Resumen de Mesura de la conductivitat de l'aigua amb elèctrodes capacitius
Electric conductivity is a good estimator of water quality. It can be measured by using cells with direct contact between the electrodes and the electrolyte, inductive cells or cells with capacitive electrodes. The use of capacitive electrodes is a need when measurements are made from the outside of an insulating container, and it is advisable when electrode degradation is to be avoided. Electrical impedance models for measurements using capacitive electrodes often obviate parasitic capacitances that may affect the measured values. The instruments commonly used for measuring the electrical impedance of materials are insensitive to some of these capacitances, but not to all of them. Some authors have pointed that parasitic capacitances may lead to unpredicted results, suck as inductive effects, but most of the described phenomena are related to direct contact electrodes. Regarding capacitive electrodes there are few descriptions of stray impedances, even fewer analysis and no known solution. The main objective of this work is to define and to verify a method to measure the conductivity of water in non-conductive containers using capacitive electrodes on the outer side of the recipient. The impedance of water (Rx, which contains the information about conductivity, and Cx), the impedance of the electrode (Re and Ce), and the impedance between electrodes (Rhl and Chl) are modelled by a resistance in parallel with a capacitance. The parasitic capacitance Cg between the material under test and erath ground is an element with distributed parameters, and its effects are different depend on the measuring system. This study is focused on those systems that apply a voltage to the material and that measure the output current at the material, in particular on auto-balancing bridges. Cg disminishes the measured current, which is shown as an inductive effect as the calculated impedance increases with frequency. The analysis performed shows the gyration undergone by Cg and the effect is an ¿apparent¿ impedance connected in series with the impedance under test and the impedance of the electrodes. For materials modeled by Rx in parallel with Cx, besides the inductive effect, there are resistive and capacitive effects also due to Cg. This effects due to Cg depend on the ratio Cg/Ce. When direct contact electrodes are used, the impedance of the electrodes and the leakage impedance have no significant effect. Electrodes have influence at the lower frequencies whereas the effects of Cg appear at higher frequencies. In the middle range, there is a flat frequency band where Rx can be easily measured. If Cg is higher than Cx, peaks in the magnitude of the impedance as well as positive phases may be found. The relevant parameter is not the value of Cg but the ratio Cg/Cx. Measurements using four electrodes do not avoid the effects of Cg. In addition to inductive effects and resonances at high frequency, There is, when Cg increases, also an increment of the impedance that is independent from the frequency. The effects of Cg and Chl are more noticeable for capacitive electrodes. The leakage resistance Rhl shunts Re, and when Rhl decreases, the real part of the impedance increases at low frequencies. The effect of the parasitic capacitance between the electrodes Chl is a reduction of the real part of the impedance that is frequency independant. When Cg increases, the real part of the impedance increases, and this increment is not frequency dependant either. If Ce and Chl are known, Cg can be obtained from the measurement at low frequency of the imaginary part of the impedance, since increasing Cg makes the equivalent capacitance between the electrodes to decrease. Once Ce, Chl and Cg are known, Rx can be calculated in the frequency band where the real part of the impedance is flat. In addition, setting Chl and Cg, for instance by shielding the measurement cell, the parameters that depend on Ce, Chl and Cg can be included in an equivalent constant cell.
