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Gauge (PRESSURE an instrument)

PRESSURE, an instrument for indicating the intensity of a fluid contained in a closed vessel. Gauges are divided into three classes: the mercurial pressure gauge, the air manometer, and the spring gauge. The mercurial gauge consists of an inverted siphon or "U" tube of glass, the lower part of which contains mercury, and whose vertical legs are divided either in parts of an inch or divisions indicating pounds per square inch. One leg of the tube is open to the atmosphere and the other communicates with the vessel containing the fluid. The difference in level of the mercury indicates the difference in pressure between the atmosphere and the fluid.

The air manometer consists of a long vertical glass tube closed at the upper end, open at the lower end, containing air and immersed in a transparent liquid GAUL which communicates with the vessel in which the pressure is to be determined.

The compression of the air in the tube is proportional to the pressure. Spring gauges have metallic tubes or drums communicating with the closed vessel.

The pressure tends to straighten the tube. The motion produced in the tube or drum is transmitted to a pointer by means of levers or other gearing. The pointer travels over a graduated arc from which the pressure is read directly.