One type of thermal flow meter.
Fig. illustrates the non-linear shift in ΔT in a heated-tube-type flow meter, where the asymmetricity of the temperature distribution increases with flow.
In this type of flow meter, when a fluid flows in a pipe, a thin layer (film) exists between the main body of the fluid and the pipe wall. When heat is passing through the pipe wall to the fluid, this layer resists the flow of heat. If the heater is sufficiently insulated and if the piping material is a good conductor, the heat transfer from the heater to the fluid can be expressed as
These types of flow meters are best suited for the measurement of homogeneous gases and are not recommended for applications where the process fluid composition or moisture content is variable. In order for these flow meters to be useful in a system, both the thermal conductivity and the specific heat of the process fluid must be constant.