Venturi Tube:

The venturi tube, described in Figure, is the most accurate flow-sensing element while properly calibrated. A venturi tube has a converging conical inlet, a diverging recovery cone and cylindrical throat.  No sharp corners and no sudden changes in contour it has no projections into the fluid.

Figure: Venturi Tube

The inlet section decreases the area of the fluid stream will causing the velocity to increase and the pressure to decrease. A low pressure is measured in the center of the cylindrical throat since the pressure will be at its lowest value and neither a pressure nor the velocity is changing. The recovery cone permits for the recovery of pressure such that total pressure loss is only 10 percent to 25 percent. The high pressure is measured upstream of the entrance cone. The main drawback of this category of flow detection is the high initial costs for installation and hard in installation and inspection.