Ultrasonic Measurements and Technologies

Abstract

This book presents a very useful insight into ultrasonic technology and is based on years of practical experience at the Slovak Technical University of Bratislava. Much of the technology presented has been developed as part of a long-term programme in machine control. In the main, the technology involves the application to air-borne ultrasonics. The book is therefore particularly useful for the engineer wishing to apply ultrasonic techniques for position monitoring, flow and wind velocity measurements. The first chapter is an introduction to ultrasonic waves and this is followed by a chapter on transducers. Transducers covered include from capacitive, piezoelectric, magnetostrictive, electrical discharge (spark) and discharge microphone. In the latter cases, electrical excitation and detection circuits are presented. Chapter 4 deals with digital measuring methods. The design engineer new to ultrasonics will find this most useful as it describes methods aimed at achieving different levels of precision, using both phase and pulse methods. The principles of pulse-echo methods, especially applied to liquid level detection and measurement, are analysed. Again, practical issues are considered such as the surface quality. This chapter also considers the significant errors that can occur in level gauging caused by the variation of sound speed in air or other gases with temperature. Methods of overcoming these errors are considered including the use of parallel channels. The chapter deals at some length with coordinate position measurement using spark sources and distributed sensors, including methods of self-calibration. Liquid flow measurement in pipes is also considered in this chapter. Chapter 5 considers non-destructive testing from a fairly basic perspective. Chapter 6 analyses in greater detail the pulse methods for position and flow measurement and addresses the need for greater accuracy, in particular for the compensation of atmospheric influences. The last chapter looks at different instruments used in industrial control and monitoring, and a number of commercially available systems are described. The causal reader will find this book very useful as it addresses many practical issues and makes recommendations on topics rarely found in textbooks. For instance, section 3.1.2 discusses the capacitance transducers for air ultrasonics and mentions many critical aspects of design. However, the reader wishing to know more may be rather frustrated as many issues are dealt with rather superficially, a particular example being the electrodes and underlying substrate. Four configurations are given in figure 3.1 but no discussion about their relative merits, nor any further references, is given, a weakness in a number of other places. This book is therefore not one for the purist. This is confirmed by a more careful examination of the equations and presentation which show a number of inconsistencies. One of the major problems is the inconsistent use of symbols. For instance, on page 7 lower case p and upper case P are used for acoustic pressure, and on pages 40 and 41, the subscript is used without explanation, presumably a misprint which should read 33. The reader also needs to take care in interpreting some of the equations. For instance, equation 2.8 is intended to give the power in an ultrasonic beam based on its intensity and area. However, it uses the parameter s which is referred to as a `unit area'. This equation should take account of the actual area of the beam and its intensity distribution, and if it assumed that the beam has constant intensity over the beam area, the parameter s should then be the beam area and not unit area. These are just some of the instances where the reader needs to be careful in interpreting the text. Overall, this is a useful book for the engineer new to ultrasound and even for the more experienced it represents a very valuable collection of technical data.