Flow measurement (Flow Meter)
▶ Flow measurement(Meter) is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.
Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume
is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing stream as it
overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of
fluid over a known area.
Type of Flow measurement (Flow Meter)
♦ Ultrasonic and electromagnetic flow meters
♦ Electromagnetic flow meters
♦ Mechanical flow meters
• Turbine flow meter
• Piston meter/Rotary piston
• Pelton wheel
• Paddle wheel meter
♦ Pressure-based meters
• Venturi meter
• Orifice plate
• Pitot tube
♦ Open channel flow measurement
• Parshall flume
Principle and characteristic
♦ Ultrasonic flow meters
• Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in and
against flow direction. This time difference is a measure for the average velocity of the fluid along the path of the
ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound
can be calculated.
(Ultrasonic flow meters)
♦ Electromagnetic flow meters
• Magnetic flow meters, often called "mag meter"s or "electromag"s, use a magnetic field applied to the metering
tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines.
The potential difference is sensed by electrodes aligned perpendicular to the flow and the applied magnetic
field. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter
requires a conducting fluid and a nonconducting pipe liner. The electrodes must not corrode in contact with the
process fluid; some magnetic flowmeters have auxiliary transducers installed to clean the electrodes in place.
The applied magnetic field is pulsed, which allows the flowmeter to cancel out the effect of stray voltage
in the piping system.
(Magnetic flow meters)
♦ Turbine flow meter
• The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine
rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to
have all the flow traveling around it.
• The turbine wheel is set in the path of a fluid stream. The flowing fluid impinges on the turbine blades, imparting
a force to the blade surface and setting the rotor in motion. When a steady rotation speed has been reached,
the speed is proportional to fluid velocity.
(Turbine flow meter)
♦ Piston meter/Rotary piston
• Because they are used for domestic water measurement, piston meters, also known as rotary piston or
semi-positive displacement meters, are the most common flow measurement devices in the UK and are used for
almost all meter sizes up to and including 40 mm (1½?). The piston meter operates on the principle of a piston rotating
within a chamber of known volume. For each rotation, an amount of water passes through the piston chamber.
Through a gear mechanism and, sometimes, a magnetic drive, a needle dial and odometer type display are advanced.
♦ Venturi meter
• A Venturi meter constricts the flow in some fashion, and pressure sensors measure the differential pressure
before and within the constriction. This method is widely used to measure flow rate in the transmission of gas
through pipelines, and has been used since Roman Empire times.The coefficient of discharge of Venturi meter
ranges from 0.93 to 0.97.
♦ Orifice plate
• An orifice plate is a plate with a hole through it, placed in the flow; it constricts the flow, and measuring the pressure
differential across the constriction gives the flow rate. It is basically a crude form of Venturi meter, but with higher
energy losses. There are three type of orifice: concentric, eccentric, and segmental
♦ Pitot tube
• A Pitot tube is a pressure measuring instrument used to measure fluid flow velocity by determining the stagnation
pressure. Bernoulli's equation is used to calculate the dynamic pressure and hence fluid velocity.
• A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take
the form of a barrier across the river that causes water to pool behind the structure (not unlike a dam), but allows
water to flow over the top. Weirs are commonly used to alter the flow regime of the river, prevent flooding, measure
discharge and help render a river navigable.
♦ Parshall flume
• One of the most common weirs is the Parshall flume which was developed by Dr. Ralph L. Parshall of the U.S.
Bureau of Reclamation in 1922. It consists of three sections, a converging section, a throat section and a
diverging section. The critical depth develops in the throat section. The flow upstream and downstream of the
throat is subcritical and supercritical respectively. The hydraullic jump forms in the diverging section.