Measurement Systems

This chapter introduces the basic constituent elements of measurement systems and the performance terminology used in the specification of instrumentation, i.e. terms associated with resolution, accuracy, errors, range, precision, repeatability, reproducibility, sensitivity, dynamic characteristics, and reliability. The idea of national standards and traceability of measurements is introduced.

Keywords

Instrumentation terminology; national standards

Outline

1.1 Introduction

This chapter is an introduction to the instrumentation systems used for making measurements and deals with the basic elements of such systems and the terminology used to describe their performance in use.

1.1.1 Systems

The term system will be freely used throughout this book, and so here is a brief explanation of what is meant by a system and how we can represent systems.

If you want to use an amplifier then you might not be interested in the internal working of the amplifier but what output you can obtain for a particular input. In such a situation we can talk of the amplifier being a system and describe it by means of specifying how the output is related to the input. With an engineering system an engineer is often more interested in the inputs and outputs of a system than the internal workings of the component elements of that system.

A system can be defined as an arrangement of parts within some boundary which work together to provide some form of output from a specified input or inputs. The boundary divides the system from the environment and the system interacts with the environment by means of signals crossing the boundary from the environment to the system, i.e. inputs, and signals crossing the boundary from the system to the environment, i.e. outputs (Figure 1.1).

A useful way of representing a system is as a block diagram. Within the boundary described by the box outline is the system, and inputs to the system are shown by arrows entering the box and outputs by arrows leaving the box. Figure 1.2 illustrates this for an electric motor system; there is an input of electrical energy and an output of mechanical energy, though you might consider there is also an output of waste heat. The interest is in the relationship between the output and the input rather than the internal science of the motor and how it operates. It is convenient to think of the system in the box operating on the input to produce the output. Thus, in the case of an amplifier system (Figure 1.3) we can think of the system multiplying the input V by some factor G, i.e. the amplifier gain, to give the output GV.

Often we are concerned with a number of linked systems. For example, we might have a CD player system linked to an amplifier system, which, in turn, is linked to a loudspeaker system. We can then draw this as three interconnected boxes (Figure 1.4) with the output from one system becoming the input to the next system. In drawing a system as a series of interconnected blocks, it is necessary to recognise that the lines drawn to connect boxes indicate a flow of information in the direction indicated by the arrow and not necessarily physical connections.

1.2 Instrumentation Systems

The purpose of an instrumentation system used for making measurements is to give the user a numerical value corresponding to the variable being measured. Thus a thermometer may be used to give a numerical value for the temperature of a liquid. We must, however, recognise that, for a variety of reasons, this numerical value may not actually be the true value of the variable. Thus, in the case of the thermometer, there may be errors due to the limited accuracy in the scale calibration, or reading errors due to the reading falling between two scale markings, or perhaps errors due to the insertion of a cold thermometer into a hot liquid, lowering the temperature of the liquid and so altering the temperature being measured. We thus consider a measurement system to have an input of the true value of the variable being measured and an output of the measured value of that variable (Figure 1.5). Figure 1.6 shows some examples of such instrumentation systems.

An instrumentation system for making measurements has an input of the true value of the variable being measured and an output of the measured value. This output might be then used in a control system to control the variable to some set value.

1.2.1 The Constituent Elements of an Instrumentation System

An instrumentation system for making measurements consists of several elements which are used to carry out particular functions. These functional elements are:

Figure 1.10 shows how these basic functional elements form a measurement system.

The term transducer is often used in relation to measurement systems. Transducers are defined as an element that converts a change in some physical variable into a related change in some other physical variable. It is generally used for an element that converts a change in some physical variable into an electrical signal change. Thus sensors can be transducers. However, a measurement system may use transducers, in addition to the sensor, in other parts of the system to convert signals in one form to another form.