Novel Design of Ultrasound Flow Meter – Flow Tube Configuration
Figure 4 shows a schematic of the new flow-tube configuration. It consists of an axial tube made of stainless steel. Via the ultrasound flow meter, ultrasound is generated by PZT 5A transducers vibrating in a radial mode where the inner and outer diameters are selected to generate the required frequency. The waves generated by the transducers propagate through the liquid as a plane wave and an attenuating non-plane wave. The attenuating non-plane wave is symmetric about the central plane of the transducer. By the use of the weight function approach to the sensitivity of an ultrasound flow meter, when this wave interacts with an axi-symmetric velocity profile it provides no contribution to the transit time difference. The plane wave propagating down the flowtube provides averaging of the velocity profile irrespective of the flow profile and thus shows no change in sensitivity as the flow profile changes from laminar to turbulent.
Modelling of the Ultrasound Flow Meter
Extensive modelling of the ultrasound flow meter has been undertaken using PACSYS ultrasonic modelling software used in conjunction with a SPICE circuit simulation program. These simulations enable the design of the flowtube from an ultrasonic point of view to be optimised and the effects of the drive and receive electronics to be fully analysed.
Figure 5 shows the PACSYS model of a typical flowmeter configuration. The model is for half the meter length and the flowmeter and it is modelled with radial symmetry. Figure 6 shows the waves passing down the flow tube and Figure 7 shows typical predicted signal on the downstream transducer when the upstream transducer is excited with three cycles of a 200kHz square wave.
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