When a venture meter is placed in a pipe 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.
The following are the main parts and areas of venture meter:
1) 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.
2) After the entry, there is a converging conical section with an
included angle of 19’ to 23’.
3) Following the converging section, there is a cylindrical section
with minimum area called as the throat.
4) After the throat, there is a diverging conical section with an
included angle of 5’ to 15’.
5) 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.
CONSTRUCTION OF VENTURI METER
1) The fluid whose flow rate is to be measured enters the
entry section of the venture meter with a pressure P1.
2) As the fluid from the entry section of venture 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
3) The differential pressure sensor attached between the
entry and throat section of the venture meter records the
pressure difference(P1-P2) which becomes an indication
of the flow rate of the fluid through the pipe when
4) 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
When pressure waves are released into the flowing
fluid, their velocity and amplitude are affected by
the fluid velocity. Ultrasonic flowmeters help in
measuring these pressure wave changes,
especially in the ones having frequencies greater
than 20KiloHertz using specialized techniques.
The working of Ultrasonic flow measurement system by
measuring phase difference is shown in the figure below.
The two peizo-crystals p1 and p2 working both as
transmitter and receiver of signals alternatively are
mounted conveniently, so that the ultrasonic signals are
transmitted between them as well as through the liquid.
Switch ‘sw’ is utilized to supply p1 and p2 alternately from
an oscillator simultaneously connecting the detector to p2
and p1 respectively.
The detector is designed to measure the transit time from
upstream to downstream and vice versa via phase shift
If C is the velocity of the ultrasonic wave and v is the fluid
velocity, then for a distance b between the crystals pa and
when conductive fluids pass through a magnetic field they generate
electromotive force in proportion to flow velocity.
The operation of a magnetic flowmeter is based on Faraday's Law.
The electromotive force is
generated in a direction
perpendicular to the direction of
the fluid motion and the
magnetic field, according to
Fleming's Right-hand Rule.
An electromagnetic flowmeter can be used to
measure the flow of fluids like corrosive acids, acid
slurries, paper pulp, detergents, beer, and so on.
The magnetic flowmeter detects this electromotive
force by using a pair of electrodes installed inside the
measuring tube, calculates the flow rate, converts it to
4-20 mA and pulse signals, and outputs them.
It is one of the most accurate industrial flowmeter
It has no movable parts or other obstructions inside the
measuring tube, can be used with corrosive fluids, and
Magnetic flowmeters are therefore extensively used in
such industries as chemicals, food, iron & steel, pulp &
paper, and water supply.
When an electrically heated wire is placed in a flowing
gas stream, heat is transferred from the wire to the gas and
hence the temperature of the wire reduces, and due to this,
the resistance of the wire also changes.
This change in resistance of the wire becomes a measure of
Types of anemometer.
1. Hot wire anemometer.
2. Hot film anemometer.
The bridge arrangement along with the anemometer has been shown
in diagram. The anemometer is kept in the flowing gas stream to
measure flow rate.
A constant current is passed through the sensing wire.
That is, the voltage across the bridge circuit is kept constant, that is,
Due to the gas flow, heat transfer takes place from the sensing wire to
the flowing gas and hence the temperature of the sensing wire
reduces causing a change in the resistance of the sensing wire. (this
change in resistance becomes a measure of flow rate).
Due to this, the galvanometer which was initially at zero position
deflects and this deflection of the galvanometer becomes a measure
of flow rate of the gas when calibrated.