A multiple-channel gas delivery system, comprising: a primary channel; at least three secondary channels (122a, 122b... 122i); and a flow ratio controller coupled to the primary channel and the secondary channels so as to receive the gas through the primary channel and control the ratio of the flow rate of the gas through each of the at least three secondary channels relative to the entire flow rate, the flow ratio controller including an input for receiving flow ratio set point inputs, one for each of the secondary channels (122a, 122b...122i); wherein each secondary channel includes: a flow sensor providing a flow rate signal; and a valve actively controlled by a single input single output (SISO) feedback controller; wherein the flow ratio controller is configured to: sum the flow rate signals to generate a measured total flow; calculate a measured flow ratio for each secondary channel using the flow rate signals and the measured total flow; compare the measured flow ratios with the set of flow ratio set points thereby to generate an error signal for each secondary channel; and input the respective error signal into the single input single output controller for each secondary channel to generate the valve control command. a primary channel; at least three secondary channels (122a, 122b... 122i); and a flow ratio controller coupled to the primary channel and the secondary channels so as to receive the gas through the primary channel and control the ratio of the flow rate of the gas through each of the at least three secondary channels relative to the entire flow rate, the flow ratio controller including an input for receiving flow ratio set point inputs, one for each of the secondary channels (122a, 122b...122i); wherein each secondary channel includes: a flow sensor providing a flow rate signal; and a valve actively controlled by a single input single output (SISO) feedback controller; wherein the flow ratio controller is configured to: sum the flow rate signals to generate a measured total flow; calculate a measured flow ratio for each secondary channel using the flow rate signals and the measured total flow; compare the measured flow ratios with the set of flow ratio set points thereby to generate an error signal for each secondary channel; and input the respective error signal into the single input single output controller for each secondary channel to generate the valve control command. a flow sensor providing a flow rate signal; and a valve actively controlled by a single input single output (SISO) feedback controller; wherein the flow ratio controller is configured to: sum the flow rate signals to generate a measured total flow; calculate a measured flow ratio for each secondary channel using the flow rate signals and the measured total flow; compare the measured flow ratios with the set of flow ratio set points thereby to generate an error signal for each secondary channel; and input the respective error signal into the single input single output controller for each secondary channel to generate the valve control command. sum the flow rate signals to generate a measured total flow; calculate a measured flow ratio for each secondary channel using the flow rate signals and the measured total flow; compare the measured flow ratios with the set of flow ratio set points thereby to generate an error signal for each secondary channel; and input the respective error signal into the single input single output controller for each secondary channel to generate the valve control command.