Proportional dosimeter including a metering body with a main liquid inlet (2) and an outlet (3), a hydraulic motor (4) housed in the body, actuated by the main liquid, and connected to a plunger piston (5) that moves in a first chamber (6), the piston plunger drawing in fluid in an outbound stroke, a check valve provided to allow liquid to pass to the interior volume of the body when the pressure in the first chamber exceeds a certain pressure, during the return stroke; the dosimeter includes a second chamber (12) of variable volume bounded by a membrane subjected to the pressure in the first chamber, the second chamber including an intake orifice (20) for the auxiliary liquid and a delivery orifice (23) for the auxiliary liquid, this delivery orifice being connected by a pipe (26) to an injection chamber (S) situated downstream of the outlet of the dosimeter body.