Gas mixing apparatus

A gas mixing apparatus for providing a gas mixture containing at least first, second and third gases, the proportion by volume of the first gas in the gas mixture being maintained above a predetermined lower level, comprising first, second and third paths respectively for the passage therethrough of said first, second and third gases towards a common mixing means, the second and third paths each including a rotatable needle valve for controlling the flow of its respective gas therethrough, the first path being divided for at least a part of its length into at least first and second by-pass passages, each including a rotatable needle valve for controlling the flow of the first gas therethrough, first means interconnecting the needle valve associated with the first by-pass passage with the needle valve associated with the second path and second means interconnecting the needle valve associated with the second by-pass passage with the needle valve associated with the third path, the first and second interconnecting means both being arranged so that rotational movement to open either of the needle valves associated with the second and third paths will also cause the needle valves associated with the first and second by-pass passages respectively to rotate to open to prevent the percentage of the firs gas by volume falling below a predetermined proportion.