Novel PCB-based thermal flow sensors for air conditioning systems

This work addresses the development, characterization, and optimization of thermal flow sensors for heating, ventilation, and air conditioning (HVAC) systems. The sensors are based on printed circuit board (PCB) technology in order to obtain flexible, robust, and cost-effective devices. The layout comprises a copper heater meander located in the center of the PCB and two additional meanders placed on either side of the heater, acting as temperature sensors. Two different operating modes were investigated. In the first mode, a constant current applied to the heater leads to flow dependent voltage across the heater which depends on the fluid temperature and saturates at higher flow velocities. In the improved second mode, the difference between the mean temperature near the heater and the mean temperature at the sensor's edge is kept constant by an electronic controller. This is a novel approach compared to the common case where the difference between heater and ambient temperature is evaluated. In the proposed quasi-constant temperature mode the heater voltage adjusted by the controller serves as an output signal. It is approximately independent of the ambient temperature and features a good sensitivity over the whole range of interest up to 10 m/s. The sensor development and optimization was supported by FEM simulations.