Acoustic Flow Sensor Using a Passive Bell Transducer

Sensing based on a passive transducer that is wirelessly linked to a nearby data collection node can offer an attractive solution for use in remote, inaccessible, or harsh environments. Here, we report a pipe flow sensor based on this principle. A transducer mounted inside the pipe generates an acoustic signal that is picked up by an external microphone. The passive transducer comprises a cavity with a trapped ball that can oscillate in response to flow. Its collisions generate an acoustic signal correlated with the flow speed. The transducer is implemented on a 6-mm-diameter probe and characterized as a water flowmeter. The time-average microphone voltage output is calculated by an analog circuit, without any further signal processing. With the microphone mounted on the probe, and for flow rates in the range of 0.35-6.5 m/s, the correlation between the sensor voltage output and flow rate data from a commercial flowmeter is demonstrated with a worst case accuracy of 2%. This was achieved by simple averaging of the acoustic pulse train over a 5-s time interval. Consistent correlation with the microphone mounted on the pipe wall at distances up to 150 mm from the probe location is also reported. These results demonstrate the viability of remote acoustic flow sensing using passive structures and offer a simple and minimally invasive flow monitoring method.