Fundamentals of Instrumentation and Measurement

Chapter 2¹

General Principles of Sensors

Sensors are the first components of a measurement chain. They convert the physical and chemical variables of a process or an installation into electrical signals that almost always begin as analogical signals. This conversion must also mirror as closely as possible the involved variables. Only a thorough knowledge of sensor responses guarantees success; sometimes sensors produce faulty signals due to interference, the conditions of use, or often because of the processes themselves.

We begin this chapter by discussing some of the basic principles of sensors and how they work [NOR 99]. These principles are based on calibration, evaluation of uncertainties, calculation of response time, and conditioning. Our aim is to provide the reader with a fairly general guide. Some relevant equations and formulae, as well as many issues relating to instrumentation and signal analysis, will be discussed in later chapters.

2.1. General points

2.1.1. Basic definitions

The quantity to be measured being the measurand, which we call m, the sensor must convert m into an electrical variable called s.

The measurement s can be an impedance, an electrical charge, a current, or a difference of potential. The relation that joins s to m can be called s = F(m) and depends on:

- the physical law determining the sensor;

- the structure and purpose of the sensor;

- the sensor’s environment.

The expression F(m) is established by calibration. By using a standard or unit ...