Venturi Flow Meter Theory

Venturi Flow Meter

When a venture meter is placed in apipe carrying the fluid whose flow rate is to be measured, a pressure drop occurs between the entrance and throat of the venturimeter.

This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.

Construction of Venturi meter

The following are the main parts and areas of venture meter:

  • The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.

  • After the entry, there is a converging conical section with an included angle of 19’ to 23’.

  • Following the converging section, there is a cylindrical section with minimum area called as the throat.

  • After the throat, there is a diverging conical section with an included angle of 5’ to 15’.

  • Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venture meter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge, etc) as shown in diagram.

Operation of venturi meter:

  • The fluid whose flow rate is to be measured enters the entry section of the venturi meter with a pressure P1.
  • As the fluid from the entry section of venturi meter flows into the converging section, its pressure keeps on reducing and attains a minimum value P2 when it enters the throat. That is, in the throat, the fluid pressure P2 will be minimum.
  • The differential pressure sensor attached between the entry and throat section of the venturi meter records the pressure difference(P1-P2) which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
  • The diverging section has been provided to enable the fluid to regain its pressure and hence its kinetic energy. Lesser the angle of the diverging section, greater is the recovery.


  • It is used where high pressure recovery is required.
  • Can be used for measuring flow rates of water,gases,suspended solids, slurries and dirty liquids.
  • Can be used to measure high flow rates in pipes having diameters in a few meters.

Advantages of venturi meters

  • Less changes of getting clogged with sediments
  • Coefficient of discharge is high.
  • Its behaviour can be predicted perfectly.
  • Can be installed vertically, horizontally or inclinded.


  • They are large in size and hence where space is limited, they cannot be used.
  • Expensive initial cost, installation and maintenance.
  • Require long laying length. That is, the veturimeter has ti be proceeded by a straight pipe which is free from fittings and misalignments to avoid turbulence in flow, for satisfactory operation. Therefore, straightening vanes are a must.
  • Cannot be used in pipes below 7.5cm diameter.