Characterization and optimization of a thermal flow sensor on circuit board level

The characterization and optimization of a low-cost thermal flow sensor optimized for the use in heating ventilating and air conditioning (HVAC) systems is presented. The fabrication of the transducer is exclusively based on printed circuit board technology to keep costs low and allow for easy handling and replacement. The measurement principle utilizes a calorimetric principle with thermoresistive heat transfer. The thermistors form a Wheatstone bridge while the heating element is supplied with a constant current. The AC bridge output voltage is a function of the flow offering adequate sensitivity and a suitable measurement range for HVAC systems. The main experiments focus on flow measurements with a Lock-In amplifier technique to evaluate the measurement range and sensitivity. Based on prior results for time constants, output voltage, sensitivity, etc., the sensor design has been optimized.