Thermal Mass Flow Meters Types and Theory of Operations
Thermal mass flow meters, also known as thermal dispersion or immersible mass flow meters, are used to measure the total mass flow rate of a fluid, primarily gases, flowing through closed conduits. It is a direct measurement type mass flow meter with no moving parts.
Theory of Operation-
A Thermal Mass Flow Meter contains two temperature sensors for this purpose. One sensor measures the fluid temperature as a reference. The second sensor is heated and has a constant temperature differential relative to the first sensor. When the flow of air/gas starts, heat is taken away from the warmer sensor. The amount of heat taken away has a direct relationship with the mass velocity of the fluid. That is how it is a direct mass flow meter.
Being a direct mass flow meter, it does not require any pressure or temperature compensation.
There are two types of Thermal Mass Flow Meters,
- Thermal Dispersion/Immersible Type-
In the case of Thermal dispersion or immersible type, the heat is transferred to the layer of the flowing fluid encountering the heater surface. The heat transferred from the surface of the heating element is related to the point mass velocity of the fluid.
- Capillary Tube Type-
In the second type, i.e., the capillary-type thermal mass flow meter, the heat is transferred to the bulk of fluid passing through the small, heated capillary tube.
Although the principles of operation of both types depend on heat convection, both methods are quite different. Thermal dispersion flow meters are commonly used for general industrial gas flow applications in pipes and ducts and, hence, are more popular for such industrial applications. The capillary types are primarily used for smaller flows of clean gases or liquids in tubes.
The thermal mass flow meter is used in a variety of gas flow applications, such as
- Compressed air/gas Consumption measurement
- Gas measurement at very low-pressure flowing conditions
- Fuel Gases like CNG/PNG/LPG consumption measurement and control
- Automobile gas engine testing
- Industrial oxygen flow measurement and control
- Furnace and burner control
- Biogas measurement in digester biogas digester systems
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