Abstract: The improved gas flow control apparatus controls the flow of multiple gases to a tonometer for calibration of a blood gas or pH sensor and compensates for the failure of gas supply solenoid valves to operate properly in controlling gas flow. Intermediate gas supply solenoid valves select between gas flow of the supplied gas in an open position, and venting to the atmosphere in a closed position. A common selector valve is also provided in the gas flow path to the tonometer to select gas flow from one of the intermediate gas flow valves. A control unit is provided for controlling the operation of the intermediate and common selector valves.