Compact Bidirectional Acoustic Airflow Meter for Aviation Applications (CBAAM)

The Comtech Bidirectional Acoustic Air Meter uses tuned piezoelectric transducers to send pulse trains of approximately 250 cycles of ultrasonic (40-100 kHz, typ.) acoustic energy diagonally across the air stream. Every ~10 ms, the transducers change roles and send the same acoustic energy in the opposite direction, defined as “upstream” and “downstream”. We precisely measure the phase shift between the sender and receiver in both directions. If there is no air flow, the upstream and downstream phase shifts are identical. If there is flow, the upstream phase shift will be slightly longer than the downstream and vice versa. Air velocity is determined using the difference between the two phase shifts. Air temperature is determined from the average of the two phase shifts. Air pressure information is measured independently with a pressure sensor as the speed of sound is virtually independent of ambient pressure. For a known cross sectional area, mass air flow is calculated every ~10 ms by combining the velocity, temperature and pressure information. High resolution is obtained by averaging the phase readings (~100) during each 10 ms reading cycle. Since acoustic energy is sent in both directions, reverse flows and temperatures can be measured with equal resolution.