CHAPTER 8MEASUREMENT AND PRESSURECONTROL DEVICESFor safe and efficient operation, fluid powersystems are designed to operate at a specificpressure and/or temperature, or within a pressureand/or temperature range.You have learned that the lubricating powerof hydraulic fluids varies with temperature andthat excessively high temperatures reduce the lifeof hydraulic fluids. Additionally, you havelearned that the materials, dimensions, andmethod of fabrication of fluid power componentslimit the pressure and temperature at which asystem operates. You have also learned of meansof automatically controlling pressure in bothhydraulic and pneumatic systems.Most fluid power systems are provided withpressure gauges and thermometers for measuringand indicating the pressure and/or the tempera-ture in the system. Additionally, various tempera-ture and pressure switches are used to warn of anadverse pressure or temperature condition. Someswitches will even shut the system off when anadverse condition occurs. These devices will bediscussed in this chapter.PRESSURE GAUGESMany pressure-measuring instruments arecalled gauges. However, this section will berestricted to two mechanical instruments thatcontain elastic elements that respond to pressuresfound in fluid power systems—the Bourdon-tubeand bellows gauges.BOURDON TUBE GAUGESThe majority of pressure gauges in use havea Bourdon-tube as a measuring element. (Thegauge is named for its inventor, Eugene Bourdon,a French engineer.) The Bourdon tube is a devicethat senses pressure and converts the pressure todisplacement. Since the Bourdon-tube displace-ment is a function of the pressure applied, it maybe mechanically amplified and indicated by apointer. Thus, the pointer position indirectlyindicates pressure.The Bourdon-tube gauge is available invarious tube shapes: curved or C-shaped, helical,and spiral. The size, shape, and material of thetube depend on the pressure range and the typeof gauge desired. Low-pressure Bourdon tubes(pressures up to 2000 psi) are often made ofphosphor bronze. High-pressure Bourdon tubes(pressures above 2000 psi) are made of stainlesssteel or other high-strength materials. High-pressure Bourdon tubes tend to have more circularcross sections than their lower-range counterparts,which tend to have oval cross sections. TheBourdon tube most commonly used is theC-shaped metal tube that is sealed at one end andopen at the other (fig. 8-1).Figure 8-1.—Simplex Bourdon-tube pressure gauge.8-1
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