736 



HYDRAULICS AND ITS APPLICATIONS 



as '2 foot per second, and is exceedingly accurate when fitted to large 

 mains, but is not suitable without careful calibration for use in small 

 pipes below about 2" diameter, because of the greater proportional effect of 

 viscosity in such pipes. It is, moreover, not well adapted for use in pipes 

 where the water is subjected to periodic pulsations, as, for example, in the 

 discharge pipe from a reciprocating pump, since this is likely to set up 

 oscillations in the recording mechanism. 



Prof. C. M. Allen ("Am. Soc. Mech. Engineers," December 1909), on tests 

 of a 2" Venturi used for boiler feed, found a factor " C " varying from '935 

 at 40 Ibs. per sq. in. to '962 at 200 Ibs. With steady flow errors were less 

 than 1 per cent. ; with pulsating flow, within 2J per cent. 



For measuring the difference of head h A li a , a differential gauge 



consisting of a U-tube containing mercury, 

 may be used (Fig. 355), 1 though where 

 small differences of pressure are to be 

 measured a preferable device is that shown 

 in Fig. 356, where the difference of pres- 

 sure head is directly measured in feet of 

 water. Here compressed air must be sup- 

 plied to the higher portion of the inverted 

 U-tube. 



If the meter tube be not horizontal, and 

 if z be the difference in level at the entrance 

 and throat, so that we have 



W~^~~2~i ~W'^ 2// " 

 it is easily shown that equation (1) becomes : 



Volume per second = c k f V(h A h a ) z, 



If the connecting tubes be shut off from the main, and connection be 

 made so that the water may attain a common level in the two tubes, and 

 if now the pencil of the recorder be put to zero, the effect is to add z 

 automatically to the observed head, and on cutting off the connection 

 between the tubes and coupling up to the mains, true readings will be 

 given on the ordinary record sheet. 



It should be noted that the converging portion of the main is the only 

 part really essential to the meter action. The diverging cone simply 

 ensures that the reconversion of kinetic into potential energy shall take 



1 Where a mercury gauge is used having the connecting pipes full of water, it is easily 



13 '6 1 

 shown that the effective gauge reading is less than the apparent in the ratio ' ' = -926. 



