Robust Ocean Salinity Sensing

Abstract

We report a low power ocean salinity sensor based on capacitance measurements. Our design compares two carefully tuned astable multivibrator oscillators to measure small frequency shifts in the sensing oscillator induced by changes in salt concentration. Comparing the difference in the number of oscillator cycles during a measurement time interval enables the calculation of a frequency shift from which a corresponding salinity change is fitted. We demonstrate a design by which this oscillator cycle-count comparison can be conducted without the need for a microcontroller which enables very low power operation. Our fabricated sensor is sensitive to a frequency shift as small as 0.024% allowing sensitivity to changes in salinity of less than 0.1 parts-per-thousand (ppt). Our sensor’s sensitivity to changes in salinity is amplified by a conductive hydrogel occupying the space between the parallel plates and whose permittivity varies with the salinity of the water absorbed.