Instrument types and performance characteristics

This chapter discusses how the instruments can be subdivided into separate classes according to several criteria. These subclassifications are useful in broadly establishing several attributes of particular instruments such as accuracy, cost, and general applicability to different applications. Instruments are divided into active or passive ones according to whether the instrument output is entirely produced by the quantity being measured, or whether the quantity being measured simply modulates the magnitude of some external power source. An analog instrument gives an output that varies continuously as the quantity being measured changes. The advent of the microprocessor has created a new division in instruments between those that incorporate a smart microprocessor and those that don't. Precision is a term that describes an instrument's degree of freedom from random errors. Tolerance is a term that is closely related to accuracy and defines the maximum error that is to be expected in some value. The range or span of an instrument defines the minimum and maximum values of a quantity that the instrument is designed to measure. The sensitivity of measurement is a measure of the change in instrument output that occurs when the quantity being measured changes by a given amount. The static characteristics of measuring instruments are concerned only with the steady-state reading that the instrument settles down to, such as the accuracy of the reading etc. The foregoing discussion has described the static and dynamic characteristics of measuring instruments with more details.