For the hydrodynamic characterisation of reactors handling aqueous liquids, especially in conjunction with the validation of numerical simulations, data with high temporal resolution is essential. This paper describes the design of a rapid, flat, and low-cost sensor array for parallel salinity measurements, which allows to perform the required measurements in nearly any reactor type with minimal impact on the flow characteristics due to its very thin (125 mu m) and flexible structure. Sample measurements for the sensor array system are shown using a step change in aqueous salinity and the resulting response of a thin-layer cascade algae reactor unit as well as performing measurements of a pulse in salinity in the circularly operated algae reactor containing a feedback loop. The system's response, mixing, and cycle time can be easily extracted from the generated data sets. The sensors provide measurements with an accuracy of about 2% for a linear range of 0-10 g l(-1) NaCl solution. In its default configuration one device consists of up to eight sensors, which are distributed on two stripes (220 x 25 mm, 125 mu m thick) each containing four sensor spots, that are positioned 5 cm apart from each other. The modular set up of the sensor array facilitates simple modification depending on individual requirements. (C) 2017 Elsevier Ltd. All rights reserved.