Chapter 7-PRINCIPLES OF MEASUREMENT 



indicate rate of flow in gallons per minute (gpm) 

 or in other flow units. 



HEAD METERS 



Head meters measure fluid flow by measur- 

 ing the pressure differential across a specially 

 designed restriction in the flow line. The re- 

 striction may be an orifice plate, a flow nozzle, 

 a venturi tube, an elbow, a pitot tube, or some 

 similar device. As the fluid flows toward the 

 restriction, the velocity decreases and the pres- 

 sure (or "head") increases; as the fluid flows 

 through the restriction, the velocity increases 

 and the pressure decreases. Figure 7-30 illus- 

 trates the pressure changes that occur as a fluid 

 flows through a line that contains an orifice 

 plate or similar restriction. Note that there is 

 a slight increase in pressure just ahead of the 

 restriction and then a sudden drop in pressure 

 at the restriction. The point of minimum pres- 

 sure and maximum velocity is slightly down- 

 stream from the restriction; this point is called 

 the vena contracta . Beyond the vena contracta, 

 the velocity decreases and the pressure in- 

 creases until eventually normal flow is re- 

 established. 



The flow nozzle, orifice plate, or other 

 restriction in the line is called the primary 

 element of the head meter. A high pressure tap 

 upstream from the restriction and a low pres- 

 sure tap downstream from it are connected to 

 a differential bellows, a diaphragm, or some 



other device for measuring differential pres- 

 sure. 



The pressure drop occurring in a fluid flow- 

 ing through a restriction varies as the square of 

 the fluid velocity; or, to put it another way, the 

 square root of the pressure differential is pro- 

 portional to the rate of fluid flow. Because of the 

 square-root relationship between the pressure 

 differential and the rate of fluid flow, a square- 

 root extracting device is usually included so that 

 the scale can be graduated in even steps or in- 

 crements. Without a device for extracting the 

 square root of the pressure differential, the 

 scale would have to be unevenly divided, with 

 wider divisions at the top of the scale than at 

 the bottom. 



AREA METERS 



An area meter indicates the rate of fluid 

 flow by means of an orifice that is varied in 

 area by variations in the fluid flow. The vari- 

 ations in the area of the orifice are produced 

 by some type of movable device which is posi- 

 tioned by the pressure of the flowing fluid. Since 

 the fluid itself positions the movable device and 

 thus varies the area of the orifice, there is no 

 significant pressure drop between the upstream 

 side and the downstream side of the variable 

 orifice. Since there is an essentially linear 

 relationship between the area of the orifice and 

 the rate of flow, there is no need for a square- 

 root extracting device in an area meter. 



ORIFICE PLATE 



DIRECTION OF FLOW 



^FLANGES 



147.59 



Figure 7-30.— Pressure changes in fluid flowing through restriction in line. 



145 



