An apparatus for controlling blending of a gas mixture containing known components, including first, second, and third control valves for controlling the flow of first, second, and third components, respectively, a first gas density sensor to measure the density of a first mixture of the first and second components, a second gas density sensor to measure the density of a second mixture of the first mixture and the third component, and a controller to determine based on data from the first and second gas density sensors the relative compositions of the first, second, and third components in the second mixture, and to control the first, second, and third control valves to obtain a desired relative composition of the first, second, and third components in the second mixture.