case that has a removable cover, an electricalconnection, and a pressure-sensing connection.The switch contains a seamless metallic bellowslocated in its housing. Changes in the measuredpressure causes the bellows to work against anadjustable spring. This spring determines thepressure required to actuate the switch. Throughsuitable linkage, the spring causes the contacts toopen or close the electrical circuit automaticallywhen the operating pressure falls below or risesabove a specified value. A permanent magnet inthe switch mechanism provides a positive snap onboth the opening and closing of the contacts. Theswitch is constantly energized. However, it is theclosing of the contacts that energizes the entireelectrical circuit.Another pressure switch is an electric-hydraulic assembly that is used for shutting offthe pump’s motor whenever the system pressureexceeds a pre-determined maximum value (fig.8-10). The switch is mounted on the pump housingso that the former’s low pressure ports draindirectly into the pump housing.This pressure switch principally consists of aflange-mounted hydraulic valve to which is fixeda normally closed electrical limit switch.The valve consists of two hydraulicallyinterconnected components, the pilot valve sub-assembly, which bolts on the bottom of thebody (l), functions to sense system pressurecontinuously and initiates pressure switch actionwhenever this pressure exceeds the adjusted settingof the pilot adjustment. System pressure isdirected into the bottom port and is appliedagainst the exposed tip of the pilot piston (5). Thispiston is held on its seat by compression from thepiston spring (6) which is dependent on theposition of the adjusting screw (8). Whenever thepressure causes a force sufficiently large enoughto raise the pilot piston from its seat, fluidflows through an interconnecting passage to theactuating piston (2) chamber. The accompanyingfluid force raises the actuating piston against theforce of spring 3 and causes depression ofthe extended switch plunger. This, in turn,disconnects the contained electrical switch, whichmay be connected into the pump motor’s electricsupply system.Pressure switches come in many sizes andconfigurations depending on how they will beused.Figure 8-10.—Electric-hydraulic pressure switch.TEMPERATURE-MEASURINGINSTRUMENTSTemperature is the degree of hotness orcoldness of a substance measured on a definitescale. Temperature is measured when a measuringinstrument, such as a thermometer, is broughtinto contact with the medium being measured.All temperature-measuring instruments usesome change in a material to indicate temperature.Some of the effects that are used to indicatetemperature are changes in physical properties andaltered physical dimensions. One of the moreimportant physical properties used in temperature-measuring instruments is the change in the lengthof a material in the form of expansion andcontraction.Consider the uniform homogeneous barillustrated in figure 8-11. If the bar has a given8-6
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