Positive Displacement Flow Meters

Technology and Operating Principle of Positive Displacement Flow Meters

A positive displacement meter determines the actual quantity of liquid transferred as the fluid passes through a measuring chamber. Within the broader family of flow meters, this technology is distinctive because it does not calculate flow solely from fluid velocity. Instead, it divides the liquid into defined geometric volumes and counts them as they pass through the instrument.

The movement of an internal element is converted into an indication of volume, instantaneous flow rate or totalized quantity. In oval gear meters, the fluid rotates two meshing oval gears. In oscillating-piston meters, the volume is measured through the reciprocating movement of the piston inside the measuring chamber. In gear flow meters, rotation of the toothed elements provides continuous measurement in applications that require resolution, repeatability and integration with process control.

The positive displacement meters in our catalog can be equipped with a mechanical register, electronic display, pulse output, analog or digital signals, and interfaces for automation systems. This allows the instrument to operate either as a local readout device or as an integrated component in control, supervision or batching architectures.

Types of Riels Positive Displacement Meters

The Riels range of positive displacement meters combines several measurement principles to address different application conditions: from industrial fluids and solvents to fuel monitoring, from fuel oils and viscous liquids to hydraulic applications and demanding process conditions.

Oval gear flow meters are among the most widely used solutions for industrial liquids, oils, fuels, solvents and viscous fluids, provided that the selected model and materials are compatible with the process. MXP oval gear flow meter and MXS flow meter for solvents use sensor-detected oval gear rotation to determine the quantity of liquid passing through the instrument. Configurations with a display and ATEX certification are also available on request. The RIF400 for low flow rates of viscous liquids applies the same principle to low measuring ranges, with a pulse output and a compact design suitable for confined spaces.

Fuel meters are used where volume measurement must be stable, repeatable and suitable for accounting requirements. The MC5XX fuel meter uses positive displacement rotors to measure diesel, gasoline, kerosene, biofuels and aviation fuels, depending on the configuration. It is available with intrinsically safe ATEX protection and MID certification for applications requiring legal-for-trade metering.

Oscillating-piston meters are used for consumption monitoring and the measurement of fuels or fuel oils in mobile and stationary applications. The VZO fuel oil flow meter and the VZF-II with display for viscous fuels are designed for oils, heavy oils, fuel oil and biofuels, with a mechanical register or electronic functions depending on the version. The DFM for fuel consumption monitoring in diesel systems is specifically designed to measure consumption in diesel engines, using an oscillating-piston positive displacement principle and a compact installation footprint.

Gear-type positive displacement flow meters complete the range for specific industrial applications, including hydraulic fluids, lubricants, ATF, paints, high-viscosity fluids, and aggressive or abrasive liquids when the selected configuration is compatible. They are chosen when applications require continuous measurement, good resolution and integration with control systems.

How to Select a Positive Displacement Flow Meter

Selecting a positive displacement flow meter depends on the liquid to be measured and the actual operating conditions of the system. Fluid type, viscosity, minimum and maximum flow rate, pressure, temperature, chemical compatibility, contaminants, connections and readout method determine the most suitable configuration.

For complex applications, Riels technical team supports companies, installers and engineering firms in verifying materials, signals, certifications and operating conditions, reducing the risk of sizing errors or solutions that are not aligned with the process.

Compatible Fluids and Operating Limits

The Riels range of positive displacement meters includes versions for oils and lubricants, fuels, fuel oils, solvents, hydraulic fluids, paints, adhesives, detergents and chemical liquids compatible with the selected configuration.

The positive displacement principle requires particular attention to fluid cleanliness. Contaminants and solid particles can affect measurement and increase wear on internal components. Flow rate, viscosity, temperature, pressure, filtration and materials must therefore be compatible with the application.

Application Areas

Positive displacement flow meters are used in processes where control of the transferred quantity affects consumption, batching and product management. Applications include the transfer of oils and lubricants, fuel accounting, supply to burners and generators, consumption monitoring on diesel engines, and metering of industrial or viscous fluids.

In hydraulic systems and test benches, gear flow meters can monitor lubricating fluids, ATF and hydraulic oils even under demanding operating conditions. In painting, coating and precision dosing applications, dedicated versions can handle low flow rates with good repeatability.

In chemical processes, the meter supports control of the transferred liquid, process repeatability and the availability of data for batching, supervision and consumption verification.

Readout, Signals and Integration

The readout method is a key selection factor. A mechanical register provides a simple local indication. An electronic display offers more advanced flow and volume readings. Pulse, analog or digital outputs enable integration with PLCs, supervisory systems, totalizers and batching systems.

The meter can therefore operate as a standalone instrument or as part of an automation system, supplying data to control, recording, accounting and batching functions.

Accuracy, Calibration and Certifications of Riels Positive Displacement Meters

Choosing a Riels positive displacement meter means relying on solutions selected to deliver robust construction, accuracy and measurement continuity across a wide range of industrial applications. The availability of specific configurations and certifications, including ATEX and MID, also addresses demanding safety, compliance and legal metrology requirements.

Explore the available models and identify the solution best suited to your system. For special applications or certification requirements, Riels technical team is available to help you select the most suitable positive displacement meter.

FAQ

What is the difference between a positive displacement flow meter and a volumetric meter?

A positive displacement flow meter measures the quantity of liquid passing through the instrument using a positive displacement principle. The term volumetric meter is often used for the same type of instrument when its primary function is to measure and totalize the volume of transferred liquid.

How does an oval gear flow meter work?

In an oval gear flow meter, the liquid passes through a measuring chamber and rotates two meshing oval gears. Each rotation corresponds to a known volume of fluid. The movement is detected by a sensor or counting mechanism and converted into volume, flow rate or an output pulse.

Which fluids can be measured with a positive displacement meter?

Positive displacement meters can measure oils, lubricants, fuels, diesel, solvents, hydraulic fluids, paints, adhesives, detergents and chemical liquids that are compatible with the instrument configuration. Compatibility always depends on wetted materials, seals, viscosity, pressure, temperature and fluid condition.

Are positive displacement flow meters suitable for viscous liquids?

Yes. Many positive displacement meters are suitable for viscous liquids. The Riels range includes configurable models for oils, lubricants, high-viscosity fluids, fuel oils and viscous fuels, subject to the flow range and materials specified for each version.

When is an ATEX or MID version required?

An ATEX version is required when the instrument operates in a potentially explosive atmosphere or in an environment subject to specific safety requirements. MID certification is required when the measurement is used for regulated accounting or legal metrology applications. Certification availability must be checked for the individual model and selected configuration.

How should the correct flow range be selected?

The flow range must cover the actual operating conditions of the system without the instrument running continuously outside its minimum or maximum limits. Flow rate, viscosity, pressure, temperature, fluid type, required accuracy and output signal should be evaluated together to ensure stable measurement consistent with the application.