Theory and Design for Mechanical Measurements, Third Edition

Having approached this book with eager expectation, I have to admit I was soon disappointed. Firstly, it seems to me that the title is rather misleading: how many would think of `Analog Electrical Devices & Measurements' - Chapter 6; `Sampling, Digital Devices & Data Acquisition' - Chapter 7, or `Temperature Measurements' - Chapter 8, as Mechanical measurements? This aside, however, I find the coverage both patchy and uneven; there are some excellent and useful sections, obviously those where the authors have personal experience, such as for instance Chapter 11, `Strain Measurements', and also some useful work on Probability and Statistics and Uncertainty Analysis, but the appreciation of a problem is often too superficial, with many sources of error being overlooked or ignored. However, the most serious problem with the book is the number of errors, some of such a fundamental nature that one is inevitably driven to consider just how much of the rest one can consider reliable. Some instances will show what one must expect. These examples were not found by a careful trawl in depth, so must be considered as representative only. The definition of the ohm as the resistance of a column of mercury was abandoned many years ago; a standard cell must never be used to deliver 100 mA, let alone for a few minutes, and in any case, both volt and ohm have been defined for more than ten years by quantum effects. Problem 1.37 specifies an LVDT being driven by a d.c. voltage, when of course only a.c. can be used. (There are `d.c.' LVDTs but these contain electronics to generate the necessary a.c.) The accompanying diagram also shows the connections incorrectly and the text contains the sentence `outputs a voltage which is linear to the input'. Which input? The electrical one or the displacement? Such looseness abounds, even ignoring the use of `output' as a verb. Page 36: `Analog describes a signal that is continuous in time'. Alas, not so, analog describes a signal where the intelligence is represented by a continuously variable level. Page 43: Fig. 2.8 seems to show (quite clearly! ) that subtracting a d.c. offset actually causes a waveform to change shape. Page 197: `An a.c. meter indicates a true rms value for a simple periodic signal only'. Absolutely wrong: an a.c. meter (the rectified moving coil type is meant) indicates average only, whatever it may be scaled. The authors' application of mathematical analysis to the consideration of errors is both praiseworthy and necessary but reveals their obvious preference for an analysis of errors rather than for the application of careful thought to remove sources of error first, before applying analysis to those that inevitably remain. An example of this lack of thought is revealed in Fig. 6.4, a simple multirange ammeter, where the moving coil would vaporize if the switch were ever operated under load or there were ever any jump in contact resistance. Amazingly there is no discussion of the universal shunt, nor of the necessity of a swamping series resistor. There is very little discussion of what most people would actually think of when considering `Mechanical Measurement'. Importantly, there is no discussion on the fundamental and vital differences between a micrometer and a caliper - that the former indicates its own true zero, unless it is very seriously damaged, while the latter is subject to many errors caused by wear or by quite minor damage, a further indication of inattention to some vital considerations. The authors seem also to be unaware that micrometers can be used in at least as large a size as a caliper. There is no mention at all of modern precision coordinate measuring machines - only one of several omissions. The arrangement of the knife edges of a precision balance in Fig. 12.13, purporting to show a precision balance, is drastically wrong. A pencil type pressure gauge has no spring, which rather makes nonsense of the rest of the analysis. It is surprising and regrettable that so many errors (and there are many more) should have survived into a third edition. A major revision is called for before the book could be wholeheartedly recommended. This book is certainly a useful addition to one's library for its chapters on probability of statistics and analysis on uncertainty, but it would be most unwise to use it for general guidance on the theory of measurements for one not already thoroughly familiar with the design of actual instruments. J C Taylor