Fluid Flow: Flow Measurement Devices

To measure fluid flow rate, there are different flow meters like venturimeter, orifice meter, wier, notches and rotameter.

Venturimeter, orifice meter and rotameter are used to measure flow in closed pipe/channel while wier, notches are used for open flow like river. 

These flow meters are classified as variable head meter and variable area meter. venturimeter, orifice meter are variable head meters where velocity head is converted into pressure head and in rotameter the area of flow in rotameter increases along the flow. when the fluid flows in rotameter, the float starts rising due to buoyancy and drag force. As the flow rate increases, the drag force ( at earlier point) increases that takes the float up in the meter, where the area is larger, leads to decrease in velocity to the earlier value and the the float attains equilibrium.