Figure 11-23.—Revolution counter.It magnetically opens or closes the reed switch as thefloat passes over it. Figure 11-22 shows the constructionof the magnetically operated float switch. Magneticfloat switches may be constructed with more than onefloat on a stem. Magnetic float switches can be installedto detect multiple levels in the same tank; and this typeof switch can activate a high- and low-level alarm.Figure 11-24.—stroboscope tachometer.REVOLUTION COUNTERS ANDINDICATORSMeasurements of rotational speed are necessary forthe proper operation of pumps, forced-draft blowers,main engines, and other components of the engineeringplants. Various types of instruments are used to measureequipment revolutions per minute (rpm) and count thenumber of revolutions a shaft makes.PROPELLER INDICATORSPropeller indicators are mounted on the throttleboard. They indicate the speed and direction of rotationof the propulsion shaft or shafts. They also record thenumber of revolutions the propulsion shaft has made.The speed of rotation is important because it is relatedto the ship’s speed. The total number of revolutions isused to determine the total distance traveled by the ship.A typical revolution counter is shown in figure 11-23.OTHER REVOLUTION INDICATORSEquipment speed is determined by eitherpermanently installed mechanical or electricaltachometers or by portable tachometers. Portabletachometers are hand-held, mechanical types. Theyrequire access to the end of the rotating machinery shaft.Another type of tachometer is the stroboscopetachometer (fig. 11-24). This device allows rotatingmachinery to be viewed intermittently, under flashinglight, so that the rotation appears to stop.WARNINGIf you use a stroboscopic tachometer,NEVER reach into the rotating machinery.Although the machinery appears to be stopped,it is still rotating.11-13
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