Variable Area Flow Meter

A Variable Area Flow Meter is a flow measuring instrument used to monitor the flow rate of liquids and gases. It operates using a float that moves within the measuring section of the meter. As the flow rate increases, the float rises and the available flow area between the float and the metering ring becomes larger. The float position is directly related to the flow rate and can be displayed locally or transmitted electronically, depending on the meter design.

Variable area flow meters are valued for their simple measuring principle, robust construction, high reliability, and the fact that the basic measurement does not require an external power supply.

Unlike conventional rotameters that use a tapered tube and often rely on a float guide rod, Heinrichs BGN flow meters use a straight measuring tube in combination with a metering ring and a specially designed float. Float stabilization is achieved by guide stars integrated into the float design. This robust measuring principle reduces the risk of blockage caused by deposits and makes the meter less sensitive to contamination.

A Variable Area Flow Meter measures flow using a float that moves vertically within the measuring section of the meter. As the flow rate increases, the hydrodynamic forces acting on the float increase, causing it to rise. In simple terms, the flowing fluid pushes the float upward. This movement increases the flow area between the float and the metering ring until a stable equilibrium position is reached.

The position of the float is directly related to the flow rate and is used to indicate or transmit the measured value. Because the available flow area changes with the float position, the meter can accurately measure a wide range of flow rates using a simple and reliable mechanical measuring principle.

The Heinrichs BGN Variable Area Flow Meter combines a robust mechanical measuring principle with modern monitoring and communication options. Key features include:

• Direct Local Flow Indication: The flow rate is indicated directly by the float position, allowing easy local monitoring without requiring an external power supply for the basic measurement.
• Straight Measuring Tube and Metering Ring Design: Unlike conventional rotameters with a tapered tube, the BGN uses a straight measuring tube in combination with a metering ring. This design provides reliable operation and contributes to reduced sensitivity to contamination and deposits.
• Float Stabilization with Guide Stars: The float is stabilized by integrated guide stars instead of a float guide rod. This design minimizes the risk of blockage caused by deposits and supports reliable long-term operation.
• NAMUR Inductive Limit Switches: The BGN can be equipped with up to two NAMUR inductive limit switches for flow monitoring and alarm functions. SIL-capable versions are available for applications with functional safety requirements.
• Reed Switches: Up to two reed switches can be installed for simple switching and monitoring applications.
• Analog and Digital Communication: Depending on the configuration, the BGN can be equipped with:
  1. 4-20 mA output signals
  2. HART® communication
  3. FOUNDATION™ Fieldbus communication
  4. This allows integration into PLC, DCS, and process automation systems.
• Totalizer Function: The electronic transmitter can be equipped with a totalizer function that records the accumulated flow volume over time. This allows operators to monitor both instantaneous flow rate and total consumption.
• Optional Digital Display: An optional digital display provides local indication of the measured flow rate and can also display accumulated totalizer values.
• Low Maintenance Design: The simple and robust measuring principle minimizes maintenance requirements and supports long service life in industrial applications.

The Heinrichs BGN Variable Area Flow Meter can be used to measure a wide range of liquids, gases, and steam in industrial applications. The suitability of the meter depends on factors such as the process medium, operating pressure, temperature, viscosity, and material compatibility.

• Liquids: The BGN is suitable for measuring a wide variety of liquids, including water, process water, chemicals, acids, alkalis, solvents, and many other industrial fluids.
• Viscous Liquids: Viscous liquids can also be measured, provided that the meter is properly selected and calibrated for the specific fluid properties and operating conditions.
• Gases: The BGN can be used for a wide range of industrial gases. The meter can be configured for different gas densities, pressures, and temperatures to ensure reliable measurement performance.
• Hydrogen: The BGN is also suitable for hydrogen applications. High-pressure versions are available for demanding operating conditions in hydrogen production, storage, and process systems.
• Steam: Selected BGN configurations are suitable for steam measurement. The suitability depends on the operating pressure, temperature, and material selection of the flow meter.

