ABSTRACT
Electric resistance thermometers utilize the fact that the electric resistance of conductors (i.e., metals) increases with increasing ambient temperature. The electric resistance of semiconducting materials also changes with the ambient temperature. In the latter case, the concentration and mobility of the charge carrier in the materials increase with increasing temperature, and therefore the resistance decreases. The temperature dependence of the electric resistance of semiconducting materials is expressed by
R aT cT b 2 exp
(5.1)
where R is the resistance (V), T is the temperature (K), and a, b, and c are calibration constants. Resistance thermometers constructed of semiconducting materials are known as thermistors (i.e., thermally sensitive resistors). Thermistors are less stable and accurate than metal thermoresistors but offer the advantages of a lower manufacturing cost and a much higher resistivity than that of a metal thermoresistor.2 Table 5.2 lists the types of resistance thermometers and their operating temperature ranges.
