Flow measurement refers to the determination of the quantity of a medium (liquid, gas or vapor) flowing through a pipe or system. It can be measured in units of volume or mass and is of great importance in numerous industrial applications such as the chemical, food, automotive and process industries.
How is flow measurement defined?
Flow measurement refers to the determination of the quantity of a medium (liquid, gas or vapor) flowing through a pipe or system. The aim of this measurement is to obtain an overview of the flow rate and to use it in industrial processes.
Flow measurement differs from pure counting methods in that it records the amount of medium that flows per unit of time, for example in liters per second (l/s), liters per minute (l/min) or cubic meters per hour (m³/h). A meter, on the other hand, only records a total quantity within any given period of time and is calibrated to a quantity measure such as liters. While the display of a meter runs continuously in the same direction, the display of a flow meter oscillates on a scale depending on the current flow rate. In many industrial applications, knowledge of the instantaneous flow rate is crucial for the control and further processing of processes.
Why is flow measurement important?
Flow measurement is essential in many industrial applications as it ensures the efficiency, safety and quality of processes.
- In the chemical and petrochemical industry, it is used for precise dosing in production processes and for monitoring.
- The food industry uses them to accurately measure liquids in production lines.
- In the pharmaceutical and medical technology industries, flow measurement is crucial for controlling gases and liquids in devices and processes.
- In the energy and power plant sector, it monitors the flow of steam, natural gas or water.
- In the automotive industry, the flow of fuel, oil and cooling water is controlled.
By accurately recording the flow rate, companies can optimize their processes, reduce energy consumption and ensure product quality. Monitoring the flow makes it possible to detect leaks at an early stage and thus ensure the safety of systems and processes.
Basics of flow measurement
Flow measurement is an integral part of many industrial processes and is based on various physical principles and measurement methods. In essence, the aim is to determine the quantity of a medium that flows through a closed system or an open section per unit of time. Either the volume flow or the mass flow can be measured.
Physical principles of flow measurement :
Flow measurement is based on various physical principles that make it possible to quantify the volume of a flowing medium. These principles use different physical effects that are related to the flow rate of the medium. The choice of the appropriate physical principle is decisive for the function and accuracy of the respective flowmeter and depends heavily on the properties of the medium to be measured, the operating conditions and the requirements of the application.
Volume flow measurement
The volume flow measurement records the volume of the medium flowing through per unit of time, typically in units such as liters per second (l/s) or cubic meters per hour (m³/h). This measurement method is particularly relevant if the density of the medium is approximately constant, as is the case with water, for example.
Various flow meters use the principle of volumetric flow measurement. These includes
- Impeller flow meters: These devices convert the flow movement of the fluid into a mechanical rotary movement of the impeller. The rotary movement is recorded and converted into a frequency signal proportional to the flow rate. They are well suited for neutral, slightly aggressive and low-solids liquids.
- Measuring turbines: Similar to impeller flow meters, measuring turbines use the flow energy to set an impeller in rotation. The speed is proportional to the flow rate. Due to the axial flow and better mounting of the turbine wheel, larger measuring range spreads can be achieved.
- Oval wheel meters: These displacement meters work on the principle that two intermeshing oval wheels in a measuring chamber are set in rotation by the flowing medium. Each rotation transports a known volume, allowing the flow rate to be determined precisely. They are also suitable for highly viscous liquids.
- Ultrasonic flow meters: These devices measure the flow rate by recording the transit time of ultrasonic waves in the medium. The time difference between signals sent with and against the direction of flow is proportional to the flow velocity. They are suitable for conductive and non-conductive liquids.
- Electromagnetic flow meters (EMF): These measuring devices use Faraday’s law of electromagnetic induction. When a conductive liquid moves through a magnetic field, a voltage is induced that is proportional to the flow velocity.
Mass flow measurement
Mass flow measurement records the mass of the medium flowing through per unit of time, typically in units such as kilograms per second (kg/s) or tons per hour (t/h). This measurement method is particularly relevant when the density of the medium is not constant and changes with changes in pressure or temperature, for example, as is often the case with gases. In such cases, measuring the mass flow provides a more accurate indication of the transported quantity of the medium than volume flow measurement. Mass flow measurement is therefore more suitable for precise applications, particularly in the chemical and gas industries.
