MEMS capacitive flow sensor for natural gas pipelines

This paper presents the design, fabrication, and experimental results of an in-plane MEMS capacitive flow sensor that uses the displacement of a micro-fabricated paddle caused by dynamic gas pressure for measuring the velocity of the flow of surrounding gas. The fabrication process is simple; the prototype is fabricated on 100-μm device Silicon-On-Insulator wafers using only three photo-lithographic mask layers. The device area is 5.5 mm by 5.5 mm. A comb-drive capacitance is used as the transducer for the flow sensor. Measurements show that the output capacitance C is a quadratic function of the gas velocity v , C = k1 v 2 + k2 v + Cp, where k1 = −8.5 fF/(m/s)2, k2 = 73.6 fF/(m/s) and Cp = 16 pF. The advantage of using a capacitive sensing mechanism is that it is virtually insensitive to changes in ambient temperature. Experimental results show that the output capacitance changed only slightly, about 0.21–0.34%, when the temperature changed from 23 °C to 43 °C. Simplicity of fabrication, combined with insensitivity to variations in ambient temperature makes this sensor ideal for widespread deployment to monitor the flow in natural gas pipelines.