Material Options for Challenging Media

To ensure compatibility with demanding process media, the BGN is available in a variety of material options. In addition to standard stainless steel construction, versions with Hastelloy® wetted parts, PTFE lining for aggressive media, seawater-resistant stainless steels, and Monel are available. Additional material options can be evaluated for specific applications.

To ensure reliable measurement performance and long service life, the flow meter should always be selected according to the specific process conditions and medium properties.

Variable area flow meters (rotameters) are widely used for flow measurement because they combine a simple operating principle with reliable performance and low operating costs. Modern variable area flow meters such as the Heinrichs BGN offer several advantages for industrial applications:

• Simple and Proven Measuring Principle: Variable area flow meters operate using a float that moves in response to the flow rate. This simple mechanical principle has been proven in industrial applications for decades and provides reliable flow measurement without complex sensor technology.
• Direct Local Flow Indication: The flow rate can be read directly on the instrument, allowing operators to monitor the process at a glance. Local indication remains available even when no control system or power supply is present.
• No External Power Supply Required
  The basic flow measurement and local indication operate without an external power supply. This makes the instrument suitable for remote locations and applications where maximum reliability is required.
• Easy Installation: In many applications, variable area flow meters do not require straight inlet or outlet pipe runs, simplifying installation and reducing space requirements. This can lower installation costs compared to other flow measurement technologies.
• Robust Construction: The Heinrichs BGN features an all-metal design with a straight measuring tube, metering ring, and guide-star stabilized float. This construction is designed for demanding industrial environments and harsh operating conditions.
• Reduced Sensitivity to Contamination: Unlike conventional guide-rod designs, the BGN uses integrated guide stars for float stabilization. This design reduces the risk of blockage caused by deposits and makes the instrument less susceptible to contamination.
• Wide Application Range: Variable area flow meters can be used for liquids, gases, and selected steam applications. They are suitable for a broad range of industries, including chemical processing, water treatment, energy, and general process industries.
• Flexible Material Options: To match demanding process conditions, the BGN is available in a variety of materials, including stainless steel, Hastelloy®, PTFE-lined versions for aggressive media, seawater-resistant materials, and other special material options upon request.
• Advanced Monitoring and Communication: The BGN can be equipped with NAMUR inductive limit switches, reed switches, 4–20 mA transmitters, HART® communication, FOUNDATION™ Fieldbus, and totalizer functions. This allows seamless integration into modern automation systems.
• Suitable for Hazardous Areas and Safety-Related Applications: The BGN is available with approvals for hazardous areas and can be equipped with safety-oriented NAMUR switching outputs. Depending on the selected configuration, the instrument is suitable for operation in safety-related measuring circuits in accordance with the applicable conformity documentation.
• Low Maintenance Requirements: The simple mechanical measuring principle and robust construction minimize maintenance requirements and support a long service life, helping to reduce operating costs over the lifetime of the instrument.

Variable area flow meters are a reliable and versatile flow measurement technology. However, like any measuring principle, they have certain application limits that should be considered during instrument selection

• Installation Orientation: Most variable area flow meters are designed for vertical installation with flow from bottom to top, as the measuring principle relies on the balance between flow forces and the weight of the float. For applications where installation orientation is limited, spring-loaded designs such as the Heinrichs BGF can provide greater flexibility and can be installed independently of flow direction.
• Dependence on Process Conditions: Measurement performance depends on factors such as density, viscosity, pressure, and temperature. Significant changes in these parameters may require recalculation or recalibration of the flow meter. This is particularly important for gas applications, where pressure and density variations can influence the indicated flow rate.
• Viscous Media: Viscous liquids can be measured successfully with variable area flow meters when the instrument is properly selected and calibrated for the specific application. Fluid properties such as viscosity, density, temperature, and operating conditions should be considered during sizing and calibration to achieve reliable results.
• Extremely High Accuracy Applications: Variable area flow meters provide reliable and repeatable measurements for a wide range of industrial applications. However, for applications requiring extremely high measurement accuracy, mass flow measurement, or advanced density measurement capabilities, other flow measurement technologies may be more suitable.For such applications, Coriolis mass flow meters such as the Heinrichs TMU can be considered.
• Media with Heavy Deposits or Solid Particles: Heavy deposits, scaling, or solid particles can affect the performance of any variable area flow meter and should be considered during instrument selection and maintenance planning. The Heinrichs BGN minimizes this effect through its straight measuring tube, metering ring design, and guide-star stabilized float, making it less susceptible to contamination than conventional guide-rod designs.

