Design, fabrication and evaluation of zero external field conductivity cell for CTD sensor

An indigenous four electrode conductivity cell configuration which has no external electric field so that measurement is not affected by presence of nearby external objects for measuring ocean water salinity is investigated. Finite element modeling is used to optimize the cell dimension for achieving zero external field effect at both ends of the cell. The conductivity cell is fabricated with double side copper clad PCB. The cell impedance as a function of frequency is studied in the 1 to 40 kHz range and at lower frequency range the cell behavior is found to be ohmic. The conductivity is measured for 0.1 N and 0.01 N standard KCl solutions and compared with ASTM D 1125. A standard solution of 32 ppt is evaluated and conductivity is found to be 52.4 mS/cm at room temperature (25°C).