Positive displacement pump, comprising at least a cylinder (11), provided with at least an entrance aperture (14) and at least an exit aperture (15) for the liquid and a plunger (12), mobile inside the cylinder (11) by means of a first actuator (20) and configured to transfer the liquid from the entrance aperture (14) to the exit aperture (15); the cylinder (11) is provided with at least a first inactive position (PI) and at least a second working position (P2) between which it can be selectively positioned in an automatic and adjustable manner by at least one second actuator (25).
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