There are a number of flow measuring devices available in the market.
- Differential pressure flowmeters: includes Orifice plate, venturimeter, flow nozzle and variable area or rotameter. These devices are based on Bernoulli's principle and measure the change in pressure between two points, which is then converted to velocity.
- Velocity flowmeter: includes, pitot tube, calorimeteric flowmeter, turbine flowmeter, vortex flowmeter, electromagnetic flowmeter, ulrasonic Doppler flowmeter. Here, the flow velocity is measured and integrated over the entire flow area.
- Positive displacement flowmeter: includes, reciprocating piston meter, nutating disk meter, rotary vane meter, etc. It is based o the rotation of rotors which is directly proportional to amount of fluid displaced.
- Mass flowmeter: includes, thermal flowmeter, Coriolis flowmeter, etc. These devices measure the mass flowrate directly by thermal sensing or Coriolis effect.
- Open Channel Flowmeter: includes weirs (V-notch, sharp crested, Cipolletti, rectangular notch) and flumes (such as Parshall flume or Venturi flume). It is based on measurement of flow height as it passes over an obstruction and since discharge is a function of this flow height, it can be converted to the flowrate.
The schematics can be obtained by simple wen search.
hope this helps.