A low cost signal processing scheme has been developed. The method of driving and receiving used in the flowmeter employs reciprocal action. This is essentially driving the transducers from and receiving the signal into the same impedance. In this case we are driving from, and receiving into, a low impedance. This approach ensures that the transducers at zero flow have identical receive signals independent of matching of the transducers, coupling and liquid temperature. The only requirement for this to work is that the transducers and associated circuitry should be linear and that during reception the transducers do not see any signal generated by themselves since this signal is not reciprocal.
The transducers are thus driven from the output of a low impedance linear amplifier and the signal received from the transducers is received into a low impedance transimpedance amplifier. The signal is then passed through a band pass filter centred at the drive frequency before being input to a 2MHz 12bit ADC. Electronic switches in the system change the roles of the transducers from transmitter to receiver and the upstream and downstream transducer are excited sequentially. The data from the ADC in both directions is averaged over a number of transmissions and stored.
A novel time measurement algorithm then processes the signals to extract the time difference between them. This signal is then processed to compensate the measurement for the speed of sound in the fluid which enables the flowmeter to be used with a range of fluids and also to be independent of the speed of sound in the fluid. The electronics is capable of providing the user with a visual indication of flow, a pulse output giving a pulse per unit volume of flow or a 4-20mA output.
Figure 8 shows a typical performance of a 6mm flowmeter. The calibration shows that the flowmeter is capable of measuring from 1.5m3/hr to approximately 3l/hr. Over a range of 100: 1 it is well within +/-2%.
By changing the bore of the meter to 1mm flows as low as 0.2l/hr have been measured.
Calibration Characteristics of Flowmeter