How does a thermal mass flow meter work?

Flow

How does a thermal mass flow meter work?

A thermal mass flow meter measures the movement of a fluid in a pipe or duct. So in this meter’s simplest configuration, gas flows past a heated velocity sensor and a temperature sensor. Then the device works to maintain a constant difference of 50 degrees Celsius between the gas and the heated sensor.

As the gas flows past the heated sensor, its molecules steal some of the heat, creating a cooling effect. So the watts needed to maintain the temperature difference between the two sensors is directly proportional to the mass flow rate. And the amount of heat lost depends on the thermal properties and flow rate of the gas.

thermal mass flow meter in-line configuration diagram
Courtesy of Sierra

 

Fast facts

  • A thermal mass flow meter uses molecular heat transfer to measure fluid mass flow rate.
  • It contains a heated velocity sensor and a temperature sensor.
  • Fluid passing the heated sensor cools the sensor.
  • This cooling effect is proportional to the fluid’s velocity.
  • But the fluid’s thermal properties (amount and rate at which it can transport heat) influence this effect.
  • You can find the mass flow rate by measuring the temperature difference between the heated sensor and the temperature sensor.
  • The watts needed to maintain the temperature difference is directly proportional to the fluid’s mass flow rate.
  • These devices measure without needing pressure and temperature compensation or flow computers.
  • Thermal mass flow meters are the only direct gas mass flow meters other than Coriolis meters.
diagram of how a thermal mass flow meter sensor works
Courtesy of Sierra

Fast factors

Many other factors will affect how well this meter works:

  • Thermal characteristics of sensor materials
  • Construction of sensor
  • Heat loss other than that being carried away by the fluid
  • Uniformity of flow in the pipe

Original content is here.

Author: Scott Rouse, Product Line Director @ Sierra Instruments

Related tags: Article Basic English Flow Principle Sierra Instruments thermal flow meter thermal mass flow meter
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