Application of a surface-acoustic-wave device for measurement of liquid flow rate

The use of a surface-acoustic-wave (SAW) device to measure the flow rate of liquids is described. A delay-line stabilized SAW oscillator heated to a suitable temperature above ambient is cooled by the flowing fluid. This results in a change in the oscillator frequency. The frequency of a 68-MHz oscillator operated at 9 degrees C above ambient is found to vary by more than 40 kHz for variation in water flow rate from 0 to 0.8 ml/min. Attractive features of this device include the ability to measure very low flow rates (less than 0.1 mu l/min) and direct digital output. However, since this is a thermal type of flow sensor, the temperature of the fluid will be slightly elevated during its passage through the flow cell. The device should be useful in applications where low flow rates have to be monitored.<>