How does a thermal flow meter work?

Posted on: 05/02/2025

Learn about the operating principle, advantages, disadvantages, and applications of thermal flow meters in gas flow measurement. Discover how to use them effectively in industries such as oil and gas, chemicals, power, and water treatment.

Thermal flow meters utilize the thermal properties of a fluid to measure the flow rate of the fluid moving through a pipe or conduit. In a typical thermal flow meter, a measured amount of heat is applied to the sensor’s heater. A portion of this heat is lost to the flowing fluid. As the flow rate increases, more heat is lost. The amount of heat lost is detected using temperature measurement within the sensor. The transmitter uses heat input and temperature measurements to determine the fluid flow rate. Most thermal flow meters are used for gas flow measurement. Thermal flow meters account for 2% of global flow meter sales.

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The amount of heat lost from the sensor depends on the sensor design and the thermal properties of the fluid. The thermal properties of the fluid can and do change with pressure and temperature; however, these changes are usually minimal in most applications. In such applications, when the thermal properties of the fluid are known and remain relatively constant during actual operation, thermal flow meters can be used to measure the mass flow of the fluid, as thermal flow measurement is independent of the fluid’s pressure or temperature.

However, in many applications, the thermal properties of the fluid can depend on its composition. In such cases, variations in fluid composition during actual operation can affect thermal flow measurement. Therefore, it is crucial for the thermal flow meter supplier to know the fluid composition in order to apply the appropriate calibration factor for accurate flow measurement. Due to this limitation, thermal flow meters are typically used for measuring the flow of pure gases. Suppliers can provide calibration data for different gas mixtures, but the accuracy of a thermal flow meter depends on whether the actual gas mixture matches the one used for calibration. In other words, the accuracy of a thermal flow meter calibrated for a specific gas mixture will decrease if the actual flowing gas has a different composition.

Advantages and Disadvantages

Thermal flow meters have a moderate cost and are well-suited for measuring low-pressure gases. They are ideal for monitoring exhaust gases and emissions. Insertion models are an excellent choice for large pipe sizes when used as an insertion meter. Their greatest advantage is that if the gas composition is known, the meter can directly measure the actual mass flow without requiring pressure compensation. However, their accuracy is only moderate, and they are primarily used for gas flow measurement. They are not suitable for steam flow measurement.

How to Use a Thermal Flow Meter

Thermal flow meters are most commonly used to measure the mass flow of clean gases, such as air, nitrogen, hydrogen, helium, ammonia, argon, and other industrial gases. Gas mixtures, such as stack gases and biogas flow, can be measured when their composition is known. One advantage of this technology is that it relies on thermal properties that are nearly independent of gas density.

However, caution should be exercised when using thermal flow meters to measure gases with unknown and/or changing compositions, such as exhaust gases containing hydrogen and other mixtures that may disproportionately affect thermal flow measurement.

Thermal flow meters can be applied to clean gases, sanitary gases, and corrosive gases where the thermal properties of the fluid are known. They are most commonly used for measuring pure gases, such as those used in laboratory experiments and semiconductor manufacturing. They can also be utilized in chemical and petrochemical plants when the thermal properties of the gases are well understood. With appropriate attention to material selection, they can measure the flow of corrosive gases, such as hydrogen chloride and hydrogen sulfide.

Industries of Application

Thermal flow meters are used in various industries, including oil and gas, power generation, chemicals, water and wastewater treatment, metals and mining, food and beverage, pulp and paper, pharmaceuticals, and textiles.