For applications involving significant contamination, sludge, or high solids content, other flow measurement technologies such as the Heinrichs TSK target flow meter may be more suitable.

Understanding these limitations helps ensure that the selected flow meter delivers reliable performance, long service life, and the best fit for the application

Variable area flow meters provide reliable and repeatable flow measurement for a wide range of industrial applications. The achievable accuracy depends on factors such as the meter design, calibration conditions, process medium, operating pressure, temperature, and viscosity.

Accuracy of the Heinrichs BGN

Under reference conditions, the Heinrichs BGN provides the following measurement accuracy:

• BGN-S and BGN-H
  Water: ±1.6% of full-scale value
  Gas: ±1.8% of full-scale value
• BGN-P
  Water: ±2.0% of full-scale value
  Gas: ±2.2% of full-scale value

When equipped with the optional ES transmitter, an additional measurement uncertainty of only ±0.2% may apply.
The stated accuracy values are determined in accordance with recognized industry standards. For gas measurements, the Heinrichs BGN complies with the requirements of VDI/VDE 3513-2 (qG 50%), providing a transparent and standardized basis for accuracy evaluation.

Accuracy and Repeatability

• Accuracy describes how closely the measured value corresponds to the actual flow rate.
• Repeatability describes how consistently the same measurement result can be achieved under identical operating conditions. In many industrial applications, repeatability is just as important as absolute accuracy.
• The Heinrichs BGN offers a repeatability of ±0.5% of full-scale value, supporting stable and reliable long-term measurements.
• Application-Specific Calibration: Variable area flow meters are calibrated according to the specific application and operating conditions. Parameters such as density, viscosity, pressure, and temperature are taken into account during sizing and calibration.

This ensures reliable measurement performance for liquids, gases, and selected steam applications. For this reason, variable area flow meters continue to be widely used in industrial applications where reliable, repeatable, and robust flow measurement is required.

Variable area flow meters are used in a wide range of industries wherever reliable flow measurement, local indication, and process monitoring are required. Their robust measuring principle and versatility make them suitable for both liquid and gas applications.

• Chemical and Process Industry: Variable area flow meters are widely used for measuring chemicals, acids, alkalis, solvents, and other process fluids. With suitable material selections, they can also be used in corrosive and demanding applications.
• Water and Wastewater Treatment: Typical applications include water distribution systems, cooling water circuits, filtration systems, and wastewater treatment processes.
• Energy and Utility Systems: Variable area flow meters are commonly used in steam systems, boiler applications, cooling circuits, and other utility services where reliable flow monitoring is required.
• Industrial Gas Applications: They are frequently used for measuring compressed air, nitrogen, oxygen, hydrogen, inert gases, and other industrial gases.
• Mechanical Engineering and OEM Equipment: Machine builders and system integrators use variable area flow meters for process control, equipment monitoring, and flow supervision in a wide range of industrial systems.
• Process Monitoring and Automation: In addition to local flow indication, modern variable area flow meters can be equipped with limit switches, transmitters, and digital communication interfaces. This allows integration into PLC, DCS, and process automation systems for monitoring, alarm, and control functions.

The Heinrichs BGN is designed for use in demanding industrial environments and can be configured for liquids, gases, hydrogen, and selected steam applications with a wide range of material, communication, and monitoring options.

Not always. The installation orientation of a variable area flow meter depends on its measuring principle and design.

Standard Variable Area Flow Meters

• Most variable area flow meters operate using the balance between the flow forces acting on the float and the weight of the float itself. For this reason, they are typically installed vertically with flow from bottom to top.
• Installing a gravity-operated flow meter in a different orientation can affect the float position and lead to incorrect measurement results.

