The most common principals for fluid flow metering are:

• Differential Pressure Flowmeters

• Velocity Flowmeters

• Positive Displacement Flowmeters

• Mass Flowmeters

• Open Channel Flowmeters

Differential Pressure Flowmeters

In a differential pressure drop device the flow is calculated by measuring the pressure drop

over an obstructions inserted in the flow. The differential pressure flowmeter is based on the

Bernoullis Equation, where the pressure drop and the further measured signal is a function

of the square flow speed.

The most common types of differential pressure flowmeters are:

• Orifice Plates

• Flow Nozzles

• Venturi Tubes

• Variable Area - Rotameters

Orifice Plate

With an orifice plate, the fluid flow is measured through the difference in pressure from the

upstream side to the downstream side of a partially obstructed pipe. The plate obstructing

the flow offers a precisely measured obstruction that narrows the pipe and forces the

flowing fluid to constrict.

The orifice plates are simple, cheap and can be delivered for almost any application in any

material.

The TurnDown Rate for orifice plates are less than 5:1. Their accuracy are poor at low flow

rates. A high accuracy depend on an orifice plate in good shape, with a sharp edge to the

upstream side. Wear reduces the accuracy.

• Orifice, Nozzle and Venturi Meters

Venturi Tube

Due to simplicity and dependability, the Venturi tube flowmeter is often used in applications

where it's necessary with higher TurnDown Rates, or lower pressure drops, than the orifice

plate can provide.

In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area

in the flow path, generating a pressure difference. After the constricted area, the fluid is

passes through a pressure recovery exit section, where up to 80% of the differential

pressure generated at the constricted area, is recovered.

With proper instrumentation and flow calibrating, the Venturi Tube flowrate can be reduced

to about 10% of its full scale range with proper accuracy. This provides a TurnDown Rate

10:1.

• Orifice, Nozzle and Venturi Meters

Flow Nozzles

Flow nozzles are often used as measuring elements for air and gas flow in industrial

applications.

The flow nozzle is relative simple and cheap, and available for many applications in many

materials.

The TurnDown Rate and accuracy can be compared with the orifice plate.

• Orifice, Nozzle and Venturi Meters

The Sonic Nozzle - Critical (Choked) Flow Nozzle

When gases accelerate through a nozzle, the velocity increase and the pressure and the gas

density decrease. The maximum velocity is achieved at the throat, the minimum area,

where it breaks Mach 1 or sonic. At this point it's not possible to increase the flow by

lowering the downstream pressure. The flow is choked.

This situation is used in many control systems to maintain fixed, accurate, repeatable gas

flow rates unaffected by the downstream pressure.

Recovery of Pressure Drop in Orifices, Nozzles and Venturi

Meters

After the pressure difference has been generated in the differential pressure flow meter, the

fluid pass through the pressure recovery exit section, where the differential pressure

generated at the constricted area is partly recovered.

As we can see, the pressure drop in orifice plates is significant higher than in the venturi

tubes.

Variable Area Flowmeter or Rotameter

The rotameter consists of a vertically oriented glass (or plastic) tube with a larger end at

the top, and a metering float which is free to move within the tube. Fluid flow causes the

float to rise in the tube as the upward pressure differential and buoyancy of the fluid

overcome the effect of gravity.

The float rises until the annular area between the float and tube increases sufficiently to

allow a state of dynamic equilibrium between the upward differential pressure and buoyancy

factors, and downward gravity factors.

The height of the float is an indication of the flow rate. The tube can be calibrated and

graduated in appropriate flow units.

The rotameter meter typically have a TurnDown Ratio up to 12:1. The accuracy may be as

good as 1% of full scale rating.

Magnetic floats can be used for alarm and signal transmission functions.

Velocity Flowmeters

In a velocity flowmeter the flow is calculated by measuring the speed in one or more points

in the flow, and integrating the flow speed over the flow area.

Pitot Tubes

The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially

in air applications as ventilation and HVAC systems, even used in airplanes for the speed

measurent.

The pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow

into potential energy.

The use of the pitot tube is restricted to point measuring. With the "annubar", or multi-

orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and

the annubar obtains an averaging effect.

Calorimetric Flowmeter

The calorimetric principle for fluid flow measurement is based on two temperature sensors

in close contact with the fluid but thermal insulated from each other.

