The foundations of this book lie in the highly successful text Principles of Measurement and Instrumentation by the same author. The first edition of this was published in 1988, and a second, revised and extended edition appeared in 1993. Since that time, a number of new developments have occurred in the field of measurement. In particular, there have been significant advances in smart sensors, intelligent instruments, microsensors, digital signal processing, digital recorders, digital fieldbuses and new methods of signal transmission. The rapid growth of digital components within measurement systems has also created a need to establish procedures for measuring and improving the reliability of the software that is used within such components. Formal standards governing instru- ment calibration procedures and measurement system performance have also extended beyond the traditional area of quality assurance systems (BS 5781, BS 5750 and more recently ISO 9000) into new areas such as environmental protection systems (BS 7750 and ISO 14000). Thus, an up-to-date book incorporating all of the latest developments in measurement is strongly needed. With so much new material to include, the oppor- tunity has been taken to substantially revise the order and content of material presented previously in Principles of Measurement and Instrumentation, and several new chapters have been written to cover the many new developments in measurement and instru- mentation that have occurred over the past few years. To emphasize the substantial revision that has taken place, a decision has been made to publish the book under a new title rather than as a third edition of the previous book. Hence, Measurement and Instrumentation Principles has been born. The overall aim of the book is to present the topics of sensors and instrumentation, and their use within measurement systems, as an integrated and coherent subject. Measurement systems, and the instruments and sensors used within them, are of immense importance in a wide variety of domestic and industrial activities. The growth in the sophistication of instruments used in industry has been particularly significant as advanced automation schemes have been developed. Similar developments have also been evident in military and medical applications. Unfortunately, the crucial part that measurement plays in all of these systems tends to get overlooked, and measurement is therefore rarely given the importance that it deserves. For example, much effort goes into designing sophisticated automatic control systems, but little regard is given to the accuracy and quality of the raw measurement data that such systems use as their inputs. This disregard of measurement system quality and performance means that such control systems will never achieve their full
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potential, as it is very difficult to increase their performance beyond the quality of the raw measurement data on which they depend. Ideally, the principles of good measurement and instrumentation practice should be taught throughout the duration of engineering courses, starting at an elementary level and moving on to more advanced topics as the course progresses. With this in mind, the material contained in this book is designed both to support introductory courses in measurement and instrumentation, and also to provide in-depth coverage of advanced topics for higher-level courses. In addition, besides its role as a student course text, it is also anticipated that the book will be useful to practising engineers, both to update their knowledge of the latest developments in measurement theory and practice, and also to serve as a guide to the typical characteristics and capabilities of the range of sensors and instruments that are currently in use. The text is divided into two parts. The principles and theory of measurement are covered first in Part 1 and then the ranges of instruments and sensors that are available for measuring various physical quantities are covered in Part 2. This order of coverage has been chosen so that the general characteristics of measuring instruments, and their behaviour in different operating environments, are well established before the reader is introduced to the procedures involved in choosing a measurement device for a particular application. This ensures that the reader will be properly equipped to appreciate and critically appraise the various merits and characteristics of different instruments when faced with the task of choosing a suitable instrument. It should be noted that, whilst measurement theory inevitably involves some mathe- matics, the mathematical content of the book has deliberately been kept to the minimum necessary for the reader to be able to design and build measurement systems that perform to a level commensurate with the needs of the automatic control scheme or other system that they support. Where mathematical procedures are necessary, worked examples are provided as necessary throughout the book to illustrate the principles involved. Self-assessment questions are also provided in critical chapters to enable readers to test their level of understanding, with answers being provided in Appendix 4. Part 1 is organized such that all of the elements in a typical measurement system are presented in a logical order, starting with the capture of a measurement signal by a sensor and then proceeding through the stages of signal processing, sensor output transducing, signal transmission and signal display or recording. Ancillary issues, such as calibration and measurement system reliability, are also covered. Discussion starts with a review of the different classes of instrument and sensor available, and the sort of applications in which these different types are typically used. This opening discussion includes analysis of the static and dynamic characteristics of instruments and exploration of how these affect instrument usage. A comprehensive discussion of measurement system errors then follows, with appropriate procedures for quantifying and reducing errors being presented. The importance of calibration procedures in all aspects of measurement systems, and particularly to satisfy the requirements of stan- dards such as ISO 9000 and ISO 14000, is recognized by devoting a full chapter to the issues involved. This is followed by an analysis of measurement noise sources, and discussion on the various analogue and digital signal-processing procedures that are used to attenuate noise and improve the quality of signals. After coverage of the range of electrical indicating and test instruments that are used to monitor electrical measurement signals, a chapter is devoted to presenting the range of variable conver- sion elements (transducers) and techniques that are used to convert non-electrical sensor outputs into electrical signals, with particular emphasis on electrical bridge circuits. The problems of signal transmission are considered next, and various means of improving the quality of transmitted signals are presented. This is followed by an introduction to digital computation techniques, and then a description of their use within intelligent measurement devices. The methods used to combine a number of intelligent devices into a large measurement network, and the current status of development of digital fieldbuses, are also explained. Then, the final element in a measurement system, of displaying, recording and presenting measurement data, is covered. To conclude Part 1, the issues of measurement system reliability, and the effect of unreliability on plant safety systems, are discussed. This discussion also includes the subject of software reliability, since computational elements are now embedded in many measurement systems. Part 2 commences in the opening chapter with a review of the various technologies used in measurement sensors. The chapters that follow then provide comprehensive coverage of the main types of sensor and instrument that exist for measuring all the physical quantities that a practising engineer is likely to meet in normal situations. However, whilst the coverage is as comprehensive as possible, the distinction is empha- sized between (a) instruments that are current and in common use, (b) instruments that are current but not widely used except in special applications, for reasons of cost or limited capabilities, and (c) instruments that are largely obsolete as regards new indus- trial implementations, but are still encountered on older plant that was installed some years ago. As well as emphasizing this distinction, some guidance is given about how to go about choosing an instrument for a particular measurement application.
Measurement and Instrumentation Principles - 3rd Edition
Alan S Morris
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