ACOUSTIC-WAVE DEVICES TO MEASURE GAS-FLOW - COMPARISON BETWEEN SURFACE-ACOUSTIC-WAVE (SAW) AND SHEAR HORIZONTAL ACOUSTIC PLATE MODE (SH-APM) OSCILLATORS

In recent years there has been growing interest in using acoustic wave s for a broad spectrum of physical and chemical sensor applications. I n 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 crystallograp hic 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 induc es a change in the oscillator frequency. The sensitivity of the two co nfigurations (SAW and SH-APM) to flow rate at various heating power le vels is presented and discussed. Taking into account simplified assump tions, we obtain an evaluation of the different heat-loss sources (the rmal conduction, natural and forced convections). The interest of the SH-APM oscillator in terms of sensitivity and response time is demonst rated. This is all the more interesting as SH-APM devices are able to work in a liquid environment.