Gauge, also called gage, is an instrument used for measurement. Gauges are used to measure such quantities as pressure, temperature, water level, and thickness. Two of the most common types of gauges are pressure gauges and water-level gauges. But people working in a great many different fields commonly use other kinds of specialized gauges in their work.
Pressure gauges
measure pressure in enclosed vessels or containers, such as boilers or pipes. For pressure greater than that of the atmosphere, most gauges measure in units of pounds per square inch (psi) or kilograms per square centimeter. Pressures below that of the atmosphere are usually measured in inches or centimeters of mercury.
A pressure gauge consists of a tube with a cross section shaped like an ellipse (see Ellipse ). This tube is called a Bourdon tube. It is bent in a hook-shaped curve. When the pressure inside the tube becomes greater than the pressure outside of it, the elliptical cross section tends to become circular. At the same time, the pressure tends to straighten the Bourdon tube. Levers, gears, or other mechanisms transfer this motion to a pointer, which rotates around a fixed dial calibrated in pressure units.
Vacuum gauges
measure pressure below the pressure of the atmosphere. Such gauges are constructed in the same manner as the gauges described above. However, when the pressure inside the Bourdon tube becomes less than the pressure outside the tube, the tube has a tendency to curl. This motion is also transmitted to a pointer that moves across a dial. The dial of a vacuum gauge is calibrated in inches or centimeters of mercury.
Water-level gauges
usually consist of a glass tube connected to the side of a container. The level of the water in the container and the level of the water in the tube are the same. Markings on the tube indicate the height of water in the container.
Other types of gauges.
Wire gauges are used to measure the thickness of wire. Thickness gauges are used by automobile mechanics to determine the clearance between the pistons and cylinders of automobile engines.
