Disclosed is a monitored proportional metering device including a liquid metering pump having an inlet, an outlet, a suction nozzle, and a mechanism for adjusting the flow in the nozzle. The metering device is also includes a detection assembly, the assembly including at least a unit for detecting the pressure variation in the nozzle, which unit is arranged between the first suction valve and the mixing chamber, a water meter at the inlet, a probe for measuring the level in the container of material to be suctioned, a unit for determining the position of the mechanism for adjusting the volume to be suctioned, and a man/machine interface for processing, recording, and displaying the data from the detection unit. Also disclosed are methods for monitoring a metering pump implementing the proportional metering device.