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？k？ 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. 1. Introduction In the field of oceanography, salinity is an important property of water. Simply put, it is defined as the total amount of dissolved salts in one kilogram of water. The degree of salinity can be expressed in parts per thousands, where the oceans have an average salinity close to 35‰, and freshwater has salinity below 0.5‰. Salinity is constantly changing through events like evaporation, precipitation, and ice formation and melting. It is an essential ecological and environmental factor that has a great influence on which kind of life can prevail and which plants can grow. It affects water usage all over the world, determining which waters are potable and which can be used for irrigation. It is therefore important to be able to measure and monitor the salt content of water. Furthermore, the salinity of water plays a critical role in the climate of the planet, where the density, which is usually not measured directly but calculated from salinity, temperature, and pressure measurements, is the driving force behind the world’s ocean circulation through convection, the rising and sinking of water [1]. The salinity and temperature of water also affect other phenomena, such as the solubility of oxygen (O2) and carbon dioxide (CO2), the vaporization and ionization of water, and the dissolution of solid calcium carbonate (CaCO3) [2]. As CO2 from
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