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As observed before now in: Theory of Fluid Flow meters, there are various classification of flow meters. No single classification is by any means exhaustive or sufficient if I may say. However, each classification attempts to present the flow meters in a logical grouping. The following flowmeters have been grouped as electronic flow meters:
(a) Magnetic Flowmeters
(b) Vortex Flowmeters
(c) Ultrasonic Flowmeters

They are not entirely electronic in nature but they represent a logical grouping of flow measurement technologies. All these meters have no moving parts(though they may experience vibration in operation). Their functionality is made possible through highly sophisticated electronic devices and circuits.
Magnetic Flowmeters
Magnetic flowmeters operate on basis of Michael Farady’s electromagnetic induction principle. Magmeters as they are also called can only work with conductive fluid.
The magnetic flowmeter consists of a non-magnetic pipe lined with an insulating material. A pair of magnetic coils positioned such that they are at right angle to the flow stream and a pair of electrodes penetrates the pipe and its lining. When a conductive fluid flows through a pipe under the influence of a magnetic field of specific density generated by the magnetic coils, the amount of voltage (E) developed across the electrodes as predicted by Faraday’s law will be proportional to the velocity (V) of the liquid.

E = KV

Where:
E = voltage developed across the electrodes
V = velocity of the fluid flowing through the pipe
K = calibration constant of the meter

Vortex Flowmeters
When a non-streamlined object (also called a bluff body) is placed in the path of a flow stream with high Reynolds number, the fluid will alternately separate from the object on its two downstream sides, and, as the boundary layer becomes detached and curls back on itself, the fluid forms vortices. These vortices cause areas of fluctuating pressure that are detected by a piezoelectric or capacitive sensor. The fluid velocity depends on frequency of generated vortices and on Strouhal’s number as given by:

V = (f x h)/S

Where:

V = average velocity of fluid flow stream
f = frequency of the vortices generated
h = Width of bluff body
S = Strouhal’s number

S varies with Reynolds’ number; but is virtually constant over a broad flow range.
In practice a k-factor, which represents the number of vortices generated per volume unit replace S. Thus volumetric flow is then:

Q = f/K

Ultrasonic Flow meters
The speed at which sound propagates in a fluid is dependent on the fluid’s density. If the density is constant, however, one can use the time of ultrasonic passage (or reflection) to determine the velocity of a flowing fluid.

Ultrasonic flow meters measure fluid velocity by passing high-frequency sound waves along the fluid flow path. Fluid motion influences the propagation of these sound waves, which may then be measured to infer fluid velocity. Two major types of ultrasonic flow meters exist: Doppler and transit-time. Both types of ultrasonic flow meter work by transmitting a high-frequency sound wave into the fluid stream (the incident pulse) and analyzing the received pulse.

Doppler flowmeters exploit the Doppler Effect, which is the shifting of frequency resulting from waves emitted by or reflected by a moving object. Doppler flowmeters bounce sound waves off of bubbles or particulate material in the flow stream, measure the frequency shift, and infer fluid velocity and therefore flow from the magnitude of that shift.

In the design of transit time ultrasonic flowmeters, the time of flight of the ultrasonic signal is measured between two transducers, one upstream and one downstream. The difference in elapsed time going with or against the flow determines the fluid velocity from where the flow rate is inferred.

Note that the following flow meters:Turbine Flowmeters, Magnetic Flowmeters, Vortex Flowmeters and Ultrasonic flowmeters can also be classified as velocity-based flow meters. This is because velocity is used to infer the flow rate in these flow meters.