Figure 8-3.—Bourdon-tube differential pressure gauge.example, some hydraulic gauges have a specialtype of spring-loaded linkage that is capable oftaking overpressure and underpressure withoutdamage to the movement and that keeps thepointer from slamming back to zero when thepressure is suddenly changed. A hydraulic gaugethat does not have such a device must be protectedby a suitable check valve. Some hydraulic gaugesmay also have special dials that indicate both thepressure (in psi) and the corresponding total forcebeing applied, for example tons of force producedby a hydraulic press.Spiral and Helical Bourdon TubesSpiral and helical Bourdon tubes (figs. 8-4 and8-5) are made from tubing with a flattened crossFigure 8-4.—Spiral Bourdon tube.section. Both were designed to provide more travelof the tube tip, primarily for moving the recordingpen of pressure recorders.BELLOWS ELASTIC ELEMENTSA bellows elastic element is a convoluted unitthat expands and contracts axially with changesin pressure. The pressure to be measured can beapplied to either the outside or the inside of thebellows; in practice, most bellows measuringFigure 8-5.—Helical Bourdon tube.8-3
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