A gas mixing system for providing mixed gas for intraocular injection. In some embodiments, a first fixed-volume chamber is automatically purged and filled with gas from a first gas supply input, to a first predetermined pressure. A second fixed-volume chamber is purged and filled with gas from a second gas supply input, to a second predetermined pressure. The first and second predetermined pressures are determined based on a desired concentration of gases in the final mix, and the respective volumes of the first and second fixed-volume chambers and of a third fixed-volume chamber. Gas from the first fixed-volume chamber is then allowed to mix with gas in the third fixed-volume chamber, which was previously purged. Next, gas from the second fixed-volume chamber is allowed to mix with gas in the third fixed-volume chamber. Finally, the mixture of gases in the third fixed-volume chamber is expressed into an intraocular syringe.
