Abstract: A method for the control of proportion of two gases which are delivered to a mixed gas line for use for example in a respiratory breathing apparatus in which the gases are delivered to a mixed gas line from a pressure control vessel, comprises continuously monitoring the pressure in the control vessel and when the pressure therein is at an initial predetermined pressure admitting a first gas into the pressure control vessel until the pressure therein increases to a predetermined higher pressure and then stopping the admission of the first gas and initiating the admitting of a second gas into the pressure control vessel, continuing to admit the second gas until a predetermined additional higher pressure is achieved in the pressure control vessel, discontinuing the admission of the second gas when the predetermined additional higher pressure is reached and initiating the withdrawal of the mixed gas through the mixed gas line, continuing to withdraw the mixed gas through the mixed gas line until an initial press