Ds1054z

To test the power usage of the sensor, a Multicomp Pro MP730026 ammeter was placed in series with the ground connection. The ammeter was then used to measure the sleep current of the sensor, which is the current while the sensor is not performing any measurements. This was found to be 100 nA. To measure the duration and charge needed for a single velocity and depth measurement, a Rigol DS1054Z oscilloscope was connected across the ammeter. As the shunt resistance of the ammeter was 4 Ω, this allowed for high time-resolution measurement of current consumption. A single measurement (which, as described above, is the average of 6 individual scans) was found to take 7.02 s and require 0.0742 mAh of charge. This results in an average current of 38.1 mA for a single measurement. The peak current could also be measured to be 64 mA. For a typical logging rate of 10 measurements per hour, the average current draw of the sensor is 740 μA. This provides 196 days of operation on a typical 3500 mAh single 18,650 Li-ion cell. For a logging rate of 1 measurement per hour, over 5 years of operation could be optimally achieved. This low power consumption supports the use case of the sensor in battery-operated installations with infrequent maintenance.

The acoustic pressure was measured with a condenser microphone (Aco, TYPE4157N) connected to a 20 dB amplifier (Aco, TYPE2127). To suppress a noise from the rotating shutter and the motor in the acquired signals, the emitted ultrasound from the cavity was guided by a hollow tube made of silicone rubber with an inner diameter of 8.8 mm and a length of 33 cm. As shown in Fig. 4b, the use of this tube allows the microphone to be placed at a distance from the shutter and the motor. The data was acquired with an oscilloscope (Rigol, DS1054Z) as a voltage sequence of ${10}^6$ points for 1 s, i.e. a temporal resolution of $1\mathrm{\mu }\text{s}$ . From the sequence, 90 pulses were processed by time-synchronous averaging. The room temperature of the experimental environment was kept at ${22}^{\circ }\text{C}$ .