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 (A) substantially parallel to a main direction wherein the fluid is intended to flow in the passage , the rotor comprising a plurality of blades , - 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 (A) 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 (A) substantially parallel to a main direction wherein the fluid is intended to flow in the passage , the rotor comprising a plurality of blades , - 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 (A) 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 . <EOS>