Various flowmeters use the principle of mass flow measurement. These include
- Coriolis flow meters: These measuring devices measure the mass flow directly by using the Coriolis effect. An external energy source causes one or more measuring tubes to vibrate. The flow of the medium through the tubes causes a phase shift in the oscillation, which is proportional to the mass flow. Coriolis flow meters are independent of changes in pressure, temperature or density of the medium and are suitable for liquids, gases and steam. Due to their high accuracy, they are used in various industries, for example for measuring oils in food production or chemicals.
- Thermal mass flow meters: These devices determine the mass flow by determining the heat transfer to a flowing gas. A heated element in the flow path transfers heat to the passing medium. The energy required to maintain a constant temperature difference is a measure of the mass flow. Thermal mass flow meters are often used to measure gases such as air or natural gas.
What is the difference between volumetric flow and mass flow measurement?
Der grundlegende Unterschied zwischen der Volumenstrom- und der Massenstrom-Durchflussmessung lThe fundamental difference between volumetric flow measurement and mass flow measurement lies in the quantity measured. Volume flow measurement measures the volume of the medium that flows through a system per unit of time, for example in cubic meters per hour (m³/h). Mass flow measurement, on the other hand, directly measures the mass of the medium that is transported per unit of time, for example in kilograms per hour (kg/h). The choice between the two measurement types depends on the specific requirements of the application and the properties of the medium to be measured.
Measured variables and units :
In flow measurement, various measured variables are used to quantify the flow of a medium. The most common are the volume flow and the mass flow.
The typical units for the volume flow are
- Cubic meters per hour (m³/h)
- Liters per second (l/s)
- Liters per minute (l/min)
- Other units such as cubic feet per minute (CFM) or gallons per minute (GPM) can also be used depending on the application.
The usual units for mass flow are
- Kilogram per second (kg/s)
- Kilograms per hour (kg/h)
- Tons per hour (t/h)
- Other units such as pounds per hour (lb/h) may also be used here.
Measuring principles and flowmeters
Various common measuring principles and the flowmeters based on them are presented below:
Differential pressure principle
The differential pressure principle in flow measurement is based on the generation of a pressure drop via a flow resistance. This flow resistance can be an orifice, a Venturi nozzle or a measuring nozzle, for example. According to Bernoulli’s equation, if the cross-section narrows, the flow velocity increases and the static pressure decreases. The resulting pressure difference between two measuring points is proportional to the flow velocity and can be used to calculate the volume flow.
This principle is mainly used in industrial applications as it is robust, reliable and suitable for high flow rates.
Measuring devices based on the differential pressure principle include:
- Pitot tube
- Aperture measurement
- Variable area flow meter
- Orifice plate
- Dynamic pressure probe
- Dynamic pressure probe (Annubar principle)
Indirect measuring devices
Indirect measurement in flow measurement refers to methods in which the flow rate is not measured directly via the volume or mass of the flowing medium, but via a physical variable that is related to the flow rate, such as the flow velocity.
Measuring devices that work according to this principle include:
- Turbine flow meter
- Impeller meter
- Ultrasonic flow meter
Displacement meter
A positive displacement meter is a flow meter that divides the flowing medium into discrete volume units and measures these mechanically. The medium is displaced by moving elements such as gear, oval gear, piston or screw rotors, which move in proportion to the volume flowing through. The number of rotation cycles or piston movements serves as a measure of the volume flowed through. Displacement meters are suitable for both liquids and gases.
Measuring devices based on the principle of displacement include:
- Oval gear meter
- Gear counter
- Piston counter
Vortex flow meter
A vortex flow meter is a flow measuring device that uses the Kármán vortex street principle to determine the flow velocity of a medium. A baffle is introduced into the flow, behind which periodic vortices form alternately on both sides. The frequency of this vortex shedding is proportional to the flow velocity and therefore to the volume flow. The number of vortices is recorded using a sensor, for example a piezo element, which reacts to changes in pressure. The transmitter of the vortex flow meter then converts the detected pulses into a flow rate using a K-factor, which is mainly determined by the pipe cross-section and the type of medium (liquid, gas or steam).
