Mass flow sensing using surface acoustic waves

Abstract

A novel approach for measuring mass flow of gases is reported. A SAW (surface acoustic wave) delay line is heated above the ambient by absorbing RF power from an oscillator loop in which it also serves as the frequency-control element; forced convective cooling due to gas flow induces substantial frequency shifts. Utilization of RF power in the sensor is analyzed by applying a simple equivalent-circuit model. A mass-flow sensing configuration that utilizes dual SAW delay-line oscillators to provide temperature compensation and a convenient low-frequency output corresponding to mass-flow rate is described. For low flow rates, oscillator frequency increases linearly with the rate, and a normalized sensitivity greater than 4*10/sup -6//sccm at 150 mW input power has been obtained for nitrogen gas. Simplicity, high sensitivity, digitally compatible output, and low fabrication cost are some of the attractive features of this sensor.<>