SURFACE ACOUSTIC-WAVE DEVICE FOR MEASURING RATE OF GAS-FLOW

The use of a surface acoustic wave (SAW) device for measuring rate of gas flow is described. A delay-line stabilized SAW oscillator is heated above the ambient by using a novel approach in which radio frequency (rf) energy is converted to heat via acoustic dissipation. This "selfheating" arrangement eliminates the need for a separate heater and associated circuitry. Convective cooling caused by gas flow results in a change in the substrate temperature and thereby the oscillator frequency. The frequency of a 75-MHz oscillator fabricated on a 128-degrees-C rotated Y-cut, X-propagating lithium niobate substrate is found to vary by more than 55 kHz for variation in flow rate from 0 to 500 cm3/min. A dual-oscillator configuration is implemented to provide temperature compensation and a convenient low-frequency output corresponding to flow rate. Adequate flow-sensing response is achieved with only modest (15 mW) oscillator loop power.