Since the late 80s ultrasonic gas meters have been pushed into the market. They show a number of advantages of the classical diaphragm meters. They are much more precise and show much higher capability with respect to flow ranges. They have no mechanically moving parts and therefore show an extended lifetime. In addition, they come in smaller outlines and with less noise.
The basic principle of the measurement is the following: you send an ultrasonic burst once in direction of gas 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 gas meter is quite a challenge. First, you
need a high fire voltage and a high amplification of the receive signal
to manage the weak sonic coupling to air. Then you have to measure time-of-flight in the range 200 μs to 500 μs with a precision in the 300 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. In addition, the amplitude of the receive signal will show strong variations in real applications.
Figure 2: Receive signal
ScioSense flow converters manage that challenges: An integrated charge pump allows to fire with up to 17V. Within the amplified received signal they measure multiple zero crossings. 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.
By means of a split burst option it is possible to detect a phase jump as a marker within the bust and to get a precise absolute ToF even with strong amplitude variations.
Integrated charge pume for 17 V fire voltage
Measuring time intervals from 10 to 500μs with picosecond resolution
For residential meter sensors offset stability has to be in the range of 300 ps and less
Current consunption needs to be as low as a few μA for many years operation from battery
High fluctuations in amplitude due to various gas mixtures, including 5 to 39% of hydrogen