Design and Performance of Capacitive Coupled Conductivity Sensor for Marine Applications

Conductivity is the intrinsic property of sea water from which the salinity can be determined, and the measurement of salinity is a regulatory requirement. This work is aimed at developing a conductivity sensor for Ocean Observation applications. The concept of capacitive coupling is attempted in combination with contact electrodes while designing the conductivity probe which eliminates the polarization and lead, or electrode contamination issues associated with regular electrode type conductivity measurement methods. This approach also confines fixed the cell constant and hence the defined current path in the probe. In this method, the probe design combines the advantages of contact type and capacitively coupled method. The probe has three circular contact type electrodes (exciting electrode, receiving electrode and dummy electrode) and two circular capacitively coupled voltage electrodes (dielectric coating over the metal electrodes). The contact type electrodes act as the current electrodes similar to the four-electrode method; Contactless electrodes act as the potential electrodes. When the exciting electrode is excited, the current will start flowing into the water and collected by the receiver electrode. If used for conductivity measurement of sea water the entire probe will be surrounded by sea water and insulator over the electrode will avoid direct contact of electrode with surrounding water. In such an application, dummy electrode will make sure that no current flows in the surrounding water and ensures that the path of current includes only the water inside the pipe between the excitation and receiver electrodes. The insulated voltage electrodes are metal electrodes coated with a dielectric material. Thus, a capacitance is formed between the metal electrodes and the outer surface of water in the tube. Suitable sensitive electronic circuitry is developed to measure the resistance of the water column in-turn the conductivity of liquid is measured.

Keywords: Conductivity, Ocean observation, Contact electrodes, Capacitive coupling, Dielectric material.