One of the two sensors is constantly heated and the cooling effect of the flowing fluid is

used to monitor the flowrate. In a stationary (no flow) fluid condition there is a constant

temperature difference between the two temperature sensors. When the fluid flow

increases, heat energy is drawn from the heated sensor and the temperature difference

between the sensors are reduced. The reduction is proportional to the flow rate of the fluid.

Response times will vary due the thermal conductivity of the fluid. In general lower thermal

conductivity require higher velocity for proper measurement.

The calorimetric flowmeter can achieve relatively high accuracy at low flow rates.

Turbine Flowmeter

There is many different manufacturing design of turbine flow meters, but in general they are

all based on the same simple principle:

If a fluid moves through a pipe and acts on the vanes of a turbine, the

turbine will start to spin and rotate. The rate of spin is measured to calculate the flow.

The turndown ratios may be more than 100:1 if the turbine meter is calibrated for a single

fluid and used at constant conditions. Accuracy may be better than +/-0,1%.

Vortex Flow Meter

An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a

critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance

where alternating low pressure zones are generated in the downstream.

These alternating low pressure zones cause the obstruction to move towards the low

pressure zone. With sensors gauging the vortices the strength of the flow can be measured.

• The Vortex Flowmeter Principle - An introduction to the vortex flowmeter principle.

Electromagnetic Flowmeter

An electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that

states that a voltage will be induced when a conductor moves through a magnetic field. The

liquid serves as the conductor and the magnetic field is created by energized coils outside

the flow tube.

The voltage produced is directly proportional to the flow rate. Two electrodes mounted in

the pipe wall detect the voltage which is measured by a secondary element.

Electromagnetic flowmeters can measure difficult and corrosive liquids and slurries, and

they can measure flow in both directions with equal accuracy.

Electromagnetic flowmeters have a relatively high power consumption and can only be used

for electrical conductive fluids as water.

• The Electromagnetic Flowmeter Principle - An introduction to the electromagnetic

flowmeter principle

Ultrasonic Doppler Flowmeter

The effect of motion of a sound source and its effect on the frequency of the sound was

observed and described by Christian Johann Doppler.

The frequency of the reflected signal is modified by the velocity and direction of the fluid

flow

If a fluid is moving towards a transducer, the frequency of the returning signal will increase.

As fluid moves away from a transducer, the frequency of the returning signal decrease.

The frequency difference is equal to the reflected frequency minus the originating frequency

and can be use to calculate the fluid flow speed.

• The Ultrasonic Doppler and Time of Flight Flowmeter

• An Ultrasonic Flowmeter Tutorial - A basic tutorial about ultrasonic flowmeters.

Positive Displacement Flowmeter

The positive displacement flowmeter measures process fluid flow by precision-fitted rotors

as flow measuring elements. Known and fixed volumes are displaced between the rotors.

The rotation of the rotors are proportional to the volume of the fluid being displaced.

The number of rotations of the rotor is counted by an integral electronic pulse transmitter

and converted to volume and flow rate.

The positive displacement rotor construction can be done in several ways:

• Reciprocating piston meters are of single and multiple-piston types.

• Oval-gear meters have two rotating, oval-shaped gears with synchronized, close

fitting teeth. A fixed quantity of liquid passes through the meter for each revolution.

Shaft rotation can be monitored to obtain specific flow rates.

• Nutating disk meters have moveable disks mounted on a concentric sphere located in

spherical side-walled chambers. The pressure of the liquid passing through the

measuring chamber causes the disk to rock in a circulating path without rotating

about its own axis. It is the only moving part in the measuring chamber.

• Rotary vane meters consists of equally divided, rotating impellers, two or more

compartments, inside the meter's housings. The impellers are in continuous contact

with the casing. A fixed volume of liquid is swept to the meter's outlet from each

compartment as the impeller rotates. The revolutions of the impeller are counted and

registered in volumetric units.

The positive displacement flowmeter may be used for all relatively nonabrasive fluids such

as heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing ink,

freon, and many more.

Accuracy may be up to ?0.1% of full rate with a TurnDown of 70:1 or more.

Mass Flowmeters

Mass meters measure the mass flow rate directly.