Flexible Installation with the Heinrichs BGF

• While most variable area flow meters require vertical installation, Heinrichs offers the BGF series for applications where installation flexibility is required.
• In the Heinrichs BGF, the float movement is controlled by a spring rather than gravity. This allows the flow meter to operate independently of mounting orientation and flow direction.
• As a result, the BGF can be installed in horizontal pipelines as well as in vertical pipelines with upward or downward flow, providing maximum flexibility for machine builders and plant designers.

Selecting the Right Design

• For standard vertical installations, gravity-operated flow meters such as the Heinrichs BGN provide a simple and reliable solution.
• Where installation constraints prevent vertical mounting or where flow direction may vary, the Heinrichs BGF offers a flexible alternative without the orientation restrictions associated with conventional gravity-operated designs.
• Selecting the correct flow meter design helps ensure reliable measurement performance and long-term operational stability.

Variable area flow meters are known for their simple and robust measuring principle and generally require very little maintenance. The required maintenance depends on the operating conditions, process medium, and application.

• Regular Visual Inspection: The flow meter should be inspected periodically for signs of mechanical damage, corrosion, leakage, or excessive contamination. Inspection intervals should be determined according to the operating conditions and plant maintenance procedures.
• Cleaning When Required: In applications involving deposits, scaling, crystallization, or contaminated media, periodic cleaning may be necessary to maintain reliable operation. The required cleaning frequency depends on the process conditions and medium properties.
• Functional Testing of Accessories: If the flow meter is equipped with limit switches, transmitters, digital displays, or communication interfaces, these components should be checked periodically to ensure proper operation.
• No Routine Wear Parts Replacement: The measuring principle does not require lubrication or routine replacement of mechanical wear parts under normal operating conditions, contributing to low maintenance costs over the lifetime of the instrument.
• Low-Maintenance Design of the Heinrichs BGN: The Heinrichs BGN is designed for long-term operation with minimal maintenance requirements. Its straight measuring tube, metering ring design, and guide-star stabilized float eliminate the need for a float guide rod and reduce the likelihood of performance issues caused by contamination and deposits compared to conventional designs.
• Long-Term Reliability: With proper installation and operation, variable area flow meters can provide reliable service over many years with minimal maintenance effort. For critical applications, periodic performance verification may be included as part of the plant maintenance strategy.

Yes. Variable area flow meters can be used in hazardous areas when they are designed, manufactured, and certified according to the applicable explosion protection requirements.

Why Are Special Certifications Required?

Hazardous areas may contain flammable gases, vapors, mists, or combustible dusts. In these environments, equipment must not become a source of ignition under normal operating conditions or foreseeable fault conditions. Special certifications verify that the flow meter has been assessed and approved for safe operation in the specified hazardous area classification.

Mechanical and Electrical Equipment Requirements

Explosion protection requirements do not apply only to electrical components. Depending on the application and regional regulations, mechanical equipment may also require certification for use in hazardous areas. This means that both the mechanical flow meter and any associated electrical components, such as NAMUR limit switches, reed switches, transmitters, or displays, may require appropriate hazardous area approvals.

Hazardous Area Solutions from Heinrichs

The Heinrichs BGN is available in versions approved for hazardous area applications. Depending on the project requirements and installation location, certifications may include:

• ATEX
• IECEx
• KCS
• NEPSI
• PESO

Appropriate certified switching devices, transmitters, and accessories are also available for use in hazardous environments.

Selecting the Correct Certification

The required certification depends on factors such as the installation location, applicable regulations, hazardous area classification, and process conditions. Therefore, hazardous area requirements should always be considered during the selection and specification of the flow meter. Using the correct certified equipment helps ensure safe operation, regulatory compliance, and reliable long-term performance in hazardous environments

Variable area flow meters measure volumetric flow. The measuring principle is based on the relationship between the flow rate and the position of the float within the measuring section of the flow meter.

Typical volumetric flow units include:

• l/h
• l/min
• m³/h
• ft³/h

What Is Volumetric Flow?

Volumetric flow describes the volume of fluid passing through the flow meter during a specific period of time.

For many liquid applications, volumetric flow measurement is sufficient because the density of the fluid remains relatively constant under operating conditions.

What Is Mass Flow?

