Flow sensing using surface acoustic waves

Abstract

A sensor employs a SAW oscillator heated above the ambient by converting RF energy to heat via acoustic dissipation in energy absorbers located outside the path of propagation. The oscillator utilizes a delay line designed to support single mode operation, fabricated on a substrate. Low insertion loss results from placing the energy absorbers outside the propagation path. Thermally conductive paths around the substrate periphery reduce thermal gradients. Used to measure fluid flow, fluid is directed across the SAW delay line lowering the substrate temperature which is elevated by the heat generating energy absorbers outside the propagation path. The temperature reduction changes the oscillator frequency, indicative of the gas velocity. In operation, gases are passed across the top of the delay-line while liquids are passed across the bottom of the delay-line in a back-side sensing operation. To measure gas or vapor contaminants, chemically sensitive coatings placed on the substrate in the propagation path can be used to sense contaminants in a gas flowing over the substrate. In mass flow sensing applications a difference frequency between an oscillator with a delay-line having end absorbers and operating at elevated temperatures and a reference oscillator with a delay-line operating at ambient is determined.