Chapter 7-PRINCIPLES OF MEASUREMENT 



and gaging tapes or by some form of permanently 

 installed, remote reading gaging system. (Al- 

 though remote reading gaging systems are often 

 referred to as "tank level indicators," it should 

 be noted that the scale may be calibrated to 

 show level, volume, or weight; frequently there 

 are two scales— one to show volume and one to 

 show level.) 



A static head gaging system of a type com- 

 monly used for measuring liquid level in fuel 

 oil tanks aboard ship is shown in figure 7-32. 

 This system balances a head of liquid in the 

 tank against a column of liquid in a manometer 

 or against a bellows or diaphragm differential 

 pressure unit; the system illustrated uses a 

 mercury manometer. The balance chamber is 

 located so that its orifice is near the bottom 

 of the tank; a line connects the top of the bal- 

 ance chamber to the mercury-filled bulb of the 

 indicator gage, and another line connects the 

 space above the mercury column to the top of 

 the tank. Since the height of the liquid in the 

 tank bears a definite relationship to the pres- 

 sure exerted by the liquid, the scale can be 

 calibrated to show height (or liquid level). When 

 the size of the tank is known, the measurement of 

 height can readily be converted to measurement 



'w/>/m/?M wy7; - AM 



COMPRESSED AIR SUPPLY 

 BALANCE CHAMBER 



-ORIFICE 



61. 5X 



Figure 7-32.— Tank gaging system. 



of volume; and, when the volume of the tank and 

 the specific weight of the liquid are known, the 

 scale can be calibrated to indicate weight. The 

 reading on this type of tank gaging system is 

 always taken after compressed air has been 

 admitted to the balance chamber through the 

 control valve; to ensure proper readings, a 

 sufficient amount of compressed air must be 

 admitted to force the liquid down to the level of 

 the bottom of the standpipe. 



MEASUREMENT OF ROTATIONAL 

 SPEED 



The rotational speed of propeller shafts, 

 turbines, generators, blowers, pumps, and other 

 kinds of shipboard machinery is measured by 

 means of tachometers. For most shipboard 

 machinery, rotational speed is expressed in 

 revolutions per minute (rpm). The tachometers 

 most commonly used aboard ship are of the 

 centrifugal type, the chronometric type, and the 

 resonance type. Stroboscopic tachometers are 

 also used occasionally. 



Some types of machinery are equipped with 

 permanently mounted tachometers, but portable 

 tachometers are used for checking the rpm of 

 many units. A portable tachometer of the cen- 

 trifugal type or of the chronometric type is 

 applied manually to a depression or a projec- 

 tion at the center of a moving shaft. Each port- 

 able tachometer is supplied with several hard 

 rubber tips; to use the instrument, the operator 

 selects a tip of the proper shape, fits it over the 

 end of the tachometer drive shaft, and holds the 

 tip against the center of the moving shaft. Some 

 tachometers are also supplied with a small 

 wheel which can be fitted to the end of the drive 

 shaft and used to measure the linear velocity 

 (in feet per second) of a wheel or a journal; with 

 this type of instrument, the wheel is held against 

 the outer surface of the moving object. Portable 

 tachometers are used only for intermittent read- 

 ing, not for continuous operation. 



CENTRIFUGAL TACHOMETERS 



As the name implies, a centrifugal tachom- 

 eter utilizes centrifugal force for its operation. 

 The main parts of a centrifugal tachometer are 

 shown in figure 7-33 and the dial of the instru- 

 ment is shown in figure 7-34. Centrifugal force 

 acts upon weights or flyballs which are connected 

 by linkage to an upper and a lower collar. The 

 upper collar is fixed to the drive shaft, but the 



147 



