Overview Some measurement and control technologies have evolved rapidly over the past few years@ while others have almost disappeared. Instruments presently in use may become obsolete as newer and more efficient measurement techniques are introduced. The emergence of new techniques is driven by the ongoing need for better quality@ by the increasing cost of materials@ by continuous product changes@ by tighter environmental requirements@ by better accuracies@ by improved plant performance@ and by the evolution of the microprocessor-based devices. These technical developments have made possible the measurement of variables judged impossible to measure only a few decades ago. Effective measurement requires a solid understanding of the process conditions. Selecting the correct measuring and control device is sometimes a challenge even for the most seasoned engineers@ technicians@ and sales personnel. This handbook provides the tools to enable users to correctly implement measuring and control systems@ which in many cases is an activity not well understood and therefore not successfully implemented. Given the ever-growing demand for measurement and control applications and the wide range of devices on the market@ the user must be able to assess different methods of measurement and control and select the most appropriate one. It is not wise to consider only one type of measurement or control since each has its own advantages and disadvantages. The user must compare the different types in terms of which best fits the user's application since many techniques are available for measuring a parameter (such as flow@ level@ etc.). Making the optimum selection involves considering the requirements of the process@ the desired degree of accuracy@ the installation@ dependability factors@ maintenance@ and economic factors. Since there is probably no one best method for measuring a specific variable this guide should help the user decide which method is more appropriate for the application. One final note: when describing the function of instrumentation it is important to ensure that we are using uniform terminology. ANSI/ISA-51.1-1979 (R1993)@ Process Instrumentation Terminology@ includes definitions for many of the specialized terms used in the industrial process industries@ such as accuracy@ dead band@ drift@ hysteresis@ linearity@ repeatability@ and reproducibility.