Thermal Dispersion Flow Switch Operating Principle

The operating principle of the FS-600 flow switch is based on the calorimetric principle.  The FS-600 uses the cooling effect of a flowing fluid or gas to monitor the flow rate.  The amount of thermal energy that is removed from the tip determines the local flow rate.  This temperature-based operating principle can reliably sense the flow of virtually any liquid or gas.

The sensor tip of the FS-600 flow sensor houses two transistors and a heater element.  One transistor is located in the sensor tip, closest to the flowing fluid.  This transistor is used to detect changes in the flow velocity of the liquid.  The second transistor is bonded to the cylindrical wall and is a reference for ambient fluid conditions.

In order to make the sensor sense flow, it is necessary to heat one of the transistors in the probe.  When power is applied, the tip of the probe is heated.  As the fluid starts to flow, heat will be carried away from the sensor tip.  Cooling of the first transistor is a function of how fast heat is conducted away by the flowing liquid.  The difference in temperature between the two transistors provides a measurement of fluid velocity past the sensor probe.  When fluid velocity is high, the temperature differential is small.  As fluid velocity decreases, there is an increase in the temperature differential.