Miniature Towed Oceanographic Conductivity Apparatus

A miniature oceanographic conductivity measuring system has been built and sea tested. It consists of a four-electrode planar sensor cell and a circuit that maintains a fixed-magnitude alternating potential across the two receiving electrodes. The cell and electronics mount onto a single 2 cm by 10 cm by 20 cm fairing on an oceanographic instrument array (or chain). Various annular electrode arrangements have been successfully used. The electrode surfaces are a platinum-gold alloy, black platinized to reduce electrode contact impedance. The planar electrodes have never fouled with detritus at sea. The circuit ground is directly coupled to the seawater through the sensor electrodes, and is isolated from grounds in the oceanographic chain by a miniature transformer-coupled power isolator. An optical analog isolator floats the output signal. The preemphasized output has a useful bandwidth of 100 Hz. Long-term stabilities better than 0.005 S/m and high frequency sensitivities in the order of S/m per root Hertz have been achieved.