Abstract: An apparatus and method are provided for determining the relative density of a flowing fluid based upon the ratio of the fluid's flow rate before and after a material is added. Using analog or digital components, two signals are generated, representing the two flow rates, a first ratio of the two signals is derived, the ratio is compared to a constant, a second ratio of a second constant to the first ratio is derived, and the result of the comparing step is multiplied by the second ratio to generate a density signal representative of the relative density of the fluid after the material has been added.