Temperature Switches

as long as 125 feet, which separates the sensing bulb from the Bourdon tube and dial (fig. 8-14). There are three basic types of distant-reading thermometers: the liquid filled, the gas filled, and the combination liquid-vapor filled. The thermometers are filled with fluid (liquid or gas) at some temperature and sealed. Almost the entire volume of the fluid is in the sensing bulb. As the temperature of the bulb changes, the volume of the fluid tries to change. Since the volume of the thermometer (sensing bulb, capillary, and Bourdon tube) is constant, a pressure change occurs within the thermometer. This pressure change causes the Bourdon tube to straighten out (with an increase in pressure), working a system of levers and gears, which causes the thermometer pointer to move over the dial and register temperature. TEMPERATURE SWITCHES Temperature switches operate from tempera- ture changes occurring in an enclosure, or in the air surrounding the temperature-sensing element. The operation of the temperature switch is similar to the operation of the pressure switch shown in figure 8-9; both switches are operated by changes in pressure. The temperature element is arranged so a change in temperature causes a change in the internal pressure of a sealed-gas or air-filled bulb Figure 8-14.—Distant-reading, Bourdon-tube thermometers. or helix, which is connected to the actuating device by a small tube or pipe. Figure 8-15 shows a temperature switch and two types of sensing elements. A temperature change causes a change in the volume of the sealed-in gas, which causes movement of a bellows. The movement is transmitted by a plunger to the switch arm. The moving contact is on the arm. A fixed contact may be arranged so the switch will open or close on a temperature rise. This allows the switch contacts to be arranged to close when the temperature drops to a predetermined value and to open when the temperature rises to the desired value. The reverse action can be obtained by a change in the contact positions. GAUGE SNUBBERS The irregularity of impulses applied to the fluid power system by some pumps or air compressors causes the gauge pointer to oscillate violently. This makes reading of the gauge not only difficult but often impossible. Pressure oscillations and other sudden pressure changes existing in fluid power systems will also affect the delicate internal mechanism of gauges and cause either damage to or complete destruction of the Figure 8-15.—Temperature switch with two types of sensing elements. A. Bulb unit. B. Helix unit. 8-8