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Introduction 1. Techniques and applicaTions We can look at instrumentation work in two ways: by tech- niques or by applications. When we consider instrumenta- tion by technique, we survey one scientific field, such as radioactivity or ultrasonics, and look at all the ways in which it can be used to make useful measurements. When we study instrumentation by application, we cover the various tech- niques to measure a particular quantity. Under flowmetering, for instance, we look at many methods, including tracers, ultrasonics, or pressure measurement. This book is mainly applications oriented, but in a few cases, notably pneumat- ics and the employment of nuclear technology, the technique has been the primary unifying theme. probability grows steadily wider as the range where it might be also grows wider. When we consider a measurement chain with several links, the two approaches give increasingly different figures. For if we think of possibilities/impossibilities, we must allow that the errors in each link can be extreme and in the same direction, calling for a simple addition when calculating the possible total error. On the other hand, this is improbable, so the "chain error" that corresponds to a given probability, e c , is appreciably smaller. In fact, statistically, e c = e 2 + e 2 + g 1 2 2. accuracy The most important question in instrumentation is the accu- racy with which a measurement is made. It is such a uni- where e 1 , e 2 , and so on are the errors in the different links, each corresponding to the same probability as e c . We can think of influence quantities as the causes of random errors. Most devices that measure a physical quan- tity are influenced by other quantities. Even in the simple case of a tape measure, the tape itself is influenced by tem- perature. Thus, a tape measure will give a false reading