An Introduction to Electrical Instrumentation and Measurement Systems An Introduction to Electrical Instrumentation and Measurement Systems A guide to the use, selection, and limitations of electrical instruments and measurement systems B.A. Gregory Senior Lecturer Specialising in Electrical Instrumentation, Department of Electrical and Electronic Engineering, Brighton Polytechnic Second Edition M MACMILLAN © B.A. Gregory 1973, 1981 All rights reserved. No reproduction, copy or transmission of this pUblication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with written permission or in accordance with the provisions of the Copyright Act 1956 (as amended). Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages. First edition 1973 Reprinted 1975 (with corrections), 1977 Second edition 1981 Reprinted 1981, 1982 (twice), 1984 (with corrections), 1985 Published by MACMILLAN EDUCATION LTD Houndmills, Basingstoke, Hampshire RG21 2XS and London Companies and representatives throughout the world ISBN 978-0-333-29384-3 ISBN 978-1-349-16482-0 (eBook) DOI 10.1007/978-1-349-16482-0 Contents Preface ix 1. Introduction 1.1 Methods of Measurement 4 1.2 Display Methods 16 1.3 Accuracy 22 1.4 Input Characteristics 37 1.5 Waveform 41 1.6 Interference 49 1.7 Selection 51 2. Analogue Instruments 53 2.1 Moving Coil Instruments 53 2.2 The Electrodynamic Instrument 97 2.3 Other Pointer Instruments 103 2.4 Energy Meters 113 2.5 Solid State Indicators 114 2.6 The Cathode Ray Oscilloscope 115 2.7 Instrumentation Tape Recorders 134 3. Comparison Methods 146 3.1 D.C. Potentiometer 146 3.2 A.C. Potentiometer 155 3.3 D.C. Bridges 156 3.4 A.C. Bridges 166 vi CONTENTS 4. Digital Instruments 186 4.1 Counters 186 4.2 Multi·function Digital Voltmeters 205 4.3 'Intelligent' Instruments 215 4.4 Hybrid Instruments 221 5. Transducers 234 5.l Resistance Change Transducers 238 5.2 Reactance Change Transducers 255 5.3 Semiconductor Devices 261 5.4 Self·generating Transducers 264 5.5 Ultrasonic Transducers 272 5.6 Digital Transducers 272 6. Signal Conditioning 277 6.l Voltage Scaling 277 6.2 Current Scaling 287 6.3 Attenuators 294 6.4 Filters 299 6.5 Probes 308 6.6 Modulation and Sampling 316 6.7 Analogue Processing 319 6.8 Digital-Analogue Conversion 329 7. Interference and Screening 331 7.l Environmental Effects 331 7.2 Component Impurities 334 7.3 Coupled Interference 341 7.4 Noise Rejection Specifications 349 8. Instrument Selection and Specification Analysis 360 8.1 Instrument Selection 360 8.2 Specification Analysis 363 9. Instrumentation Systems 379 9.l System Design 379 9.2 Analogue Systems 380 9.3 Digital Systems 384 CONTENTS vii 10. Problems and Exercises 398 10.1 Principles 398 10.2 Analogue Instruments 400 10.3 Null or Comparison Measurements 404 10.4 Digital Instruments 408 10.5 Transducers 410 10.6 Signal Conditioning 411 10.7 Interference 413 10.8 Selection 415 10.9 Systems 421 10.1 0 Answers 422 Appendix I: Units, Symbols and Conversion Factors 425 Appendix II: Dynamic Behaviour ofM oving Coil Systems 430 Appendix III: Equations to Determine the components of a Resistive 'T' Attenuator Pad 437 Index 439 Preface Our ability to measure a quantity determines our knowledge of that quantity, and since the measuring of electrical quantities-or other parameters in terms of electrical quantities-is involved in an ever expanding circle of occupations of contemporary life, it is essential for the practising engineer to have a thorough knowledge of electrical instrumentation and measurement systems. This is especially so since in addition to his own requirements, he may be called upon to advise others who have no electrical knowledge at all. This book is primarily intended to assist the student following an electrical or electronic engineering degree course to adopt a practical approach to his measure- ment problems. It will also be of use to the engineer or technician, who now fmds himself involved with measurements in terms of volts, amperes, ohms, watts, etc., and faced with an ever increasing variety of instruments from a simple pointer instrument to a complex computer-controlled system. Thus, the object of this book is to help the engineer, or instrument user, to select the right form of instrument for an application, and then analyse the performance of the com- petitive instruments from the various manufacturers in order to obtain the optimum instrument performance for each measurement situation. During that period of my career when I was employed in the research depart- ment of an industrial organisation I was, at times, appalled by the lack of ability exhibited by some graduates in selecting a suitable, let alone the best, instrument to perform quite basic