Measuring devices based on the principle of displacement include:
- Vortex flow meter
- Also abbreviated as VDM (vortex mass flow meter)
Speed measuring devices
Velocity-measuring flow meters are measuring devices that determine the flow rate of a medium by directly measuring the flow velocity. The measurement is based on physical principles such as the ultrasonic transit time difference, the Doppler effect, magnetic induction or mechanical rotors (as with turbine meters).
The measuring devices in this category include
- Ultrasonic flow meter
- Magnetic inductive flow meter:
- Turbine flow meter:
- Impeller meter
Mass flow meters
Mass flow meters are flow meters that measure the mass flow of a medium directly, regardless of pressure, temperature or density changes. They measure the actual mass per unit of time (e.g. kg/s or t/h) and are particularly accurate for applications in which the weight of the medium is decisive, such as in the chemical and food industries.
Measuring devices:
- Coriolis mass flow meter
- Thermal mass flow meters
Media and applications of flow measurement
In the following paragraphs, we will take a closer look at which media can be measured using flow measurement and in which areas of application this technology is used.
Which media are measured during flow measurement?
Liquids, gases, steam and multiphase media can be measured in flow measurement, whereby the aggregate state and viscosity play a decisive role.
- Low-viscosity liquids (e.g. water, fuels) are often measured with magnetic-inductive, ultrasonic or turbine flow meters.
- Highly viscous liquids (e.g. oils, honey) can be measured using positive displacement meters or Coriolis mass flow meters. Oval gear flow meters are also suitable for highly viscous liquids such as adhesives, oils or honey.
- Gases (e.g. air, natural gas) are often determined using thermal mass flow meters, vortex or differential pressure flow meters.
- Steam requires vortex or differential pressure methods, as pressure and temperature changes can vary greatly. Vortex flow meters are often used to measure steam flow, for example in power plants.
- Multiphase media (e.g. emulsions, slurries) require specialized Coriolis or ultrasonic flow meters with multiphase compensation.
The choice of measuring method depends on the physical properties of the medium, in particular pressure, temperature, viscosity and flow behavior. For example, magnetic inductive flow meters require a certain minimum electrical conductivity of the medium. Changes in conductivity can lead to measurement errors. Non-conductive liquids, on the other hand, can be measured with ultrasonic flow meters.
What applications are there for flow measurement?
Flow measurement has become an integral part of numerous applications in industry and everyday life. It enables the precise monitoring and control of liquid and gas flows in a wide variety of processes.
- In the chemical and pharmaceutical industries for precise dosing of liquids, gases and steam in production processes.
- In the energy and power plant sector for measuring steam, natural gas or water in power plants and pipelines.
- In the automotive industry for monitoring fuel and oil flows in engines and test systems.
- In the water and wastewater industry for monitoring the water flow in wastewater treatment plants and supply systems.
- In semiconductor production to control high-purity gases for microchip production.
Selecting the right flowmeter
Measuring range
The measuring range of a flow meter specifies the range of flow values within which the device measures according to specification and with the specified accuracy. The correct dimensioning of the measuring range is crucial for reliable measurement results. A measuring device with a measuring range that is too small may not be able to measure the maximum flow rate or may be overloaded. If, on the other hand, the measuring range is too large, the measuring accuracy may suffer in the lower range of the measuring range.
The ratio between the maximum and minimum measuring range in which the flow meter still works precisely is referred to as the turndown ratio or dynamic range. A high turndown ratio is advantageous as it enables the device to reliably measure both small and large flows. For example, a turndown ratio of 10:1 means that a device can reliably measure flows between 1 and 10 l/min.
When selecting a flow meter, it is therefore important to know exactly the expected minimum and maximum flow rate of the application and to select a device with a suitable measuring range and, if necessary, a suitable turndown ratio.
Medium
The properties of the medium to be measured are a decisive factor when selecting the right flowmeter. Different media have different physical and chemical properties, which can influence the functionality and accuracy of different measuring principles.
- For low-viscosity liquids such as water or fuels, electromagnetic, ultrasonic or turbine flow meters, for example, are well suited. Impeller flow meters are also suitable for neutral, slightly aggressive and low-solids liquids.
