Design and Experiments for a Kind of Capacitive Type Sensor Measuring Air Flow and Pressure Differential

A kind of capacitive type air flow sensor is designed and fabricated for respiratory flow measurement. Major sensitive elements of the sensor are two parallel placed capacitance plates which are perpendicularly fixed on the sidewall of the air channel. The flow and pressure differential of air can be calculated by measuring the change of the micro capacitance, which is initiated by air pressure. The Bernoulli equation and the continuous equation of the fluid are used for theoretical analysis. The mechanical analysis and FEM (finite element method) is used to make the optimum design for the sensor. Then, the micro capacitance measuring circuit using the so called peak detection method is fabricated and corresponding A/D convert digital circuit are also made. Experimental result indicates the capacitance measuring circuit has little error. The accuracy, sensitivity, and resolution of the circuit reach 0.2% FS, 1.4089 V/pF, 0.003 pF respectively, which indicates this kind of system is feasible in practical applications.