Today, residential water meters are smart and static, that means without moving parts. Ultrasonic time-of-flight technology is a common approach for that, and ScioSense offers the right front-end IC that manages the complete measuring process. The flow converters drive the piezoelectric transducers, pick-up the receive signal, amplify if necessary, and do the challenging time-of-flight measurement with picosecond precision and stability for multiple zero-crossings with the receive bursts. Doing this with only a few microampere of current allows to run those meters for up to 20 years from battery.
The basic principle of the measurement is the following: you send an ultrasonic burst once in direction of water flow and once in the opposite direction and you measure the time-of-flight. In flow direction the time-of-flight is shorter than in the opposite one, and the difference between the two is proportional to the flow. Taking into account some non-linear and temperature corrections, the time difference can therefore be used to calculate the flow. v = c² + DIFTOF/(2L) for v << c
Figure 1: Common sensor designs
The design of an ultrasonic water meter is quite a challenge. You have to measure time-of-flight in the range 30 to 200 µs with a precision in the picosecond range. Not only achieving such low noise is a challenge. Having a low offset and offset stability in the range is even a bigger one. For DN15 meters with R800 the target is already 12ps error limit. ScioSense flow converters are based on time-to-digital converter technology that perfectly solves the time measurement task. In addition, the analog front-end is designed with special focus on symmetry and reciprocity so that the offset is small and stable over temperature.
Figure 2: Receive signal
ScioSense flow converters measure multiple zero crossings within a single receive burst. This improves the resolution of the total time-of-flight measurement. With the right choice of the zero crossing – not too early to catch stable periods only, not too late to avoid interference effects – the measurement will be as precise and temperature stable as possible.