C-shaped Bourdon TubeThe C-shaped Bourdon tube has a hollow,elliptical cross section. It is closed at one end andis connected to the fluid pressure at the other end.When pressure is applied, its cross sectionbecomes more circular, causing the tube tostraighten out, like a garden hose when the wateris first turned on, until the force of the fluidpressure is balanced by the elastic resistance ofthe tube material. Since the open end of the tubeis anchored in a fixed position, changes in pressuremove the closed end. A pointer is attached to theclosed end of the tube through a linkage arm anda gear and pinion assembly, which rotates thepointer around a graduated scale.Bourdon-tube pressure gauges are oftenclassified as simplex or duplex, depending uponwhether they measure one pressure or twopressures. A simplex gauge has only one Bourdontube and measures only one pressure. The pressuregauge shown in figure 8-1 is a simplex gauge. Ared hand is available on some gauges. This handis manually positioned at the maximum operatingpressure of the system or portion of the systemin which the gauge is installed.When two Bourdon tubes are mounted ina single case, with each mechanism actingindependently but with the two pointers mountedon a common dial, the assembly is called a duplexgauge. Figure 8-2 shows a duplex gauge with viewsof the dial and the operating mechanism. Notethat each Bourdon tube has its own pressureconnection and its own pointer. Duplex gaugesare used to give a simultaneous indication of thepressure from two different locations. Forexample, it may be used to measure the inlet andoutlet pressures of a strainer to obtain thedifferential pressure across it.Differential pressure may also be measuredwith Bourdon-tube gauges. One kind of Bourdon-tube differential pressure gauge is shown infigure 8-3. This gauge has two Bourdon tubesbut only one pointer. The Bourdon tubes areconnected in such a way that they indicate thepressure difference, rather than either of twoactual pressures.As mentioned earlier, Bourdon-tube pressuregauges are used in many hydraulic systems. In thisapplication they are usually referred to ashydraulic gauges. Bourdon-tube hydraulic gaugesare not particularly different from other types ofBourdon-tube gauges in how they operate;however, they do sometimes have special designfeatures because of the extremely high systempressures to which they may be exposed. ForFigure 8-2.—Duplex Bourdon-tube pressure gauge.8-2
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