TABLE OF CONTENTS This chapter describes sensors and electrical circuits for temperature measurements. The International Temperature Scale of 1990 (ITS-90) is described first. It forms a standard for temperature measurements. Resistive sensors, thermocouples, and semiconductor devices are presented next.
The temperature scale fulfills the same function in thermometry as standards of other physical quantities (e.g., the standard of electric voltage) in electric metrology. At present, ITS-90 is obligatory. This scale was established in the same way as every previous international temperature scale, beginning with the International Temperature Scale of 1927:
A certain number of defining fixed points for temperature values assigned to them were determined; each fixed point is a well-reproducible equilibrium state of an element (or water).
Interpolative thermometers, calibrated in defining fixed points, were determined.
Equations were formulated to enable the interpolation of readings of interpolative instruments in temperature measurements between the defining fixed points.
ITS-90 takes over the range from 0.65K to the highest measurable temperature by means of the monochromatic pyrometer. The 17 Defining Fixed Points, listed in Table 2.1, were determined for ITS-90. The fixed points given in the table were defined at the pressure p a = 101,325 Pa, if different data was not given in the description of the equilibrium state. In the upper range of ITS-90, three equivalent defining fixed points were determined: the freezing point of silver (961.78 C), the freezing point of gold (1,064.18 C), and the freezing point of copper (1,084.62 C).
