A gauge, in science and engineering, is a device used to make measurements or in order to display certain dimensional information. A wide variety of tools exist which serve such functions, ranging from simple pieces of material against which sizes can be measured to complex pieces of machinery. Depending on usage, a gauge can be described as "a device for measuring a physical quantity", for example "to determine thickness, gap in space, diameter of materials, or pressure of flow", or "a device that displays the measurement of a monitored system by the use of a needle or pointer that moves along a calibrated scale".
All gauges can be divided into four main types, independent of their actual use.
The two basic types with an analogue display are usually easier for the human eyes and brain to interpret, especially if many instrument meters must be read simultaneously. An indicator or needle indicates the measurement on the gauge. The other two types are only displaying digits, which are more complex for humans to read and interpret. The ultimate example is cockpit instrumentation in aircraft. The flight instruments cannot display figures only, hence even in the most modern "glass-cockpits" where almost all instruments are displayed at screens, few figures are visible. Instead the screens display analogue meters.
More in detail
Various types of gauges include:
Richard Talman, Geometric Mechanics (2008), p. 255-56: "a "gauge" is a device for measuring a physical quantity—a thermometer is a temperature gauge, a ruler is a length gauge"..
Ray Herren, Agricultural Mechanics: Fundamentals & Applications (2009), p. 109: "A gauge is a device used to determine thickness, gap in space, diameter of materials, or pressure of flow".
Barry Hollembeak, Today's Technician: Automotive Electricity and Electronics (2010), p. 539: "A gauge is a device that displays the measurement of a monitored system by the use of a needle or pointer that moves along a calibrated scale".