A Highly Sensitive Dual-Mode Thermal Flow Sensor Based on Calorimetric Mode

This article reports a dual-mode switching of highly sensitive thermal flow sensors based on calorimetric mode that is more suitable for microflow measurements. Upstream heating and downstream heating modes (which are both calorimetric) are adopted in this article. The sensor consists of two vertically stacked suspended membrane MEMS chips. A gas microchannel is fabricated inside the chip that is mounted perpendicular to the flow direction. Because of the Laval effect, the gas flowing through the internal channels of the chip increases the gas flow velocity and improves the heat exchange efficiency between the chip and the gas; moreover, the dual-chip stacked design increases the thermal resistance of solid heat conduction and the heat exchange area of the gas, thus improving its sensitivity. Simulation results show that the gas flow velocity and heat exchange efficiency increased seven times and three times, respectively. Experimental results show that the sensor achieves high sensitivity measurements of 0.77 mV/(mL/min) for 0-18 mL/min and 0.16 mV/(mL/min) for 18-100 mL/min over the entire range by dual-mode switching. The response time is 25 ms. To the best of the authors' knowledge, this is the first report of a thermal flow sensor with dual-mode switching of upstream and downstream heating in calorimetric mode. In addition, the sensor can be widely used in semiconductors, hydrogen energy, scientific instruments, and other fields.