Find out more about the following measurement principles:
Paddle wheelOval gearVortexUltrasonicElectromagneticCoriolisSurface acoustic waves
What is flow measurement?
Flow measurement is a method of determining the amount of fluid flowing through a pipeline or other system per unit of time. When measuring the flow rate, either the volume flow rate (e.g. l/h, cbm/h) or the mass flow rate (e.g. kg/s, lb/h) can be determined.
Volumetric flow measurement is used when the density of the medium is approximately constant (e.g. water). If the density of the medium is influenced by pressure or temperature (e.g. gases), the mass flow rate should be measured.
How does a flow meter work?
A flow meter is a measuring device that measures the flow of a medium in a pipeline. It consists of a transmitter and a sensor.
Sensor
The sensor uses physical principles to provide a signal, proportional to a flow velocity.
Transmitter
The transmitter converts basic measured variables such as voltage into flow measurement values. The measured values are converted into standard signals and output in analogue (as current values in the range from 4 to 20 milliamperes),digital (as pulses) or digital communication (e.g. PROFINET, EtherCat, IO-Link). A counter function totals the flow rate values. The transmitter displays measurement data and communicates it via interfaces.The transmitter transforms a raw signal into a usable flow rate, which is then displayed and communicated via interfaces.
Why is the flow rate measured?
The flow rate of liquids, gases or vapour is measured at various points in a process – e.g. at inlets and outlets or before and after various process steps.
Possible tasks are:
- Monitoring (status information, e.g. when transferring liquids from one tank to another)
- Control (e.g. in heating and cooling applications in steel production)
- Filling/dosing (e.g. filling soft drinks)
- Mixing (e.g. flocculant supply in water treatment lines)
In what industries is a flow meter used?
Flow meters are used particularly frequently in the following industries:
- Water and wastewater (e.g. municipal drinking water treatment)
- Oil and gas (e.g. monitoring of refinery processes, petrol stations)
- Heating and cooling (e.g. cooling monitoring in metal die casting)
- Food and beverages (e.g. mixing soft drinks)
- Pharmaceuticals and medical technology (e.g. WFI cycle)
- Cosmetics (e.g. bottling of perfumes)
Important measuring principles for flow measurement
Paddle wheel
Functionality
Flow measurement using a paddle wheel works by converting the flow movement of the fluid in a pipeline into a mechanical rotary movement of the paddle wheel.A paddle wheel arranged perpendicular to the direction of flow is set in rotation by the liquid flow. A sensor element recognises the rotary movement and generates a frequency signal from it. The pulse frequency is proportional to the flow velocity: v ~ f
The sensor elements used can be optical or magnetic (coil or Hall sensors).
A transmitter calculates the flow rate from the pulse signal and a specific conversion factor (K-factor). The K-factor is largely dependent on the nominal pipe diameter and is specified by the manufacturer. The calculated flow rate is then displayed, for example, in litres per minute and output via an analogue or digital output.
Designs
- Inline: The paddle wheel is part of the fitting (the connecting piece that is installed in the pipe).
- Insertion: The paddle wheel is attached to a “finger”, which is part of the transmitter. This can be inserted into an existing pipe thanks to spigot or saddle connection).
Typical areas of application
Paddle wheel flow meters are suitable for neutral, slightly aggressive and low solid content liquids and are used, for example, for cooling machines, in water treatment or for irrigation.
Advantages of paddle wheel flow meters
+ Comparatively small and light
+ Low energy consumption
+ Independent of the conductivity of the medium
Disadvantages of paddle wheel flow meters
- Low accuracy
- Installations in the piping
- Not suitable for low flow rates
- Inlet/outlet sections required
- Contains moving parts
Oval gear
Functionality
An oval gear flow meter works according to the displacement principle.Two interlocking oval gears are set in rotation in a flowmeasurement chamber by the medium flowing through. The medium fills the space between the oval gears and the chamber wall. With each rotation, a constant, known volume flowsfrom the inlet to the outlet of the chamber.
