Paraphrase:
Venturi tube:
Is a tube of non-constant diameter (see Fig. 1). To minimize disturbances to the flow, the edges of the venture meter have the same diameter as the pipe into which the meter is inserted. Variation of the tube diameter leads to variation of the fluid pressure inside the meter. There are two pressure taps located at the widest and the narrowest locations of the tube. Therefore, we can determine the flow rate by measuring pressures p1 and p2 at these locations and substituting them into the Bernoulli equation. For an incompressible fluid, the pressure drop is related to the flow rate .
Orifice plate:
Is a plate with a machined hole in the center (see Fig. 2). The flow rate is determined by measuring the pressure drop as the flow passes through the plate. Eq. (7) still holds for orifice meter with D1 and D2 being diameters of the pipe and the orifice hole, respectively. The frictional losses in the orifice meter are much larger than in the venturi meter and a typical value of the discharge coefficient Cd is 0.6. Precise value Cd should be determined experimentally.