as long as 125 feet, which separates the sensingbulb from the Bourdon tube and dial (fig. 8-14).There are three basic types of distant-readingthermometers: the liquid filled, the gas filled,and the combination liquid-vapor filled. Thethermometers are filled with fluid (liquid or gas)at some temperature and sealed. Almost the entirevolume of the fluid is in the sensing bulb.As the temperature of the bulb changes, thevolume of the fluid tries to change. Sincethe volume of the thermometer (sensing bulb,capillary, and Bourdon tube) is constant, apressure change occurs within the thermometer.This pressure change causes the Bourdon tube tostraighten out (with an increase in pressure),working a system of levers and gears, which causesthe thermometer pointer to move over the dial andregister temperature.TEMPERATURE SWITCHESTemperature switches operate from tempera-ture changes occurring in an enclosure, or in theair surrounding the temperature-sensing element.The operation of the temperature switch is similarto the operation of the pressure switch shown infigure 8-9; both switches are operated by changesin pressure. The temperature element is arrangedso a change in temperature causes a change in theinternal pressure of a sealed-gas or air-filled bulbFigure 8-14.—Distant-reading, Bourdon-tube thermometers.or helix, which is connected to the actuating deviceby a small tube or pipe. Figure 8-15 shows atemperature switch and two types of sensingelements.A temperature change causes a change in thevolume of the sealed-in gas, which causesmovement of a bellows. The movement istransmitted by a plunger to the switch arm. Themoving contact is on the arm. A fixed contact maybe arranged so the switch will open or close ona temperature rise. This allows the switch contactsto be arranged to close when the temperaturedrops to a predetermined value and to open whenthe temperature rises to the desired value. Thereverse action can be obtained by a change in thecontact positions.GAUGE SNUBBERSThe irregularity of impulses applied to thefluid power system by some pumps or aircompressors causes the gauge pointer to oscillateviolently. This makes reading of the gauge notonly difficult but often impossible. Pressureoscillations and other sudden pressure changesexisting in fluid power systems will also affect thedelicate internal mechanism of gauges and causeeither damage to or complete destruction of theFigure 8-15.—Temperature switch with two types of sensingelements. A. Bulb unit. B. Helix unit.8-8
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