A displacement pump to be used on knapsack, used in agriculture and gardening for spraying, weeding gardens, vegetable gardens and small crops, operated via a hand lever (P) which sucks the liquid contained in a tank (K) and ejects it atomised under pressure. Such a displacement pump is of the type including a cylinder (2), within which a piston (3) is slidable, two one-way check valves that allow, alternatively, the inflow of liquid from the tank (K), through two holes present on the base, to a lower chamber or suction/compression chamber and feed it, through two holes made on the piston, into an upper chamber or accumulation chamber connected to an external duct (W). Such a displacement pump (1) is characterised in that on the base there is at least one calibrated hole, which connects the inside of the tank (K) to the suction chamber.
