An apparatus for adding at least one added gas (G n ), comprising at least one component (DA n ), to an inspiratory gas (G 0 ), to formulate a respiratory gas (G R ) for delivery to a subject and for maintaining a targeted concentration of the component in a volume of the respiratory gas (DA n T ) , characterized by comprising: a control system including a processor configured to: (a) obtain input of confirmed incremental volumes and confirmed cumulative volumes of the inspiratory gas (G 0 ) and the respiratory gas (G R ) flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (G n ) flowed to the subject with respect to the time points of interest; and (c) compute for successive respective time intervals between contiguous time points of interest, an error signal (e n ) representing a volume of the added gas (G n ) to be made available for inspiration with respect to the respective time interval so that the cumulative volume of added gas (G n ) in the cumulative volume of respiratory gas (CVG R ) makes up the target concentration of the component gas (DA n T ); wherein the control system is operable to provide an output signal to a gas delivery device (GD n ) for the respective time interval based on the error signal (e n ) computed for the respective time interval such that the cumulative volume of added gas (G n ) in the cumulative volume of respiratory gas CVG R is controlled to target the target concentration of the component gas (DA n T ). a control system including a processor configured to: (a) obtain input of confirmed incremental volumes and confirmed cumulative volumes of the inspiratory gas (G 0 ) and the respiratory gas (G R ) flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (G n ) flowed to the subject with respect to the time points of interest; and (c) compute for successive respective time intervals between contiguous time points of interest, an error signal (e n ) representing a volume of the added gas (G n ) to be made available for inspiration with respect to the respective time interval so that the cumulative volume of added gas (G n ) in the cumulative volume of respiratory gas (CVG R ) makes up the target concentration of the component gas (DA n T ); (a) obtain input of confirmed incremental volumes and confirmed cumulative volumes of the inspiratory gas (G 0 ) and the respiratory gas (G R ) flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (G n ) flowed to the subject with respect to the time points of interest; and (c) compute for successive respective time intervals between contiguous time points of interest, an error signal (e n ) representing a volume of the added gas (G n ) to be made available for inspiration with respect to the respective time interval so that the cumulative volume of added gas (G n ) in the cumulative volume of respiratory gas (CVG R ) makes up the target concentration of the component gas (DA n T ); wherein the control system is operable to provide an output signal to a gas delivery device (GD n ) for the respective time interval based on the error signal (e n ) computed for the respective time interval such that the cumulative volume of added gas (G n ) in the cumulative volume of respiratory gas CVG R is controlled to target the target concentration of the component gas (DA n T ). <EOS>