- Highly viscous liquids such as oils or honey can be measured effectively with positive displacement meters or Coriolis mass flow meters. Oval gear flow meters are also suitable for highly viscous media such as adhesives, oils or honey.
- When measuring gases such as air or natural gas, thermal mass flow meters, vortex or differential pressure flow meters are often used. If the density of the gas is influenced by pressure or temperature, it is advisable to measure the mass flow.
- Vortex or differential pressure methods have proven themselves for steam, as they can handle the variable pressure and temperature changes. Vortex flow meters are often used to measure steam flow in power plants.
- Multiphase media such as emulsions or slurries require specialized Coriolis or ultrasonic flow meters with multiphase compensation.
Other properties of the medium also play a role:
- Electrical conductivity: Electromagnetic flow meters (EMF) require a certain minimum conductivity of the medium. Changes in conductivity can lead to measurement errors. Ultrasonic flow meters are a suitable alternative for non-conductive liquids.
- Solid particles or air bubbles: Media with solid particles can impair certain measuring methods. Displacement meters or special Coriolis or ultrasonic flow meters, for example, could be used for such media. Air bubbles should be avoided with oval gear flow meters. SAW flow meters are sensitive to gas bubbles.
- Chemical composition: The chemical composition of the medium can influence the material resistance of the flow meter. It is important to choose materials that are compatible with the measured medium to avoid corrosion or other damage.
Precise analysis of the media properties is therefore essential in order to select a flowmeter that is suitable for the respective application and provides reliable measurement results.
Application
The specific application is another decisive criterion when selecting a flowmeter. In addition to the measuring range and the medium, the installation conditions, the required temperature range and the type of connection must also be taken into account.
- The installation size of the flow meter must match the available space in the system. Compact designs can be advantageous in confined spaces. There are both compact devices, in which the sensor and transmitter are physically connected, and separate versions, in which the sensor and evaluation electronics can be installed separately.
- The temperature range of the measuring device must match the temperature of the medium to be measured and the environment. Some measuring principles and materials are better suited to extreme temperatures than others.
- The type of connection (e.g. threaded, flanged or clamp connection) must be compatible with the existing connections in the pipe.
- Specific application requirements can also influence the choice of meter. In potentially explosive areas, for example, ATEX-certified flow meters must be used. For applications in which hygienic aspects play an important role, such as in the pharmaceutical and food industries, measuring methods with non-contact measurement such as SAW flow meters are particularly suitable.
- The need for inlet and outlet sections can also be a selection criterion. While some flow meters, such as oval gear flow meters, do not require inlet and outlet sections, others, such as impeller flow meters, require certain minimum lengths of straight piping before and after the measuring point to ensure accurate measurement results.
A detailed analysis of the application requirements helps to select a flowmeter that is not only technically suitable, but can also be optimally integrated into the existing system.
Measuring accuracy
Die Messgenauigkeit eines Durchflussmessers ist ein entscheidendes Kriterium bei der Auswahl, da sie angibt, wie nah der vom Gerät gemessene Wert an dem tatsächlichen Durchfluss liegt. Die erforderliche Messgenauigkeit hängt stark von der jeweiligen Anwendung ab. In einigen Prozessen, wie beispielsweise in der Abrechnung oder bei der präzisen Dosierung von teuren oder kritischen Stoffen, sind sehr hohe Genauigkeiten unerlässlich. In anderen Anwendungen, wie der einfachen Überwachung von Durchflüssen, kann eine geThe measuring accuracy of a flow meter is a decisive criterion in the selection process, as it indicates how close the value measured by the device is to the actual flow rate. The required measuring accuracy depends heavily on the respective application. In some processes, such as billing or the precise dosing of expensive or critical substances, very high accuracies are essential. In other applications, such as the simple monitoring of flow rates, a lower accuracy may be sufficient.
The measurement accuracy is defined by various key figures:
- Measurement deviation (accuracy class / error limit): Indicates the maximum deviation between the measured and the actual flow value, often as a percentage of the measured value (% of the actual value) or percentage of the full scale value (% of the full scale value).
- Repeatability: Describes how consistently a flow meter delivers the same value for repeated measurements under the same conditions.
