WATER METERS 



735 



Owing to viscosity, the true velocity accompanying a given fall of 

 pressure (p A p^) is less than this, being given by 



V - 



W T here c is a coefficient of velocity which depends slightly on the diameter 

 and material of the pipes and on the velocity of flow, increasing with the 

 diameter and velocity of 



L._2fe</._.J L. - 7'/zd. 



I 



flow and diminishing as 

 the surface roughness 

 increases. 



Experiments by Her- 

 schel show that c varies 

 from '94 to unity, the 

 great majority of tests 

 giving values between 

 96 and '99. In a 48" 

 meter c had the value 

 995. 



For any meter ( ) is fixed, the ratio commonly being 9:1, so 

 that 



Angle of Divergence 56' 3 

 FIG. 355. Venturi Meter. 



V 



(~Y- I{ 



\ a / 



is constant = k. 



A 



Taking -= 9, then k = v = '8972. 

 a o(J 



Then the volume in cubic feet) _ y ^ = c k A 

 per second through meter j 



W 





(1) 



Here 







a or h A h a is the difference in pressure at the throat (1) 



and entrance (2) expressed as a head in feet of water, and is directly 

 measured in the meter. The constants c and k being determined for the 

 instrument, the volume passing per second may be directly inferred. 



This meter has the advantage of registering at almost any velocity, the 

 permissible range of velocities depending on the permissible loss of head 

 in passing the meter. Generally, a maximum velocity of up to sixteen 

 times the minimum is permissible. It will register with velocities so low 



