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A pressure gauge is a device that measures pressure. There are two main types of pressure gauges. One measures the effect of pressure on the shape of a hollow metal part, called the element. The other type measures the pressure of a fluid by the height of a column of liquid that the pressure will support. This type includes the manometer and the mercury barometer, one of the two basic kinds of barometers.
The bourdon gauge, the most common pressure gauge, is of the first type. It consists chiefly of a bourdon tube, which is a flattened metal tube sealed at one end and bent to form part of a circle. The open end is connected to the region where the pressure is to be measured. The other end is connected to an indicator. An increase in pressure tends to uncurl the tube, while a decrease in pressure causes it to curl more tightly. These changes are registered on the indicator. Bourdon gauges may be used to measure pressures from 15 pounds per square inch to 100,000 pounds per square inch (1.1-7,050 kg per sq cm). Other instruments of this type are the diaphragm gauges, of which the aneroid barometer is an example, and the bellows-type gauge. Such gauges measure very small changes in pressure by the expansion or contraction of a hollow metal box or capsule.
In most pressure gauges the effect of the pressure being measured is opposed by atmospheric pressure. The pressure indicated on the gauge, therefore, is the pressure being measured minus atmospheric pressure. This is called the gauge pressure. Absolute pressure may be measured by pumping the air out of the part of the instrument that is normally exposed to the atmosphere.