Flow sensing using surface acoustic waves

A surface acoustic wave delay line sensor comprising:

• (a) a planar piezoelectric substrate having first and second ends;
• (b) transmitting and receiving interdigital transducers disposed on a surface of the substrate and separated by a propagation region;
• (c) a first acoustic end absorber disposed between the first substrate end and the transmitting transducer;
• (d) a second acoustic end absorber disposed between the second substrate end and the receiving transducer;
• (e) the first and second acoustic end absorbers absorbing mechanical and electrical energy components from the acoustic wave, thereby heating the delay line above a surrounding ambient temperature;
• and
• (f) means for distributing heat generated in the end absorbers around the substrate.

A method of monitoring selective contaminants in a gas with a surface acoustical wave delay line having interdigital transmitting and receiving transducers separated by a propagation region coated with a said contaminant sensitive coating on a top surface of a substrate comprising the steps of:

• (a) passing a sample of the gas to be monitored across the contaminant sensitive coating on the top surface of the surface acoustical wave delay line
• (b) heating the delay line above a surrounding ambient temperature by absorbing energy in absorbers disposed between ends of the substrate and the interdigital transducers;
• (c) coupling an amplifier with the delay line to form a stabilized oscillator;
• (d) measuring a resonant frequency of the oscillator and determining the presence of said contaminants in the gas.

A method of mass flow sensing comprising the steps of:

• (a) providing a first surface acoustical wave delay line with interdigital transmitting and receiving transducers separated by a propagation region on a surface of a substrate, the delay line also having acoustic end absorbers disposed between ends of the substrate and the interdigital transducers for absorbing acoustical energy and heating the delay line above a surrounding ambient temperature;
• (b) coupling a first amplifier with said first delay line to form a first stabilized oscillator with elevated delay line temperature;
• (c) providing a second delay line having a temperature coefficient of delay substantially equal to a temperature coefficient of delay of said first delay line;
• (d) coupling a second amplifier with said second delay line to form a second stabilized oscillator, said second oscillator having substantially ambient temperature operation;
• (e) providing a flow conduit of substantially fixed cross sectional area with mounting means and thermal isolation means for said delay lines;
• (f) mounting said delay lines with said thermal isolation means and said mounting means to pass a fluid across said delay lines;
• (g) measuring a resonant frequency of both said oscillators when said fluid is passed over the delay lines and computing a difference frequency;
• (h) determining a mass flow rate from said difference frequency.

A method of measuring a velocity of gas comprising the steps of:

• (a) providing a surface acoustical wave delay line with interdigital transmitting and receiving transducers separated by a propagation region on a surface of a substrate;
• (b) heating the delay line with acoustic end absorbers, disposed between ends of the substrate and the interdigital transducers, said end absorbers absorbing acoustical energy and heating the delay line above a surrounding ambient temperature;
• (c) coupling an amplifier with the delay line to form a stabilized oscillator with elevated delay-line temperature and passing a fluid across said delay-line;
• and
• (d) measuring a resonant frequency of the oscillator and thereby determining the velocity of the gas from said resonant frequency.