Thermal mass flow meters measure gas mass flow in various processes and offer distinct advantages, such as repeatability, low maintenance, and operating over a wide flow range. Here we discuss the meter’s fundamentals, the working principle of thermal mass flow measurement, and the two approaches to measuring heat dissipation.
What is a thermal mass flow meter?
Thermal mass flow meters are precision instruments that directly measure gas mass flow (instead of volumetric flow) for industrial gases, compressed gas, and other gases. The device is used in a wide range of applications (in many industries), including:
- Air and biogas measurement (e.g., in wastewater plants)
- Carbon dioxide (for beverage production and chilling)
- Argon (in steel production)
- Compressed air
- Natural gas (for burners and boiler feed control)
- Nitrogen and oxygen (production)
How does a thermal mass flowmeter work?
The thermal mass meter measures gas flow based on convective heat transfer.
The Sage flow meters are available in either inline flow bodies or insertion-style. In either case, the meter’s probe inserts into a gas stream of a pipe, stack, or duct. Toward the tip of the meter’s probe are two sensors. These sensors are resistance temperature detectors (RTDs) or resistance thermometers that measure temperature. The RTDs consist of durable reference-grade platinum windings clad in a protective 316 SS or Hastelloy C sheath.
One RTD is heated by an integrated circuit and functions as the flow sensor, while a second RTD acts as the reference sensor and determines the gas temperature. The Sage proprietary circuitry maintains a continuous overheat between the flow and reference sensors. As gas flows by the heated RTD, drifting gas molecules transport heat away from it, and as a result, the sensor cools, and the energy escapes. The circuit balance disrupts, and the temperature difference (ΔT) between the heated RTD and the reference RTD changes. The circuit restores the lost energy within a second by heating the flow sensor to adjust the overheat temperature.
The electrical power required to sustain this overheat denotes the mass flow signal.
Are thermal flow meters for gas or liquid applications?
Thermal mass flow meters are used almost entirely for gas flow applications. As the name implies, the meters use heat to measure flow, and they introduce heat into a flowing stream and measure how much heat dissipates using one or more temperature sensors.
This method works best with gas mass flow measurement. Due to heat absorption considerations, it is challenging to get a strong signal using thermal mass flow meters in liquids.
What are the two approaches to measure heat dissipation?
While all thermal mass flowmeters use heat to make their flow measurements, there are two different methods for measuring how much heat dissipates.
Constant Temperature Differential Meters
One approach is called the constant temperature differential thermal gas mass flowmeter. Sage Metering uses this method which utilizes two temperature sensors: a heated sensor and another that measures the temperature of the gas. The mass flow rate is computed based on the amount of electrical power or energy expressed in milliwatts required to maintain a constant difference in temperature between the heated sensor and the passive temperature sensor.
Constant Current Differential Meters
Because of its slow response characteristics, a second less popular concept is called a constant current thermal mass flow meter. Meters using this method also have a heated sensor and another sensor that senses the temperature of the flow stream. The power or energy delivered to the heated sensor is kept constant. The mass flow is measured as a function of the difference between the temperature of the heated sensor and the flow stream.
Both methods are based on the principle that higher velocity flows result in a more significant cooling effect as heat redirects from the heated sensor.
While all thermal flow meters use heat to make their flow measurements, there are two different methods for measuring how much heat dissipates.
Why is measuring mass flow important?
What are the advantages of thermal mass flowmeters?
- Thermal flowmeters have no moving parts, reducing maintenance and permitting the use in demanding application areas, including saturated gas.
- Gas mass meters calculate mass flow rather than volumetric flow. They do not require temperature or pressure correction, so there is no additional expense for purchasing and installing other equipment.
- Thermal flowmeters provide excellent accuracy and repeatability over a wide range of flow rates.
- Thermal flow meters can measure flow in large pipes.
Also, see our post “9 Thermal flowmeter Advantages.“