WATER-FLOW METERS 



263 



ally less than the pressure at A. This difference in pressure 

 between A and B can be accurately measured, and bears an exact 

 ratio at all times to the rate onflow through the throat B. After 

 passing the throat, the velocity begins to decrease with an accom- 

 panying rise in pressure, and when C is reached the pressure tem- 

 porarily lost at B has been almost entirely regained. Therefore, a 

 properly proportioned tube not only provides a basis for accurate 



Main 



FIG. 142. Method of Installing Venturi Meter. 



FIG. 143. Principle of Venturi Meter Tube. 



measurement of the flow, but it will deliver practically the same 

 amount of water as a straight pipe of equal length and diameter. 



Commercial tubes are made in two or more sections, as seen 

 from Fig. 142, and near the inlet and at the throat are annular 

 chambers communicating with the interior of the pipe by numer- 

 ous ventholes. The throat portion is lined with bronze accurately 

 bored to a definite diameter and contour. 



