A densitometer for measuring a concentration of matter in a fluid system comprised of liquids and suspended and/or floating matter such as, for example, a densitometer for measuring the concentration of oil in bilge water that includes suspended and/or floating solid material. Two emulsifying conditions of the system are produced by first emulsification with a relatively high power emulsifying action and then a relatively low power emulsification. The amount of light which penetrates the system is a function of the turbidity which varies with the intensity of the emulsification. The concentration of oil in water, for example, in bilge water, is a function of the ratio of the amount of light which passes through the system in the two turbid states.