated heating and cooling within the thermometer’s specified temperature range of operation. The material’s specific resistance in ohms per cubic centimeter must be within limits that will permit fabrication of practical-size elements. The material must exhibit relatively small resistance changes for nontemperature effects, such as strain and possible contamination which may not be totally eliminated from a controlled manufacturing environment. The material’s change in resistance with temperature must be relatively large in order to produce a resultant thermometer with inherent sensitivity. The metal must not undergo any change of phase or state within a reasonable temperature range. Finally, the metal must be commercially available with essentially a consistent resistance-temperature relationship to provide reliable uniformity. Industrial resistance thermometers, often referred to as RTDs, are commonly available with elements of platinum, nickel, 70% nickel–30% iron (Balco), or copper. The entire resistance thermometer is an assembly of parts, which include the sensing element, internal leadwires, internal supporting and insulating materials, and protection tube or case (Fig. 11 and Table 4).

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Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

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