Toroidal Seawater Conductivity Sensors and Instrumentation for Antisubmarine Warfare

This article describes the development of toroid-based seawater conductivity sensors and instrumentation for 0–60-mS/cm conductivity range and 50-bar hydrostatic pressure operational capability which is equivalent to 500 m of ocean depth. Double core toroidal transformer concept was used and investigated for Manganese–Zinc (Mn–Zn) and Fe-based nanocrystalline cores, turn’s ratio, and temperature dependence. Different excitation and sense coil turn numbers were fabricated by keeping the turn’s ratio 1:2, 1:4, 1:6, and 1:8. Optimum frequency of Mn–Zn-based conductivity sensors with lower and higher turn numbers were observed for turn ratios 1:8 and 1:6 at 120 and 90 kHz, respectively. Sensors with turn ratio ${N}_{{1}}$ : ${N}_{{4}} = {1}{:}{8}$ in “lower turn ratio” offered a high sensitivity value of 20.6-mV/mS $\cdot $ cm−1 conductivity. Analytical sensor responses versus the conductance plot developed are in good agreement with the experimental plot. Fe-based nanocrystalline core with a turn ratio of 1:8 shows an optimum frequency of 70 kHz and gives a sensitivity of 18 mV/mS $\cdot $ cm−1. Temperature dependence of Fe-based nanocrystalline core sensors was linear compared with Mn–Zn-based sensors. The mechanical design and frequency of operation of developed conductivity sensors give inherent immunity to the antisubmarine warfare (ASW) band of frequency interference. The efficacy of conductivity sensors and measuring electronics was proved experimentally up to 500 m in sea profiling with commercial sensors and obtained a root-mean-square error (RMSE) of 0.42 for profile data.