A flow meter using heat transfer principle described above has many limitations. The temperature sensors and the heater must protrude into the fluid stream. Thus these components (particularly the heater) are easily damaged by corrosion and erosion. Furthermore, the integrity of the piping is sacrificed by the protrusions into the fluid stream, increasing the danger of leakage.
To overcome these problems, the heater and the upstream and downstream temperature sensors can be mounted outside of the piping, which is shown in Fig.
In this type of construction the heat transfer mechanism becomes more complicated and the relationship between mass flow and temperature difference becomes non-linear and to overcome this non-linear relationship, heated tube type is introduced.