Chapter 4 - Temperature sensors and thermal transducers

Abstract

This chapter focuses on temperature sensors and thermal transducers. Although they may seem synonyms, they vary in their usage as the temperature sensors depend on changes that take place in materials as their temperatures change. An older principle used in temperature sensing is liquid expansion in conjunction with a pressure switch, making use of the principles of the familiar mercury thermometer. The simplest sensor of this type is an adaptation of a mercury thermometer with two wire electrodes inset into the capillary. The principle is that two dissimilar metals always have a (small) contact potential between them, and this contact potential changes as the temperature changes. The platinum-resistance thermometer was formerly used only as a laboratory standard, but advances in the construction of these and resistance thermometers generally have led to resistance thermometers being used in many applications for which only thermocouples were once considered. The familiar transducer for electrical to thermal conversion is the heating element, and for most purposes this is composed of a nickel alloy wire such as Nichrome. This alloy of nickel, chromium, and iron has good resistance to oxidation even when red hot, and its resistivity is high, allowing a high resistance to be achieved without the need for very long lengths of narrow-gauge wire.