A gas supply apparatus with improved control is provided. The gas supply apparatus provides gas at an outlet junction. The apparatus includes a gas mixer for gasses from primary and secondary supplies to a given ratio. The apparatus also includes a gas reservoir supplied by the gas mixer. A tertiary supply valve is also included and connected in parallel with the reservoir. The tertiary supply valve is adapted to connect the outlet junction to a tertiary gas supply when gas is not being supplied to a mixer by the primary and/or secondary supply.