Resistance thermometry metals

For pure metals, resistance thermometry is governed by Equation (5), where R( is the resistance at temperature t, Rq is the resistance at the reference temperature, usually 0°C, and a, h, c are coefEcients based on calibration points [3]. 

The resistance thermometry is based on the temperature dependence of the electric resistance of metals, semiconductors and other resistive materials. This is the most diffused type of low-temperature thermometry sensors are usually commercial low-cost components. At very low temperatures, however, several drawbacks take place such as the low thermal conductivity in the bulk of the resistance and at the contact surface, the heating due to RF pick up and overheating (see Section 9.6.3)... 

Since the resistivity of pure metals varies with change in temperature, metals have been used as simple and reliable temperature-measuring devices. Many elements or compounds, however, are not suitable for use in low-temperature resistance thermometry since they lack one or more of the desirable properties of an ideal resistance thermometer. Such properties include high sensitivity, linear resistivity with temperature, high stability over time and with thermal cycling, and mechanical workability. 

Even though there are a number of metals that appear suitable for resistance thermometry, platinum has come to occupy a predominant position, partly because of excellent characteristics, such as chemical inertness and ease of fabrication, and partly because of custom that is, certain desirable features such as ready availability in high purity and the existence of a large body of knowledge about its behavior have come into being as its use grew and have tended to perpetuate that use. Its sensitivity down to 20 K and its stability are excellent. Its principal disadvantages are low resistivity, insensitivity below about 10 K, and a variation of the form of the resistance-temperature relation from specimen to specimen below about 30 K. 

The basis for resistance thermometry is the feet that most metals and some semiconductors change in resistivity with temperature in a known, reproducible manner. Several materials are commonly employed for resistance thermometers. 

In the last few years a program of precision secondary thermometry in the 2-4 K range has been greatly facilitated by the use of metallic storage dewars which contain a few liters of liquid helium. The temperature distribution in these nearly "constant temperature" baths have been explored with both resistance and vapor-pressure thermometers. The results of these investigations, and also the reproducibilities of resistance thermometers, are presented. 

Temperature can be measured from heat transfer by conduction, convection, or radiation. Household thermometers use either the expansion of metals or other substances or the increase in resistance with temperature. Thermocouples measure the electromotive force generated by temperature difference. Pyrometers measure infrared radiation from a heat source. Spectroscopic thermometry compares the spectrum of radiation against a blackbody spectrum. Temperature-sensitive paints and liquid crystals change intensity of radiation in certain wavelengths with temperature. 

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