Metering mechanism for a proportioning pump, and associated pump and usage method

A metering mechanism, comprising:

• a jacket (3) mounted in an adjusting sleeve (4);
• a metering body (10) mounted to move in translation in the jacket (3), said adjusting sleeve being adapted to cooperate by way of a screwing movement with said jacket, and the screwing movement driving the metering body in translation,
• said metering body being provided at a first end thereof with a shut-off check valve (11) and accepting at a second end thereof a plunger piston (9), an alternating movement in translation of said plunger piston generating a suction at a level of the first end of the metering body and then expulsion into a volume (V) surrounding the second end of the metering body via a passage formed in at least one sealing device (J1, J93, J91),
• the metering body including a central first cylinder (101) and a peripheral second cylinder (102) that are concentric and fastened to one another,
• the plunger piston being fastened to a first sealing device (J1) and a second sealing device (J93) respectively mounted to bear against the first and second cylinders, so that a reduced pressure zone is created inside the first and second cylinders, respectively, when the first and second sealing devices (J1, J93) are moved away from the end of the body provided with the check valve,
• the reduced pressure zone of the central first cylinder being adapted to communicate with the reduced pressure zone of the peripheral second cylinder, and the reduced pressure zone of the peripheral second cylinder being adapted to communicate with the volume (V) surrounding the second end of the metering body;
• and
• a selector (6) mounted on the central first cylinder and mobile relative thereto over a stroke delimited by a first position, in which a first communication (e1) between the reduced pressure zones of the first and second cylinders is cut off while a second communication (e2) between the reduced pressure zone of second cylinder and the volume (V) surrounding the second end of the metering body is open, and a second position, in which the first communication (e1) between the reduced pressure zones of the first and second cylinders is open while the second communication (e2) between the reduced pressure zone of the second cylinder and the volume (V) surrounding the second end of the metering body is cut off,
• wherein the passage formed in the at least one sealing device (J1, J93, J91), which enables expulsion into the volume (V) surrounding the second end of the metering body, is provided at one of i) a level of the second sealing device (J93), said second sealing device (J93) being a seal that opens when the plunger piston (9) moves toward the check valve, or ii) a level of a lip seal (J91) mounted on the plunger piston (9), said lip seal (J91) being open when the plunger piston moves toward the check valve.