Abstract: The proportioning valve apparatus includes a cylindrical valve body having a first inlet for receipt of a first fluid, a second inlet for receipt of a second fluid, a mixing chamber and an outlet for releasing a mixture of the two respective inputs. A rotary barrier element is positioned adjacent to the first and second inlets and operates to selectively impede the flow of fluids through first and second inlets and into the valve chamber. Rotation of the rotary valve barrier in a first direction is operative to close the first inlet while simultaneously opening the second inlet. The present invention further includes a closed loop control system utilizing measurement of the density of the output mixture to control the activation of the proportioning valve.