Abstract: A system for detecting the concentration of hydrocarbons (petroleum products) or other pollutants in water having a differential sound velocimeter which compares the velocity of sound in the liquid under study with the velocity of sound in the liquid (largely water) from which the oil or other pollutant has been separated by mechanical and/or solvent extraction means. A suitably calibrated frequency-to-analog converter is utilized to indicate the amount of the oil concentration and/or shutdown the water transporting system if the concentration exceeds a predetermined value.