A system for measuring the flowrate of a fluid including liquid comprising: - a body comprising at least one inlet, at least one outlet and at least one passage for the flow of fluid from the inlet to the outlet, - at least one rotor disposed inside the passage and configured to be rotated, by the flow of the fluid, around an axis substantially parallel to a main direction wherein the fluid is intended to flow in the passage, the rotor comprising a plurality of blades (1101b, 1101b'), - a device for measuring a rotational speed of the rotor, the measuring device comprising: - a module for emitting incident light radiation, - an optical module configured to o transmit the incident light radiation from the emission module onto the blades of the rotor, and o receive reflected light radiation, the reflected light radiation coming from the reflection, on the blades of the rotor, of the incident light radiation, - a conversion module configured to o determine the rotational speed of the rotor according to the reflected radiation and o determine the flowrate of the fluid according to the rotational speed of the rotor, and wherein the optical module is configured so that the incident light radiation is incident on the blades of the rotor in a direction substantially perpendicular to the axis of rotation of the rotor, and in that the measuring device further comprises a module for connecting the optical module to the body, the connection module being configured to establish a fluid-tight junction between the optical module and the passage, and characterised in that at least one blade of the rotor has a reflection coefficient of light radiation different from the reflection coefficient of light radiation of at least one other blade. - a body comprising at least one inlet, at least one outlet and at least one passage for the flow of fluid from the inlet to the outlet, - at least one rotor disposed inside the passage and configured to be rotated, by the flow of the fluid, around an axis substantially parallel to a main direction wherein the fluid is intended to flow in the passage, the rotor comprising a plurality of blades (1101b, 1101b'), - a device for measuring a rotational speed of the rotor, the measuring device comprising: - a module for emitting incident light radiation, - an optical module configured to o transmit the incident light radiation from the emission module onto the blades of the rotor, and o receive reflected light radiation, the reflected light radiation coming from the reflection, on the blades of the rotor, of the incident light radiation, o transmit the incident light radiation from the emission module onto the blades of the rotor, and o receive reflected light radiation, the reflected light radiation coming from the reflection, on the blades of the rotor, of the incident light radiation, - a conversion module configured to o determine the rotational speed of the rotor according to the reflected radiation and o determine the flowrate of the fluid according to the rotational speed of the rotor, and o determine the rotational speed of the rotor according to the reflected radiation and o determine the flowrate of the fluid according to the rotational speed of the rotor, and wherein the optical module is configured so that the incident light radiation is incident on the blades of the rotor in a direction substantially perpendicular to the axis of rotation of the rotor, and in that the measuring device further comprises a module for connecting the optical module to the body, the connection module being configured to establish a fluid-tight junction between the optical module and the passage, and characterised in that at least one blade of the rotor has a reflection coefficient of light radiation different from the reflection coefficient of light radiation of at least one other blade.