Thermal dispersion flow switch is a precise flow sensing device for liquids. The principle of operation is based on heat transfer.
Based on the principle of heat transfer, the thermal dispersion flow switch consists of two temperature sensors. One temperature sensor is the reference and measures the temperature of the fluid where the thermal dispersion flow switch probe is immersed. The second temperature sensor is paced near a heating element. As flow flows. there will be temperature difference between the two temperature sensors.
As the flow rate of the fluid increases there is a cooling effect due to the fluid moving past the heated sensor. The more flow, the more the cooling. A reduction in temperature difference between the two temperature sensors indicates that the flow rate exceeds the user adjustable set point. When the flow rate drops there is less cooling, which causes an increase in the temperature difference.
When liquid is flowing, there is a temperature difference between the two sensors. The temperature difference has an inverse relationship with the flow velocity (fast flowing liquids will result in greater heat differences and vice versa) and the same is used to detect flow. Since the device contains no moving parts, has no wear and tear and maintains a long lifespan.
Source : FineTek