This device is a “time of flight” type flow-meter, where a high frequency (approximately 1MHz) pressure wave is beamed at an acute angle across the pipe. The time required for the wave to mach the opposite wall depends on the whether direction moving; with or against the flow and on the speed of sound through the liquid. Flow-rate information is obtained from the difference of the measured time with and against the flow.
The device has an Analogue Module in order to generate the transmission pulses, to make the amplification of the received signals, to extract the trigger signal from the received waveform. This module provides the signals to mark the beginning of the sound pulse (marked by transmission pulse) and the end of the flying time of sound (marked by the received trigger signal).
The high-speed Digital Module is build in an EPLD circuit (ATMEL1500A or ALTERA 7030). In this circuit is implemented the synchronization circuits for trigger signals from the Analogue Module, a 24 bit binary counter to measure the flying time of sound across the pipe, and the bus drivers for the connection with the Central Unit. This module has the followings operation modes: arm, trigger, count, stop and read the counter.
The Central Unit is build around the “ATMEL89C55” processor. This module provides the control signals for the Digital Module, make the connection with the local LCD, keyboard and the specific interfaces (M-Bus and RS232). The processor performs the algorithm to measure the water flow, making a statistical mediation of flying time in bought flow senses along hundreds of sound pulses. In this way, at each second is calculated the water velocity, and then the flow.
A bloc representation of the flow-meter is represented in Figure 6.2.
The “Ultrasonic Flow-meter for Water” has the following main performances:
For a pipe with the diameter of 65mm:
Maximum Flow: 72 [m 3 /h];
Nominal Flow: 36 [m 3 /h];
Minimal Flow: 1.44 [m 3 /h];
Data displayed: Instant-Flow, Cumulated-Volume-Flow, Total-Working-Time,
Maximum-Flow Instant-Sound-Speed in water.