We work with leading authors to develop the strongest educational materials in engineering, bringing cutting-edge thinking and best learning practice to a global market. Under a range of well-known imprints, including Prentice Hall, we craft high quality print and electronic publications which help readers to understand and apply their content, whether studying or at work. To ﬁnd out more about the complete range of our publishing, please visit us on the World Wide Web at: www.pearsoned.co.uk
Principles of Measurement Systems
John P. Bentley Emeritus Professor of Measurement Systems University of Teesside
Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsoned.co.uk First published 1983 Second Edition 1988 Third Edition 1995 Fourth Edition published 2005 © Pearson Education Limited 1983, 2005 The right of John P. Bentley to be identiﬁed as author of this work has been asserted by him in accordance w th the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP. ISBN 0 130 43028 5 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data Bentley, John P., 1943– Principles of measurement systems / John P. Bentley. – 4th ed. p. cm. Includes bibliographical references and index. ISBN 0-13-043028-5 1. Physical instruments. 2. Physical measurements. 3. Engineering instruments. 4. Automatic control. I. Title. QC53.B44 2005 530.8–dc22 2004044467 10 10 9 8 7 6 09 08 07 5 4 3 06 05 2 1
Typeset in 10/12pt Times by 35 Printed in Malaysia The publisher’s policy is to use paper manufactured from sustainable forests.
To Pauline, Sarah and Victoria
Preface to the fourth edition Acknowledgements xi xiii
The 1.1 1.2 1.3 1.4 General Measurement System Purpose and performance of measurement systems Structure of measurement systems Examples of measurement systems Block diagram symbols
3 3 4 5 7 9 9 15 17 21 35 35 36 41 51 51 58 65 70 77 77 84 97 97 98 107 110 113
Static Characteristics of Measurement System Elements 2.1 Systematic characteristics 2.2 Generalised model of a system element 2.3 Statistical characteristics 2.4 Identiﬁcation of static characteristics – calibration The Accuracy of Measurement Systems in the Steady State 3.1 Measurement error of a system of ideal elements 3.2 The error probability density function of a system of non-ideal elements 3.3 Error reduction techniques Dynamic Characteristics of Measurement Systems 4.1 Transfer function G(s) for typical system elements 4.2 Identiﬁcation of the dynamics of an element 4.3 Dynamic errors in measurement systems 4.4 Techniques for dynamic compensation Loading Effects and Two-port Networks 5.1 Electrical loading 5.2 Two-port networks Signals and Noise in Measurement Systems 6.1 Introduction 6.2 Statistical representation of random signals 6.3 Effects of noise and interference on measurement circuits 6.4 Noise sources and coupling mechanisms 6.5 Methods of reducing effects of noise and interference
viii CONTENTS 7 Reliability, Choice and Economics of Measurement Systems 7.1 Reliability of measurement systems 7.2 Choice of measurement systems 7.3 Total lifetime operating cost 125 125 140 141
Typical Measurement System Elements
Sensing Elements 8.1 Resistive sensing elements 8.2 Capacitive sensing...