Chapter 2 – Instrument Types and Performance Characteristics

Publisher Summary This chapter reviews various different classes of instruments, considering how these differences affect their typical usage. Instruments are divided into active or passive ones according to whether instrument output is produced entirely by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source. The pressure gauge just mentioned is a good example of a deflection type of instrument, where the value of the quantity being measured is displayed in terms of the amount of movement of a pointer. The accuracy of these two instruments depends on different things. For the first one it depends on the linearity and calibration of the spring, whereas for the second it relies on calibration of the weights. The distinction between analogue and digital instruments has become particularly important with rapid growth in the application of microcomputers to automatic control systems. The final way in which instruments can be divided is between those that merely give an audio or visual indication of the magnitude of the physical quantity measured and those that give an output in the form of a measurement signal whose magnitude is proportional to the measured quantity.