The Series TDFS Thermal-Dispersion Flow Switch from the W.E. Anderson Division of Dwyer Instruments Inc. is a thermal flow switch that indicates whether the flowrate is above or below a user set flowrate with NO and NC NPN outputs. Setpoint is easily field-set by tapping the included magnet on the setpoint target three times at the desired flowrate. Incorporated into the unit are two LED status indicators on opposite sides of the unit providing visual switch indication, green when the flow is above setpoint, red when the flow is below setpoint. The TDFS uses an impulse thermal dispersion measurement technique to measure the flowrate where the probe is heated above the process temperature and then is allowed to cool down to the process temperature. – dwyer-inst.com