Towards Chip-Based Salinity Measurements for Small Submersibles and Biologgers

Water’s salinity plays an important role in the environment. It can be determined by measuring conductivity, temperature, and depth (CTD). The corresponding sensor systems are commonly large and cumbersome. Here, a 7.5 × 3.5 mm chip, containing microstructured CTD sensor elements, has been developed. On this, 1.5 mm2 gold finger electrodes are used to measure the impedance, and thereby the conductivity of water, in the MHz frequency range. Operation at these frequencies resulted in higher sensitivities than those at sub-MHz frequencies. Up to 14 kΩ per parts per thousand salt concentration was obtained repeatedly for freshwater concentrations. This was three orders of magnitude higher than that obtained for concentrations in and above the brackish range. A platinum electrode is used to determine a set ambient temperature with an accuracy of 0.005°C. Membranes with Nichrome strain gauges responded to a pressure change of 1 bar with a change in resistance of up to 0.21 . A linear fit to data over 7 bars gave a sensitivity of 0.1185 /bar with an of 0.9964. This indicates that the described device can be used in size-limited applications, like miniaturized submersibles, or as a bio-logger on marine animals.