Gas blending apparatus

An apparatus for supplying a gaseous mixture to an external system on demand, comprising:

• (a) a source of a first gas under pressure;
• (b) a source of a second gas under pressure;
• (c) a gas blanding means in communication with the external system and in communication with said sources of said first and second gases for adjustably blending said gases into a gaseous mixture of pre-determined proportions and for supplying said gaseous mixture to the external system on demand, said gas blending means comprising a high flow rate stage and a low flow rate stage;
• (d) pneumatic rely means in communication with said high flow rate and low flow rate stages of said gas blending means and said sources of said first and second gases for supplying said second gas to said gas blending means on the demand of the external system, said pneumatic rely means including first valving means operable by said first gas under pressure for controlling the flow of said second gas to said gas blending means upon the demand of the external system;
• (e) differential pressure control means in communication with said high flow rate stage of said gas blending means and with said external system for controlling the flow of gases between said high flow rate stage and said external system;
• and
• (f) control means in communication with said sources of said first and second gases for controlling the flow of said first gas to said pneumatic rely means, said control means including second valving means operable by said second gas under a predetermined elevated pressure for controlling the flow of said first gas to said pneumatic rely means.