Principles of naval engineering - Historic Naval Ships Association

Chapter 7-PRINCIPLES OF MEASUREMENTand gaging tapes or by some form of permanentlyinstalled, remote reading gaging system. (Althoughremote reading gaging systems are oftenreferred to as "tank level indicators," it shouldbe noted that the scale may be calibrated toshow level, volume, or weight; frequently thereare two scales— one to show volume and one toshow level.)A static head gaging system of a type commonlyused for measuring liquid level in fueloil tanks aboard ship is shown in figure 7-32.This system balances a head of liquid in thetank against a column of liquid in a manometeror against a bellows or diaphragm differentialpressure unit; the system illustrated uses amercury manometer. The balance chamber islocated so that its orifice is near the bottomof the tank; a line connects the top of the balancechamber to the mercury-filled bulb of theindicator gage, and another line connects thespace above the mercury column to the top ofthe tank. Since the height of the liquid in thetank bears a definite relationship to the pressureexerted by the liquid, the scale can becalibrated to show height (or liquid level). Whenthe size of the tank is known, the measurement ofheight can readily be converted to measurementof volume; and, when the volume of the tank andthe specific weight of the liquid are known, thescale can be calibrated to indicate weight. Thereading on this type of tank gaging system isalways taken after compressed air has beenadmitted to the balance chamber through thecontrol valve; to ensure proper readings, asufficient amount of compressed air must beadmitted to force the liquid down to the level ofthe bottom of the standpipe.MEASUREMENT OF ROTATIONALSPEEDThe rotational speed of propeller shafts,turbines, generators, blowers, pumps, and otherkinds of shipboard machinery is measured bymeans of tachometers. For most shipboardmachinery, rotational speed is expressed inrevolutions per minute (rpm). The tachometersmost commonly used aboard ship are of thecentrifugal type, the chronometric type, and theresonance type. Stroboscopic tachometers arealso used occasionally.Some types of machinery are equipped withpermanently mounted tachometers, but portabletachometers are used for checking the rpm ofmany units. A portable tachometer of the centrifugaltype or of the chronometric type isapplied manually to a depression or a projectionat the center of a moving shaft. Each portabletachometer is supplied with several hardrubber tips; to use the instrument, the operatorselects a tip of the proper shape, fits it over theend of the tachometer drive shaft, and holds theSometip against the center of the moving shaft.tachometers are also supplied with a smallwheel which can be fitted to the end of the driveshaft and used to measure the linear velocity(in feet per second) of a wheel or a journal; withthis type of instrument, the wheel is held againstthe outer surface of the moving object. Portabletachometers are used only for intermittent reading,not for continuous operation.CENTRIFUGAL TACHOMETERSCOMPRESSED AIR SUPPLYBALANCE CHAMBER-ORIFICE'w/>/m/?Mwy7;ĀM61. 5XFigure 7-32.— Tank gaging system.As the name implies, a centrifugal tachometerutilizes centrifugal force for its operation.The main parts of a centrifugal tachometer areshown in figure 7-33 and the dial of the instrumentis shown in figure 7-34. Centrifugal forceacts upon weights or flyballs which are connectedby linkage to an upper and a lower collar. Theupper collar is fixed to the drive shaft, but the147