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).