- Linearity: Shows how precisely the measured values correspond to an ideal measuring line over the entire measuring range.
- Hysteresis: The difference between the measured value with increasing and decreasing flow.
- Long-term stability (drift): Describes the change in measurement accuracy over a long period of time.
When selecting a flow meter, it is important to define the required accuracy for the specific application and select a device whose key figures meet these requirements. Higher accuracy requirements usually lead to higher costs for the meter. Therefore, an appropriate compromise between accuracy and cost should be found.
Standards and directives
Compliance with standards and directives is an important criterion when selecting a flow meter, as they can ensure safety standards, compatibility and the comparability of measurement results. Depending on the operating environment and the specific requirements of the application, flow meters must comply with certain standards.
- For protection against the ingress of dust and water, flow meters must meet the corresponding IP protection classes (e.g. IP67).
- The ATEX Directive (2014/34/EU) is relevant in potentially explosive atmospheres, such as those that can occur in chemical plants. This directive classifies devices for different zones (0, 1 or 2) depending on the frequency of occurrence of explosive atmospheres. ATEX-certified flowmeters bear special markings that define their degree of explosion protection (e.g. “Ex db IIC T6 Gb”).
- Industry standards such as DIN EN ISO 5167 (differential pressure measurement), OIML R117 (accuracy requirements for trade) and MID 2014/32/EU (measuring devices subject to mandatory calibration) regulate specific technical requirements for flow meters.
It is therefore advisable to find out about the relevant standards and directives for the planned application before selecting a flow meter and to ensure that the selected device meets these requirements.
Sources of error in flow measurement
In practice, flow measurement can be influenced by various factors that can lead to inaccuracies in the measurement results. Understanding these sources of error is important to ensure reliable measurements and to be able to take suitable countermeasures if necessary. The most common sources of error include:
- Incorrect calibration: An inaccurate or outdated calibration of the flow meter leads directly to incorrect measured values.
- Unsuitable installation conditions: Incorrect installation positions or inlet and outlet sections that are too short can negatively affect the flow pattern and impair the measuring accuracy. Some flow meters require specific inlet and outlet sections in order to function correctly.
- Temperature and pressure fluctuations: Particularly when measuring gases, changes in temperature and pressure can influence the density of the medium and thus distort the measuring accuracy of volumetric flow meters.
- Media properties: Air bubbles, particles or deposits in the measurement medium can interfere with the functioning of the sensors. Excessive viscosity or a lack of electrical conductivity in the medium can make certain measuring methods unsuitable. The chemical composition of the medium can also attack the materials of the measuring device and lead to errors.
- Vibrations and electromagnetic fields: External vibrations can affect Coriolis and electromagnetic flow meters in particular. Electromagnetic fields can also falsify measured values.
- Sensor ageing and signal interference: Over time, the accuracy of the sensor can decrease (drift). Signal interference in the measured value transmission can also lead to incorrect readings.
These sources of error can be minimized by selecting the correct measuring method, precise installation, regular calibration and maintenance.
Expert advice on flow measurement
Selecting the right flowmeter for a specific application can be a complex task, as a large number of factors need to be taken into account. In addition to technical specifications such as measuring range, medium, application and measuring accuracy, standards, guidelines and possible sources of error also play a decisive role.
To ensure that you find the optimum solution for your flow measurement requirements, it is advisable to draw on the expertise and experience of specialists. Competent advice can help you to better understand the various measuring principles and devices and weigh up the pros and cons for your individual situation.
Comprehensive advice takes the following aspects into account:
- Analysis of your specific requirements: Detailed recording of the process parameters, the medium to be measured, the ambient conditions and the desired measuring accuracy.
- Recommendation of suitable measuring principles and devices: Based on the analysis, the appropriate technologies and products are presented to you.
- Instructions for correct installation and commissioning: Avoid installation errors and ensure optimum performance of the meter.
- Information on calibration, maintenance and troubleshooting: Ensuring permanently accurate and reliable measurement.
Take the opportunity to get advice from experienced experts at DDM Sensors. DDM Sensors offers application-specific solutions for measuring pressure and flow. Contact us for a comprehensive consultation.
