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In case of a fluid flow, the relation between volumetric flow and velocity of fluid can be defined as-

Where

Q= Flow rate of the fluid

A= Cross sectional area of the pipe/meter

V= Velocity of fluid.

It can be observed that in case of differential pressure type flow meters, the flow is proportional to the square root of pressure drop. One the other hand, in case of

This linear relationship between flow and velocity enables velocity type flow meters to measure flow over a wide range. There are different types of flow meters falling under this category based on the type of mechanism used to measure the velocity of the flow.

In a fluid flow, if some obstruction is created, vortex are formed. The number of vortex formed can be used to find out the velocity of the fluid based on which the actual flow can be calculated. The typical construction of these flow meters involves an obstruction (termed as a shredder bar) which obstructs the fluid flow. As a result of this obstruction, vortex are formed. The number of vortexes formed is measured with the help of transmitter and receiver transducer. Actual flow is calculated based on the number of vortexes formed.

Magnetic flow meters are also velocity type flow meters which work on the Faraday’s law. As the Faraday’s law states, when a conducting substance moves across a magnetic field, an electric voltage will be induced.

Electromagnetic flow meters can be used only to measure conducting fluids. When all other parameters are kept constant, the voltage induced is directly proportional to the fluid flow. From the voltage induced, the velocity can be found out which helps to calculate the actual flow of the fluid.

Other types of flow type flow meters are turbine meters, swirl meters etc. In case of a turbine meter, a turbine is placed in the flow path with its axis of rotation perpendicular to the direction of flow. The number of rotations of the turbine gives the velocity of the flow from which the actual flow can be calculated.