A sensor detects the movement of the oval gears via integrated permanent magnets and generates two square-wave output pulses. The transmitter converts these into a flow rate which, depending on the user’s choice and configuration, is shown on the display and sent as milliampere values via the analogue output or as pulses via the digital output.
Typical areas of application
Oval gear flow meters are also suitable for highly viscous liquids and are used, for example, to measure the flow of adhesives, oils or honey.
Advantages of oval gear flow meters
+ High accuracy and repeatability
+ Suitable for media with high viscosity
+ No inlet and outlet sections required
Disadvantages of oval gear flow meters
- Air bubbles must be avoided
- Installations in the pipeing
- Not suitable for low flow rates
- Not suitable for very fluid liquids
Vortex
Function principle
Vortex flow measurement works according to the principle of the Kármán vortex street. If a liquid or air flows past an obstacle, vortexes form on both sides behind such elements. In nature, this phenomenon occurs behind a bridge pier, for example.
In a vortex flow measurement, a disturbance body is installed in the flow path, the size and shape of which is selected in such a way that it promotes vortex formation. The vortexes that form behind this body on both sides are detected by a sensor such as a piezo element, which reacts to changes in pressure. As the flow velocity increases, the frequency of the vortexes increases. The transmitter of the vortex flow meter converts the number of pulses detected by the piezo element into a flow rate. A K-factor is also used here, which is mainly determined by the pipe cross-section and the type of medium (liquid, gas or vapour).
Typical areas of application
Vortex flow meters are often used to measure steam flows – e.g. in power stations.
Advantages of vortex flow meters
+Robust and reliable
+ Low maintenance as there are no moving parts
+ Insensitive to magnetic fields
Disadvantages of vortex flow meters
- Limited to defined Reynolds number range
Ultrasonic
Function principle
The transit time flow meter measures the time difference required for ultrasonic waves to travel in and against the flow direction of the fluid.For this purpose, two piezo elements are attached to the pipe wall, each of which acts as a receiver or transmitter. Both piezo elements simultaneously transmit an ultrasonic signal to the other piezo element. The signals are reflected by two mirrors, one of which is located in the inlet area and the other in the outlet area of the pipe. The time it takes for the ultrasonic signal to travel with the flow is shorter than the time it takes for the signal to travel against the flow direction. The transit time difference is directly proportional to the flow velocity. The higher the flow velocity, the greater the time difference. Together with the known pipe cross-section, the flow rate can be calculated from this.
In addition to the transit time method, there is also the Doppler method, which analyses the change in frequency of the reflected waves. It is used less frequently and is suitable for fluids with particles or air bubbles.
Typical areas of application
Ultrasonic flow meters are often used for irrigation, for measuring the flow of industrial water and for heating and cooling in the metal industry.
Advantages of ultrasonic flow meters
+ No moving parts
+ Suitable for conductive and non conductive liquids
Disadvantages of ultrasonic flow meters
- Accuracy affected by fluid properties
Electromagnetic flow meter
Function principle
Electromagnetic flow meters (EMF), also known as magmeters, measure the flow rate of conductive liquids using the principle of electromagnetic induction. The measuring principle is based on Faraday’s law. The flow rate is determined by measuring the induced voltage of a conductive liquid flowing through a controlled magnetic field. The more liquid flows through or the higher the flow velocity, the higher the induced voltage.
Electromagnetic flow meters are available in various variants. They differ mainly in terms of electrode spacing and the strength of the magnetic field.
Designs
EMF insertion
An EMF insertion is based on a so-called finger with an integrated coil and two electrodes at the end of the finger that come into contact with the liquid. The electric coil generates a magnetic field in the flow path. A conductive liquid flowing through the magnetic field induces an electric current that can be measured as a voltage between two electrodes. The greater the flow rate, the higher the measured voltage. Using a K-factor, which represents the pipe cross-section, the transmitter converts the voltage signal into a flow rate, which can then be displayed or sent via an analogue output in milliamperes or a digital output as pulses.