Thermal Flowmeter

The thermal mass flowmeter operates independent of density, pressure, and viscosity.

Thermal meters use a heated sensing element isolated from the fluid flow path where the

flow stream conducts heat from the sensing element. The conducted heat is directly

proportional to the mass flow rate and the he temperature difference is calculated to mass

flow.

The accuracy of the thermal mass flow device depends on the calibrations reliability of the

actual process and variations in the temperature, pressure, flow rate, heat capacity and

viscosity of the fluid.

Coriolis Flowmeter

Direct mass measurement sets Coriolis flowmeters apart from other technologies. Mass

measurement is not sensible to changes in pressure, temperature, viscosity and density.

With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal

meters.

Coriolis Mass Flowmeter uses the Coriolis effect to measure the amount of mass moving

through the element. The fluid to be measured runs through a U-shaped tube that is caused

to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will

deform and an additional vibration component will be added to the oscillation. This

additional component causes a phase shift on some places of the tubes which can be

measured with sensors.

The Coriolis flow meters are in general very accurate, better than +/-0,1% with an

turndown rate more than 100:1. The Coriolis meter can also be used to measure the fluids

density.

Open Channel Flowmeters

A common method of measuring flow through an open channel is to measure the height of

the liquid as it passes over an obstruction as a flume or weir in the channel.

Common used is the Sharp-Crested Weir, the V-Notch Weir, the Cipolletti weir, the

Rectangular-Notch Weir, the Parshall Flume or Venturi Flume.

Weirs are structures consisting of an obstruction such as a dam or bulkhead placed across

the open channel with a specially shaped opening or notch. The weir results an increase in

the water level, or head, which is measured upstream of the structure. The flow rate over a

weir is a function of the head on the weir.

Common weir constructions are the rectangular weir, the triangular or v-notch weir, and the

broad-crested weir. Weirs are called sharp-crested if their crests are constructed of thin

metal plates, and broad-crested if they are made of wide timber or concrete.

Water level-discharge relationships can be applied and meet accuracy requirements for

sharp-crested weirs if the installation is designed and installed consistent with established

ASTM and ISO standards.

• Common Standards and Specifications for Weir Flow Measurements

Rectangular weirs and triangular or v-notch weirs are often used in water supply,

wastewater and sewage systems. They consist of a sharp edged plate with a rectangular,

triangular or v-notch profile for the water flow.

Broad-crested weirs can be observed in dam spillways where the broad edge is beneath the

water surface across the entire stream. Flow measurement installations with broad-crested

weirs will meet accuracy requirements only if they are calibrated.

Other available weirs are the trapezoidal (Cipolletti) weir, Sutro (proportional) weir and

compound weirs (combination of the previously mentioned weir shapes).

Rectangular Weir

The flow rate measurement in a rectangular weir is based on the Bernoulli Equation

principles and can be expressed as:

q = 2/3 cd b [2 g]1/2 h3/2 (1)

where

q = flow rate

h = head on the weir

b = width of the weir

g = gravity

cd= discharge constant for the weir - must be determined

cd must be determined by analysis and calibration tests. For standard weirs - cd - is well

defined or constant for measuring within specified head ranges.

Triangular or V-Notch Weir

For a triangular or v-notch weir the flow rate can be expressed as:

q = 8/15 cd b [2 g]1/2 tan(θ/2) h5/2 (2)

where

θ = v-notch angle

• An Online V-notch - Triangular - Flow Rate Calulator

Broad-Crested Weir

For the broad-crested weir the flow rate can be expressed as:

q = cd h2 b [2 g (h1 - h2)]1/2 (3)

Measuring the Levels

For measuring the flow rate it's obviously necessary to measure the flow levels, then use

the equations above for calculating. It's common to measure the levels with:

• ultrasonic level transmitters, or

• pressure transmitters

Ultrasonic level transmitters are positioned above the flow without any direct contact with

the flow. Ultrasonic level transmitters can be used for all measurements. Some of the

transmitters can even calculate a linear flow signal - like a pulse signal or 4-20 mA signal -

before transmitting it to the control system.

Pressure transmitters can be used for the sharp-crested weirs and for the first measure

point in broad-crested weir. The pressure transmitter outputs a linear level signal - 4-20 mA

- and the flow must in general be calculated in the control system.