MEMS thermal flow sensors
This paper presents micromachined thermal flow sensors for measuring liquid flow down to 0.05 ml/min. The sensing element is a parylene C thin film with two thin film platinum resistors as heating and sensing elements. The sensors are integrated with AWG 24 Teflon tubing and additional two external constant resistors to form a Wheatstone bridge. In this study, we investigated the efficacy of the orifice type sensor in two configurations, vertical and horizontal. In vertical configuration, the sensor was arranged perpendicular to the flow direction. The orifice flow allows maximum heat transfer from the sensor to the flow but leads to higher flow resistance. After redesigning the geometries of the sensor, the dumbbell type thermal flow sensor was further developed. The sensor is suspended in the middle of the channel to improve thermal insulation and achieve better sensitivity. The sensors have demonstrated a flow rate resolution below 0.05 ml/min. The experimental results show that the sensor has a sensitivity of 7.7 mV/(ml/min) at 0.13 mW power consumption and 0.05 ml/min volumetric flow rates. Comparison with related literature has been made to judge how good the flow sensors developed in this study are.