Pressure taps are installed upstream the throat and at the throat, and the difference in pressure at these two locations is used to calculate velocity and the total rate of fluid flowing through a venturi.

Due to the gradual decrease and increase in the cross section of the flow passage, the effect of frictional forces on decelerating the fluid velocity can be considered to be negligibly small.

ii) Orifice meter and nozzle

An orifice meter is of simple construction. A thin plate with a centrally located hole is inserted into the flow passage. The main path of flow through the orifice is same as that of venturi, but the flow contracts suddenly as fluid passes through the hole. The flow continues to contract a short distance downstream the hole as shown in the figure. Figure also shows the flow lines.  It can be seen that the region of smallest cross section, known as vena contracta, is developed downstream the orifice. In the vena contracta the kinetic energy is maximum. Note that the minimum cross section in the orifice meter is not orifice diameter, but it is cross section at vena- contracta. Accordingly pressure tap is to be  installed at vena contracta as shown in the figure.

Nozzles are similar to orifices in general but the decrease in cross section area in the direction of flow is gradual when compared with an orifice in which it is abrupt. So that the losses due to friction are lower in nozzle than in the orifice.