measurements. Since entering the field of higher education to lecture in electrical measurements and instrumentation, my philosophy has been to instruct students to consider each measurement situation on its merits and then select the best instrument for that particular set of circumstances. Such an approach must of course include descriptions of types of instruments, and be presented so that the student understands the functioning and limitations of each instrument in order to be able to make the optimum selection. There will undoubtedly be comments on and criticisms of this version and for those of the previous edition I am grateful. In this second edition, I have updated the material of the 1973 edition, taking into account the many changes that x PREFACE have occurred in instrumentation during the past six years; I have also added instructional problems (a deficiency of the first edition). I have made appreciable rearrangements to the book, largely to accommodate the changes in instrumentation that have resulted from the developments in integrated circuits, such as the microprocessor, which has made possible program- mable and calculating facilities within instruments. Hence, the general theme of the book is to describe the techniques used to produce the various types of instrument available and illustrate their description with examples of manu- facturers' specifications. Unfortunately there is a limit to the amount that can be included in a book of realistic size (and price). I have therefore omitted speciali- sed topics such as medical instruments, chemical analysis, radio frequency measurements, acoustic measurements and programming. The last of these topics it might be argued should be included, for more and more instrumentation will involve the use of programmable devices, be it the purpose-built microprocessor- controlled instrumentation system, or the computer-operated system in which a high level language is used. I would suggest that programming instruction is better documented by the expert rather than by myself. To assist the reader with difficulties of this and other kinds, there is a list of references for further reading at the end of each chapter. I would like to thank all the instrument manufacturers who have willingly assisted me in producing this volume by providing application notes, specifica- tions, reproductions of articles, and also their obliging field engineers. I have endeavoured to acknowledge all sources of diagrams and other material, but I hope that any oversights will be excused. I should also like to thank my colleagues in the Department of Electrical and Electronic Engineering at Brighton Polytechnic for their assistance and encouragement; in particular my thanks are due to Doctors R. Miller, R. Thomas and K. Woodcock, for reading and comment- ing on various parts of the manuscript, also to Brenda Foster for patience and effort in typing the manuscript. B. A. GREGORY 1 Introduction Scientific and technical instruments have been defined as devices used in observ- ing, measuring, controlling, computing or communicating. Additionally the same source* states that: 'Instruments and instrument systems refine, extend or supplement human facilities and abilities to sense, perceive, communicate, remember, calculate or reason'. The principal concern of this book is to describe instruments and their attributes so that the magnitudes of, and variations in, electrical, mechanical, and other quantities may be monitored in an optimised manner for any measurement situation. Before describing any instruments in detail it is desirable to consider the questions that must be answered before making any measurement. (a) What is the most suitable method of performing the measurement? (b) How should the result be displayed? (c) What tolerance on the measured value is acceptable? (d) How will the presence of the instrument affect the signal? (e) How will the signal waveshape affect the instrument's performance? (f) Over what'range of frequencies does the instrument perform correctly? (g) Will the result obtained be affected by external influences? These questions presuppose the possibility of being able to select an instru- ment without any restrictions-a situation unlikely to occur in practice where limitations of availability will be present, or if a new instrument is being pur- chased, financial restrictions are likely to apply. Thus, as in solving any engineer- ing problem, a compromise between the ideal and the real will provide the solution. However, so that the above questions can be honestly answered, the points they raise are discussed in the following sections. *Encyclopedia of Science and Technology (McGraw-Hill, London, 1971)