Gas mixing apparatus for respirator

A gas mixing apparatus, comprising:

• a first chamber having an inlet for connection to a supply of a first gas:
• a second chamber having an inlet for connection to a supply of a second gas;
• a third, mixing chamber having an outlet for a mixture of said first and second gases;
• the first and second chambers having first and second passageways, respectively, connecting said first and second chambers, respectively, to said third chamber;
• a first piston slidably mounted in said first passageway and having a first opening defining a first valve seat;
• a first valve member movably mounted in said first chamber for movement between a closed position seated against said first valve seat to close said opening and prevent gas flow between said first chamber and said mixing chamber, and a second position defining a maximum flow area orifice between said valve member and valve seat;
• a second piston slidably mounted in said second passageway and having a second opening defining a second valve seat;
• a second valve member movably mounted in said second chamber for movement between a closed position seated against said second valve seat to close said opening and prevent gas flow between said second chamber and said mixing chamber, and a second position defining a maximum flow area orifice between said valve member and valve seat;
• an adjustment mechanism linked to said first and second valve members to control the position of each valve member, whereby the areas of the orifices between each valve member and valve seat are controlled to select the proportions of the two gases supplied to the mixing chamber;
• the first and second pistons being connected together and being movable relative to said valve seats to vary the flow area of each orifice in response to variations in pressure drop across the respective pistons;
• and
• each valve member and valve seat having opposing surfaces of predetermined shape for forming a series of orifices of progressively increasing area as said valve members and valve seats move relative to one another from said closed position to said maximum flow area position, each series corresponding to the same geometrical progression having a predetermined progression ratio.