Thermal Flow Meters

Technology and Operating Principle of Thermal Flow Meters

A thermal flow meter detects gas flow through heat exchange between the gas and the sensor. The measurement principle is based on thermal dispersion: a heated element is cooled by the passing gas, and the amount of heat removed increases with the flow rate.

This technology is also referred to as a thermal mass flow meter, thermal dispersion flow meter or calorimetric flow sensor. In all cases, the measurement depends on the thermal properties of the gas and on correct instrument configuration for the gas and operating conditions.

In flow measurement, the distinction between volumetric flow and mass flow is fundamental. Volumetric flow indicates the volume passing through a cross-section per unit of time, whereas mass flow directly measures the quantity of fluid being transferred. In compressed-air and technical-gas systems, this parameter enables a more consistent assessment of consumption and system performance.

Depending on the model, the meter can provide instantaneous flow rate, totalized volume, mass and consumption. Accuracy and stability depend on the gas, measuring range, temperature, pressure, installation and calibration specified for the application.

Types of Thermal Flow Meters in the Riels Catalog

Thermal flow meters can be configured with different architectures according to the pipework and plant design. Inline versions are installed directly in the process line, while insertion versions introduce a probe into the pipe or duct and are often preferred when pipe diameter and installation conditions make this arrangement more suitable. In bypass configurations, part of the flow is routed through a dedicated measuring circuit.

Among the available solutions, the RIF800 thermal mass flow meter is designed for gases such as air, nitrogen, oxygen and CO2, subject to the selected configuration and operating conditions. The model measures volumetric flow, mass flow, volume and mass, with a local display and outputs including 4-20 mA, pulse and Modbus RTU in the applicable configurations.

The RIF840 thermal mass flow meter is designed for air and nitrogen applications, with direct measurement of mass flow or standardized flow and an insertion installation. It includes a local display and can be configured with analog outputs, pulse, Modbus and Bluetooth, allowing measurement data to be used by monitoring and control systems.

How to Select the Right Thermal Flow Meter

Within the range of flow meters, the selection of a thermal flow meter depends on the gas to be measured and how the instrument must integrate into the plant. Key parameters include gas type, minimum and maximum flow rate, pressure, temperature, pipe diameter, required accuracy and installation type.

Local indication on a display, consumption totalization and the output signals required by the control system should also be considered.

For applications that require sizing, Riels technical team supports the selection of the most suitable thermal meter by checking the gas type, operating range, installation and required signals.

Compatible Gases and Process Conditions

Depending on the model, thermal flow meters can be configured for different gases. Compressed air and nitrogen are among the most common applications, while oxygen, CO2 and other technical gases must be evaluated only when the instrument and configuration are suitable.

Compatibility does not depend on the gas name alone. Operating pressure, temperature, humidity, gas composition, contaminants, flow profile and pipe conditions affect measurement stability and technology selection. It is therefore more accurate to refer to compatible configurations rather than universal instruments.

Installation, Signals and Integration

The installation type has a direct effect on measurement quality. For insertion models, the probe position, orientation, mechanical connection and flow profile must be checked. For inline or bypass versions, pipe diameter, process connections and integration with the existing pipework become particularly important.

Correct installation promotes stable and repeatable measurement. Straight pipe runs, power supply, ingress protection rating, outputs and communication protocols must be compatible with PLCs, SCADA systems, data loggers and monitoring platforms.

Applications of Thermal Flow Meters

Thermal flow meters are used where continuous gas measurement supports process control, consumption management and plant supervision. In compressed-air networks, they provide data for assessing efficiency, identifying abnormal consumption and understanding gas distribution across different production areas.

They can also be used to monitor nitrogen, oxygen, CO2 and other compatible technical gases in production lines, laboratories, manufacturing plants and distribution networks.

In energy efficiency projects, the meter does not automatically guarantee savings, but it provides measurement data for analysis, verification and operational decision-making.

Benefits and Accuracy of Riels Thermal Flow Meters

Riels thermal flow meters measure gases and compressed air with continuous flow-rate monitoring and configurations suited to different process requirements. Since the technology has no moving mechanical components in contact with the gas, it can reduce wear compared with certain mechanical solutions while integrating cleanly with control systems.

The available configurations allow the instrument to be matched to the gas, flow range, operating conditions and required signals.

Explore the available models or contact the Riels technical team to identify the most suitable solution for monitoring compressed air and technical gases.

FAQ

How does a thermal flow meter work?

A thermal flow meter measures gas flow based on heat exchange between the sensor and the gas. The gas removes heat from a heated element, and the instrument processes the dissipated heat to calculate the flow rate.

What is the difference between a thermal flow meter and a Coriolis flow meter?

A thermal flow meter uses heat transfer and is typically applied to gases and compressed air. A Coriolis meter measures mass flow directly through the Coriolis effect and is often used in applications requiring high accuracy, including liquids and a wider range of process conditions.

Are thermal flow meters suitable for compressed air?

Yes. Thermal flow meters are widely used to monitor compressed air, consumption and network distribution. Model selection depends on pipe diameter, flow range, pressure, temperature, required signals and installation conditions.

What is the difference between insertion and inline thermal flow meters?

An inline meter is installed directly in the pipe as a complete measuring body. An insertion meter introduces a probe into the pipe or duct, which can be advantageous when the pipe diameter and plant configuration make this installation method preferable.

What information is required to select a thermal mass flow meter?

The minimum information required includes gas type, minimum and maximum flow rate, operating pressure and temperature, pipe diameter, installation type, required accuracy, need for a display or totalization, and output signals to PLCs, data loggers or supervisory systems.