How Does a Thermal Flowmeter Work?

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Courtesy of Sierra Instruments, Inc.

A thermal flowmeter works by measuring the total mass flow rate of a fluid flowing through a pipe or duct.

In a thermal flowmeter’s simplest working configuration, fluid flows past a heated thermal sensor and a temperature sensor. As the molecules of the fluid flow past the heated thermal sensor, heat is lost to the flowing fluid. The thermal sensor cools down while the temperature sensor continues to measures the relatively constant temperature of the flowing fluid.

The amount of heat lost depends on the thermal properties of the fluid and the flow rate of the fluid. Thus, by measuring the temperature difference between the thermal and temperature sensors, the flow rate can be determined.

Thermal Flowmeters:

    • Work by using thermal heat energy transfer to derive mass flow
    • Contain a heated thermal sensor and a temperature sensor
    • Flow past the heated thermal sensor, which cools it down as the molecules flowing by take heat away with them
    • Use a cooling effect that is proportional to the velocity of the flowing fluid and also a function of the thermal properties of the fluid (the amount and rate at which the fluid can transport heat)
    • Allow for the mass flow rate of the fluid to be determined by measuring the difference in temperature between the thermal and temperature sensors
  • Result in temperature differential that is directly proportional to the mass flow rate

Many other factors will affect how well a thermal flow meter works, including:

  • The thermal characteristics of the sensor materials
  • How the thermal sensor is constructed
  • The heat lost through means other than being carried away by the fluid flow
  • The uniformity of the fluid flow in the pipe
Other Factors

For a full explanation of how this type of meter functions, watch Sierra’s video on how thermal mass flow meter technology works. You can also learn more in this white paper on the evolution of thermal mass flow meters from Sierra Founder Dr. John Olin.

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