Thermocouples

Theory and Properties

DanielD. Pollock, Daniel D. Pollock

• Publisher: Routledge
• Year: 2018
• Print ISBN: 9780849342431
• eBook ISBN: 9781351408370
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Thermocouples: Theory and Properties provides the basis for the examination and explanation of thermoelectric phenomena and their correlations with other physical properties. These results are applied and account for the properties and deviations of commercial materials in the temperature ranges of most common industrial usage. This book is written expressly for non-scientists and is an effective tool for the busy technician or engineer working with thermoelectric thermometry in metallurgical, chemical, petroleum, pharmaceutical, and food processing areas. It is also beneficial for use in quality control and research and development applications. The book provides more than the usual superficial presentations of thermoelectric properties; it explains the ""why"" as well as the ""how"" and ""what"" of thermoelectric behaviors. These answers are important because only a suitable combination of theory and practice can lead to the understanding required for optimum thermometric applications under the multitude of applications encountered in industry and science.

1 Introduction

THIS CHAPTER is intended to introduce the reader to the fields of thermoelectricity and solid-state physics. An elementary review of some physical and thermoelectric concepts is given with some discussion in terms of the original findings of Thomson. These are presented and derived in subsequent chapters.

The inability of classical electron theory to rationalize thermoelectric phenomena is used to show the need for more adequate methods to explain these properties. The requisite explanations may be obtained by the use of elementary quantum mechanics. Since it is expected that some readers are unfamiliar with quantum mechanics, brief explanations are given of some of the findings that culminated in the quantum mechanics. These provide a basis for insight into fundamental quantum concepts and lead to modern physics.

1.1 Thermoelectricity

A thermocouple is a device whose principal application is temperature measurement. This originates from the work of Seebeck (1822), who found that small electric currents flow in a closed circuit that consists of two unlike conductors^† when a temperature difference exists between their junctions. The open-circuit voltages (electromotive force [emf]) generated this way are the relative Seebeck voltages. The pair of conductors, or thermoelements, of which the thermoelectric circuit is composed is called a thermocouple. In effect, a thermocouple constitutes a means for the conversion of thermal energy into electrical energy. The resultant electrical energy is a function of the temperature difference and thus can be used as a source of electrical energy or as a means for temperature measurement.

Selected pairs of thermoelements give voltages that vary in a reasonably regular way with the differences in temperatures between their junctions. Temperature measurements may be made by maintaining one of the junctions at a known, fixed, reproducible temperature that is called the reference temperature. The reference temperature for practical measurements is commonly taken as the melting point of ice (0°C). The other junction is known as the measuring junction. When a known reference temperature is used, the temperature difference between the junctions, and consequently the voltage output of the thermocouple, is determined by the temperature of the measuring junction. Such a voltage-temperature relationship may be expressed in the forms of tables, curves, mathematical equations, or as “memories” in temperature-measuring devices. Each of these relationships gives the voltage generated by specific pairs of thermoelements either directly or indirectly as a function of the temperature of the measuring junction. Thermoelectric thermometry is based on such relationships.

It should be noted that the reference junction need not be held at any one reference temperature, provided that its actual temperature is known. This is important because it is not always convenient to maintain reference junctions at 0°C under industrial conditions. If, for example, the reference junction is held at 25°C instead of the commonly used 0°C and the measuring junction is held at 100°C, the voltage output of the thermocouple will be that resulting from a temperature difference of 75°C rather than of one resulting from a difference of 100°C; its voltage output will be smaller because the actual reference junction is 25°C too high. This can be taken into account by adding the voltage equivalent of this 25°C difference to the smaller output of the thermocouple. Accommodations such as this may be made by either manual or automatic means, depending on the voltage-measuring device. Adjustments of this kind are known as reference junction temperature compensation.

It is important to note that thermocouples have nonlinear emf-temperature relationships. Given temperature differences do not generally yield constant emf differences when determined at different ranges of temperature. This is the same as saying that the slope, or the change in emf per degree change in temperature, or the relative Seebeck coefficient, RSC, is not a constant. The RSC is a helpful and convenient means for the description of the thermoelectric properties of a thermocouple over various ranges of temperatures. It also is a useful measure for comparing the thermoelectric response of different types of thermocouples.

1.2 Related Thermal Effects

The energies of two other thermal phenomena, the Peltier and Thomson e...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• 1 Introduction
• 2 Statistical Treatment of Electrons
• 3 Solid-State Theories
• 4 Thermoelectric Phenomena
• 5 Modern Theories of Thermoelectricity
• 6 Variations of the Fermi Energy
• 7 Some Effects of Alloying
• 8 Standard Thermoelements (Dilute Alloys)
• 9 Constantans
• 10 Thermoelectric Applications of Semiconductors
• 11 Thermoelectricity as a Research Tool
• Appendices
• Index