The measurement of electrolytic conductivity is widely applied as a control parameter and its relevance is continously increasing, not only in industrial applications but also in the environmental monitoring domain. In this work the attention is focused on the electrical behaviour of a low cost in-situ four electrode conductivity sensor for water quality monitoring in estuaries and oceans. The design of the sensor, the method used to determine the conductivity value, the circuit developed for signal conditioning and the data acquisition board that links the sensor to the computer for further signal processing are described in detail. The output values of the conditioning circuit are stored in the computer and compared with more accurate conductivity values obtained from commercial equipment. In order to obtain more accurate results algorithms for digital signal processing have been presented and implemented.