MEMS thermal flow sensors

• Five different micromachined thermal flow sensors have been fabricated and integrated with commercial Teflon tubing and needle for low flow rate measurement.
• The differences in performance of thermal flow sensor arranged in vertical and horizontal configuration were examined.
• ​To improve the thermal insulation as well as to decrease the drag of the sensor, a new dumbbell type thermal flow sensor is further proposed, fabricated and tested.
• Comparison with related literature is made to show how good the sensors developed in this study are.

This paper presents micromachined thermal flow sensors for measuring liquid flow down to 0.05 μl/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 μl/min. The experimental results show that the sensor has a sensitivity of 7.7 mV/(μl/min) at 0.13 mW power consumption and 0.05 μl/min volumetric flow rates. Comparison with related literature has been made to judge how good the flow sensors developed in this study are.