Acoustic wave devices to measure gas flow: Comparison between surface acoustic wave (SAW) and shear horizontal acoustic plate mode (SH-APM) oscillators

Abstract

In recent years there has been growing interest in using acoustic waves for a broad spectrum of physical and chemical sensor applications. In this paper we describe two types of acoustic flow sensors. For both devices, interdigital transducers (IDTs) on a piezoelectric substrate have been used to realize delay lines. The substrate is quartz with a SAW (SH-APM) propagation direction along (normal to) the crystallographic X axis. The delay line, mounted in an oscillator configuration and heated at a constant power with a screen-printed resistor, is placed in the path of the flowing fluid. Cooling caused by fluid motion induces a change in the oscillator frequency. The sensitivity of the two configurations (SAW and SH-APM) to flow rate at various heating power levels is presented and discussed. Taking into account simplified assumptions, we obtain an evaluation of the different heat-loss sources (thermal conduction, natural and forced convections). The interest of the SH-APM oscillator in terms of sensitivity and response time is demonstrated. This is all the more interesting as SH-APM devices are able to work in a liquid environment.