The invention relates to a pump including: a gas mixing housing (1); ducts (2, 3) for supplying said housing (1) with HP and IP gases and giving into said mixing housing (1); an outlet duct (3)for the mixed gases; a pneumatic actuator (10) provided in said HP-gas supply duct (2) including a mobile piston connected to a stopper (12) whose position in said duct (2) determines a passage section of the HP gas towards said mixing housing (1); a control chamber (13) of said mobile piston adapted to be fed under pressure for determining the position of the mobile piston; a pneumatic circuit provided between a pressure source and said control chamber (13); characterised in that the pneumatic circuit includes at least one leak valve (22) adapted for generating a leak in the pneumatic circuit for modifying the pressure supplied to said control chamber (13).
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