Mass flow describes the actual mass of fluid passing through the flow meter over time.

Typical mass flow units include:

• kg/h
• t/h
• lb/h

Mass flow depends on both the volumetric flow rate and the density of the fluid.

Relationship Between Volume and Mass Flow

Although variable area flow meters measure volumetric flow directly, the corresponding mass flow can be calculated when the fluid density is known. For gas applications, pressure and temperature must also be considered because they influence the density of the gas.

Application-Specific Calibration

Variable area flow meters are calibrated for the specific process conditions of the application. Parameters such as density, viscosity, pressure, and temperature are considered during sizing and calibration to ensure reliable measurement performance.

Understanding the difference between volumetric flow and mass flow helps ensure the correct interpretation of measurement data and the proper selection of measuring units for the application .

Reading a variable area flow meter is straightforward. The measured flow rate is indicated on a calibrated scale and can be read directly from the local indicator.

How the Indication Works

Inside the flow meter, the float moves in response to the flow rate. As the flow increases, the float rises. As the flow decreases, the float moves downward. In the Heinrichs BGN, the float movement is transferred magnetically to an external pointer indicator. This allows the flow value to be displayed without direct mechanical penetration of the pressure boundary.

Reading the Flow Value

The current flow rate is indicated by the pointer position on the calibrated scale. The corresponding flow value can be read directly from the scale of the instrument. Because variable area flow meters are typically calibrated for specific process conditions, the scale should always be interpreted according to the medium and operating conditions for which the instrument was calibrated.

Optional Electronic Indication

Depending on the configuration, the BGN can also be equipped with electronic transmitters and digital displays. These options allow the measured value to be displayed locally, transmitted to control systems, or integrated into process automation applications. Correct interpretation of the scale ensures accurate and reliable flow measurement.

Yes. Variable area flow meters can be designed and calibrated to measure low flow rates accurately and reliably.

1. Suitable for Low Flow Applications: The measuring principle of a variable area flow meter allows stable and repeatable measurement across a wide operating range, including many low flow applications. For example, the Heinrichs BGN is available with measuring ranges starting from as low as 0.5 to 5 l/h for water applications.
2. Proper Meter Selection Is Important: For low flow measurement, the flow meter should be selected according to the specific process requirements. Factors such as flow range, fluid properties, pressure, temperature, and viscosity all influence the selection of the appropriate meter size and configuration.
3. Application-Specific Calibration: Variable area flow meters are typically calibrated for the intended application. This helps ensure reliable performance and accurate indication, even at low flow rates.
4. Typical Low Flow Applications: Low flow measurement is commonly required in applications such as

• Chemical dosing systems
• Sampling systems
• Laboratory and pilot plant installations
• Cooling and lubrication circuits
• Industrial gas supply systems

Solutions for Very Low Flow Rates

• For applications requiring even lower flow rates, Heinrichs offers dedicated low-flow metal tube variable area flow meters such as the KDS and BGK series. Like the BGN, these instruments use the proven variable area measuring principle in a robust metal tube design and are specifically developed for accurate measurement of very small liquid and gas flow rates.
• Depending on the instrument type and configuration, measuring ranges can start as low as 0.1 to 1.0 l/h for water and 3 to 30 Nl/h for air.
• This allows reliable measurement of very small flow rates in applications such as dosing systems, sampling systems, laboratories, pilot plants, and analytical processes.

Reliable Measurement at Low Flow Rates

When correctly sized and calibrated, variable area flow meters provide stable, repeatable, and reliable flow measurement across their specified operating range, including low flow conditions. The key to successful low flow measurement is selecting a flow meter that is properly matched to the application and operating conditions.

Yes. Variable area flow meters are calibrated to ensure that the indicated flow value corresponds accurately to the actual flow rate under the specified operating conditions.

Why Is Calibration Important?

The measuring principle of a variable area flow meter depends on the interaction between the fluid properties and the float position within the measuring section.

Parameters such as density, viscosity, pressure, and temperature can influence the measurement result. For this reason, the flow meter is calibrated according to the intended application and operating conditions.