EMF full bore
An EMF full bore is based on the same measuring principle, but in a different arrangement. Two electric coils are arranged around the outside of the pipe and generate a magnetic field in the entire cross-section of the sensor pipe. Two electrodes in contact with the liquid are mounted opposite each other in the pipe wall and are often fixed in the liner with a seal. The two coils generate a constant and homogeneous alternating magnetic field in the flow cross-section. As with the EMF insertion, the transmitter uses a K-factor, which represents the pipe cross-section, to convert the voltage signal into a flow rate, which can then be displayed or sent via an analogue output milliamps or digital output pulses.
Compared to the EMF insertion, the accuracy and performance of the full bore variant are betterand smaller inlet sections are possible.
Typical areas of application
Electromagnetic flow meters are used for liquids with a certain minimum electrical conductivity, for example for feeding waste water into municipal waterworks. They are also frequently used in food and beverage production, e.g. for dosing syrup.
Advantages of EMFs
+ No moving parts in the measurement tube
+ good accuracy / price ratio
+ Stable measurements
+ Low sensitivity to inlet conditions with full bore EMF
Disadvantages of EMFs
- Certain minimum conductivity of the medium required
- Changes to the conductivity of the media can lead to measurement errors
- Various components in contact with the medium
- Pressure drop
Coriolis
Function principle
The Coriolis flow meter is based on the Coriolis force. This causes an object moving in a straight line to experience an apparent deviation from the perspective of a rotating system.
One or more pipes are installed in a Coriolis flow meter, which are set in vibrationby an external energy source. These vibrations are controlled and permanent. Sensors installed along the pipes measure the vibration movements of the pipes. In the absence of a flow, the vibration between the inlet and outlet is symmetric. The signal detected at the inlet is in phase with the signal at the outlet. A flow changes the twist of the pipes, and there is a phase shift between the signal at the inlet and the signal at the outlet. The more mass flows through, the greater the phase shift, which is proportional to the mass flow rate of the medium flowing through the pipes. This measuring principle can be used for liquids, gases and vapour.
Typical areas of application
Coriolis flow meters are used in various industries due to their high accuracy. For example, for measuring oils in food production, for measuring chemicals, for billing petrochemicals (custody transfer) and for dosing and mixing in the pharmaceutical industry.
Advantages of Coriolis flow meters
+ High accuracy
+ High stability
+ Measurement of conductive and non-conductive liquids
Disadvantages of Coriolis flow meters
- Large and heavy
- Complicated installation and set-up
- Pressure loss due to narrow pipe
- Sensitive to vibrations
- Zero-point calibration required
Surface acoustic waves (SAW)
Function principle
The abbreviation “SAW” stands for “surface acoustic waves”.It therefore means waves that propagate along surfaces. The type of propagation is comparable to the wave propagation during seismic activities such as an earthquake.
Bürkert has a patent on the use of SAW technology for flow measurement. This technology forms the basis of the FLOWave flow meter.
In the FLOWave flow meter, the surface acoustic waves are generated by so-called interdigital transducers (IDT). Starting from an initial centre of excitation, a wave front propagates along the pipe surface and through the medium.The wave front is also “reflected” by the measurement tube and therefore travels several times through the medium in the tube.
You can find a detailed explanation of how it works on our glossary page on the SAW method.
Typical areas of application
SAW technology is used particularly in hygiene-sensitive environments, in the pharmaceutical & biotech and food & beverage sectors. It is suitable for WFI loops in the pharmaceutical industry and monitoring of cleaning water in F&B container production.
Advantages of SAW flow meters
+ Contactless & hygienic measurement
+ No moving parts – 100% wear-free
+ No narrowing, meaning no pressure drop/loss in pressure
+ Compact and lightweight
+ Measurement of other parameters
Disadvantages of SAW flow meters
- Sensitive to gas bubbles
Find the best measurement technology for an application
Choosing the right flow measurement technology can be crucial for the precision and efficiency of your processes. But which method is best suited to your specific requirements?
We will guide you on the way to the right flow meter and show you step by step what you should consider when making your choice.
Here, you can request our guide and checkliston flow measurement of liquids.The guide takes you through a 4-step process, helping you select an appropriate measurement technology.
