Calibration unit for optical detector

An in-line sensor for determining a parameter of a process medium flow, the sensor comprising:

• a light source embodied to generate light along an optical path, the light source including a source aperture disposed along the optical path and a connection adjacent the source aperture;
• a detector arranged in the optical path and embodied to detect the light generated by the light source, the detector including a detector aperture disposed along the optical path and a connection adjacent the detector aperture;
• a flow cell disposed between and reversibly attachable via complementary connections to the light source and the detector, the flow cell having opposing sides including opposing apertures therethrough and in the optical path adjacent the complementary connections, the flow cell further embodied to enable a fluid to be analyzed to flow along a flow path, the flow path generally orthogonal to the optical path;
• and
• a calibration unit reversibly attachable to either side of the flow cell via the complementary connections and disposed between the light source and the detector and in the optical path, wherein the calibration unit includes:
• a housing having a first aperture and a second aperture therethrough, the first aperture and the second aperture defined by corresponding housing walls and arranged along an optical path extending through the housing by an optical path length;
• a first filter disposed within the housing such that the first filter is moveable from an operating position to a calibration position, wherein in the calibration position the first filter is arranged in the optical path;
• and
• a second filter disposed within the housing such that the second filter is moveable, independently from and adjacent to the first filter, from an operating position to a calibration position, wherein in the calibration position the second filter is arranged in the optical path,
• wherein the housing includes a connection adjoining and at least partially defining each of the first aperture and the second aperture, each connection integral with the housing wall defining its corresponding aperture and structured to enable the housing to be reversibly attached to a flow cell, a detector and a light source.