Factory Calibration

Each Heinrichs variable area flow meter is individually calibrated during manufacturing.

As a standard procedure, calibration is typically performed using water under defined reference conditions. The calibration data is then converted to the specified process medium using recognized calculation methods and standards, taking into account factors such as density, viscosity, pressure, and temperature.

Based on these application-specific calculations, the flow scale is created for the customer's operating conditions.

Is Regular Recalibration Required?

In many industrial applications, routine recalibration is not required as long as the operating conditions remain unchanged and the flow meter is functioning correctly.

However, some industries, quality systems, or regulatory requirements may require periodic verification or recalibration as part of their maintenance procedures.

Verification of Measurement Performance

For critical applications, periodic verification of the measurement performance may be performed to confirm that the flow meter continues to operate within the expected accuracy limits.

Recalibration and Service Support

If required, Heinrichs can verify the calibration of the flow meter or perform a recalibration at our factory.

This service can be beneficial after long operating periods, changes in process conditions, maintenance activities, or whenever confirmation of measurement performance is required.

Please contact our service team for further information.

Long-Term Measurement Reliability

Proper calibration and application-specific sizing are important factors in achieving reliable and accurate flow measurement throughout the service life of the instrument.

Selecting the right flow meter is essential for achieving reliable measurement performance, long service life, and cost-effective operation.

Information Required for Flow Meter Selection

To select the most suitable flow meter, the following application data is typically required:

• Process medium
• Minimum and maximum flow rate
• Operating pressure
• Operating temperature
• Density and viscosity
• Pipe size and process connection
• Hazardous area requirements
• Required signal outputs and communication interfaces

Providing accurate process information helps ensure that the flow meter is correctly sized and configured for the application.

Application-Specific Engineering

Variable area flow meters are selected and calibrated according to the specific operating conditions of the process. Factors such as fluid properties, pressure, temperature, and installation requirements all influence the final instrument configuration.

Choosing the Right Heinrichs Solution

Depending on the application, different flow meter designs may be recommended.

For example:

• BGN for a wide range of industrial liquid, gas, and steam applications
• BGF where installation orientation flexibility is required
• KDS and BGK for very low flow rates
• TSK for applications involving significant contamination or solids
• TMU for direct mass flow measurement

Selecting the most appropriate measuring principle helps ensure optimal performance and long-term reliability.

Expert Support from Heinrichs

The Heinrichs engineering team can assist with flow meter selection, sizing, material compatibility, hazardous area requirements, and application-specific recommendations.

If you are unsure which flow meter is best suited for your application, please contact us. Our specialists will be happy to help identify the most suitable solution for your process.

Variable area flow meters are available from manufacturers, authorized distributors, and specialized instrumentation suppliers. However, because industrial flow meters are typically configured according to the specific requirements of an application, technical consultation is often recommended before purchase.

Heinrichs Messtechnik – Part of the KOBOLD Group

• Heinrichs Messtechnik is a specialist manufacturer of flow measurement instrumentation and a member of the internationally operating KOBOLD Group.
• Through Heinrichs and the global KOBOLD network, customers benefit from worldwide sales and technical support, local contacts, and application expertise across a wide range of industries and flow measurement technologies.

Why Technical Consultation Is Important

Unlike standard off-the-shelf products, industrial flow meters are often configured according to the specific requirements of the application.

Factors such as:

1. Process medium
2. Flow range
3. Pressure and temperature
4. Material compatibility
5. Hazardous area requirements
6. Signal outputs and communication options

It can influence the selection of the most suitable flow meter. For this reason, technical consultation is often recommended before placing an order.

Purchasing Heinrichs Variable Area Flow Meters: Heinrichs variable area flow meters can be purchased directly from Heinrichs Messtechnik or through authorized KOBOLD and Heinrichs sales partners and representatives.

Our team can assist with:

• Product selection
• Technical sizing
• Material recommendations
• Hazardous area requirements
• Application-specific configurations

Global Availability

Heinrichs products are supplied worldwide and supported by an international network of sales partners and technical specialists.

If you are looking for a variable area flow meter for a specific application, please contact us. We will be happy to help identify the most suitable solution for your process.