Introduction 



The purpose of instrumentation is to obtain information about the 

 physical or chemical nature of an investigated object or process, or to 

 control an object or a process in accordance with such information. 

 Information is required or supplied either in a continuously variable 

 form, such as the deflection of a meter (analogue systems), or in dis- 

 continuous steps, such as by counting (digital systems). 



In general, the simpler the means it involves, the better the instru- 

 mentation is. Many problems lend themselves to a solution by direct 

 observation or by simple mechanical means. The possibility of such 

 simple and direct methods should be investigated before a decision 

 is made to use the more complex electrical methods. However, elec- 

 trical methods frequently offer a number of advantages, such as high- 

 speed operation or the possibility of simple processing of the results, 

 e.g., amplifying, filtering, differentiating, storing, or telemetering. 



All electrical instruments are composed from elements. In the 

 sense used in this book an element is a functional unit, a building 

 block in a block diagram, which performs one particular task or 

 which solves one particular problem. An element is not a component 

 part, although at times a component part can serve as an element. 

 In the field of electrical instrumentation one can distinguish three 

 groups of elements: 



A . Input transducers : converting a nonelectrical quantity into an 

 electric signal (e.g., strain gauge, photoelectric cell) 



1 



