THE COMPARISON OF THE TRANSDUCER CIRCUITS FOR THE TWO-PHASE FLOW CONDUCTIVITY PROBE

The void fraction of two-phase flow can be measured by conductivity probe due to the difference of conductivity between air and water. It's difficult to detect the conductivity of the probe directly so transducer circuits should be used to transfer the conductivity to voltage, and the voltage can be sampled by DAQ device. Two transducer circuits are designed for two-phase flow double sensor conductivity probe. One is a simple DC circuit, it's made up of a constant DC voltage and a resistance, and the resistance is cascaded with the probe. Voltage drop across the resistance is depend on conductivity of surrounding media of probe tip. The other is AC circuit that utilizes a DDS (Direct Digital Synthesizer) as an excitation power supply which links to a transimpedance amplifier through the probe. The current that flow to the amplifier would change according to the surrounding's medium of the probe. The transimpedance amplifier transfers the current to voltage. Then we use AD8302, an amplitude detection chip, to detect the amplitude of the s output. The experiments show that the DC circuit has a higher SNC (signal to noise ratio) and shorter responding time. However, the needle of probe is covered with electrochemical corrosion products and its performance degrades significantly when DC circuit is used and the probe is immersed in 80 degrees C hot water for 8 hours. This kind of phenomenon is not appeared on the probe when DDS-based circuit is used. The result of the experiment verifies the SNC and responding time of DDS-based circuit is approximately equal to that of the DC circuit.