Abstract: A device for detecting the presence of contaminating water in a fuel tank as well as for measuring the density of the fuel in the tank which includes an elongated body of optical material arranged to be mounted in a fuel tank in a vertically extending position, one side face of the body forming an interface with the fluid in the tank, and a plurality of light sensors on the opposite face of the body arranged in spaced-apart relationship, together with light-emitting means at the bottom of the body which emit a divergent group of rays of successively increasing angles of incidence upwardly so that those rays exceeding the critical angle at the interface for a fuel of a specific index of refraction are reflected to illuminate those sensors corresponding to the rays exceeding the critical angle to thereby provide a detected output representing the density of the fluid, the light-emitting means providing a second group of rays striking the lower portion of the interface which are reflected when water is present t