An Auto-Balancing Signal Conditioning Scheme for Non-contact Measurement of Conductivity of Water

摘要

This paper presents a signal conditioning approach to measure resistance and hence conductivity of water in an insulating tube through capacitively coupled electrodes. In the scheme presented, the electrodes are electrically insulated from water and measurement is taken through couple of capacitances formed between the electrode and water column. The capacitive coupling overcomes the problems of electrode polarization and contamination associated with conventional contact based approach of conductivity measurement of water. The large reactance of the coupling capacitors, compared to the resistance of the water column under measurement, is a challenge. Moreover, the variations in the coupling capacitor over time presents another challenge. The auto-balancing signal conditioning scheme presented here overcome these challenges by providing an output that is directly proportional to the resistance under measurement and is independent of the value of the coupling capacitors. Test results on a prototype of the proposed circuit show that the maximum error in the resistance measurement is less than 0.9 % and the output is independent of the coupling capacitors.