A density observer for estimating an actual density of a material with a proportional integral controller responsive to a density error determined by subtracting a feedback estimated density from a sensed density. A method of estimating a density of a material by determining a sensed density of the material, determining an estimated density of the material, determining a density error by subtracting the estimated density of the material from the sensed density of the material, and iteratively redetermining the estimated density based on the density error.