An apparatus for adding at least one added gas (Gn), comprising at least one component (DAn), to an inspiratory gas, to formulate a respiratory gas for delivery to a subject and for maintaining a targeted concentration of the component in a volume of the respiratory gas (DAnT), characterized by comprising: a control system including a processor configured to: (a) obtain input of confirmed incremental volumes of the inspiratory gas or the respiratory gas flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (Gn) flowed to the subject with respect to the time points of interest; and (c) compute for a respective time point (Tcurrent) an error signal (en) representing a volume of the added gas (Gn) to be made available for inspiration with respect to a respective time interval (ΔT), wherein the respective time interval is defined by the respective time point (Tcurrent) and an ensuing time point (Tcurrent+), so that the cumulative volume of DAn results in the target concentration of the component gas (DAnT); wherein the control system is operable to provide an output signal to a gas delivery device (GDn) for any respective time point (Tcurrent) based on the error signal (en) computed for the respective time point (Tcurrent) such that the cumulative volume of DAn is controlled to target the target concentration of the component gas (DAnT) within the respective time interval (ΔT). a control system including a processor configured to: (a) obtain input of confirmed incremental volumes of the inspiratory gas or the respiratory gas flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (Gn) flowed to the subject with respect to the time points of interest; and (c) compute for a respective time point (Tcurrent) an error signal (en) representing a volume of the added gas (Gn) to be made available for inspiration with respect to a respective time interval (ΔT), wherein the respective time interval is defined by the respective time point (Tcurrent) and an ensuing time point (Tcurrent+), so that the cumulative volume of DAn results in the target concentration of the component gas (DAnT); (a) obtain input of confirmed incremental volumes of the inspiratory gas or the respiratory gas flowed to a subject with respect to time points of interest; and (b) obtain input of confirmed incremental volumes of added gas (Gn) flowed to the subject with respect to the time points of interest; and (c) compute for a respective time point (Tcurrent) an error signal (en) representing a volume of the added gas (Gn) to be made available for inspiration with respect to a respective time interval (ΔT), wherein the respective time interval is defined by the respective time point (Tcurrent) and an ensuing time point (Tcurrent+), so that the cumulative volume of DAn results in the target concentration of the component gas (DAnT); wherein the control system is operable to provide an output signal to a gas delivery device (GDn) for any respective time point (Tcurrent) based on the error signal (en) computed for the respective time point (Tcurrent) such that the cumulative volume of DAn is controlled to target the target concentration of the component gas (DAnT) within the respective time interval (ΔT).