Cost Effective CTD for Long Term Deployments in Water Columns

Estuaries have the particularity of being in constant change concerning the salinity of the water, since the river carries fresh water to the mouth, while the force of the tide pushes salty water upstream, reaching several kilometers upstream given the right conditions. These bodies of water do not mix instantly, allowing the appearance of a distinct border between the two, known as the front. This document presents a monitoring system composed of four-probe salinity sensors, which acquire conductivity, temperature, and pressure data to calculate salinity, arranged in such a way as to make measurements along the water column, to detect the shape of the front, as well as monitoring the estuary. The sensors, and the monitoring system in general, are cost-effective, low-power, and accurate in long-term deployment, even when installed in adverse conditions such as at sea. The sensors consume $26 \mu \mathrm{A}$ in sleep mode and 10 mA during measurement, and the measurement time is less than 100 ms. The choice of a four-point configuration allows overcoming the inevitable decay of the electrodes and possible measurement errors, contributing to autonomous long-term operation.