A dispensing unit for dispensing fluid into an external container, comprising - a connector part comprising - upper fastening means, and - a bottom wall having an interior bottom surface, an exterior bottom surface, and at least one passage between the interior bottom surface and the exterior bottom surface, - a reagent cylinder for storing the fluid, the reagent cylinder being adapted to be rotatably fitted into the connector part, and comprising - a cylinder bottom, and - at least one compartment for storing the fluid, the compartment comprising at least one opening through the cylinder bottom, wherein the at least one opening is located so that for at least one mutual orientation of the reagent cylinder and the connector part, the at least one opening is aligned with the at least one passage, and fluid can flow from the compartment to the external container via the at least one opening and the at least one passage, - a locking ring comprising - fastening means adapted to be fastened to the upper fastening means of the connector part, so that when the locking ring is tightened, the cylinder bottom of the reagent cylinder is pressed towards the interior bottom surface of the connector part and creating a fluid tight sealing between the connector part and the reagent cylinder. - a connector part comprising - upper fastening means, and - a bottom wall having an interior bottom surface, an exterior bottom surface, and at least one passage between the interior bottom surface and the exterior bottom surface, - upper fastening means, and - a bottom wall having an interior bottom surface, an exterior bottom surface, and at least one passage between the interior bottom surface and the exterior bottom surface, - a reagent cylinder for storing the fluid, the reagent cylinder being adapted to be rotatably fitted into the connector part, and comprising - a cylinder bottom, and - at least one compartment for storing the fluid, the compartment comprising at least one opening through the cylinder bottom, wherein the at least one opening is located so that for at least one mutual orientation of the reagent cylinder and the connector part, the at least one opening is aligned with the at least one passage, and fluid can flow from the compartment to the external container via the at least one opening and the at least one passage, - a cylinder bottom, and - at least one compartment for storing the fluid, the compartment comprising at least one opening through the cylinder bottom, wherein the at least one opening is located so that for at least one mutual orientation of the reagent cylinder and the connector part, the at least one opening is aligned with the at least one passage, and fluid can flow from the compartment to the external container via the at least one opening and the at least one passage, - a locking ring comprising - fastening means adapted to be fastened to the upper fastening means of the connector part, so that when the locking ring is tightened, the cylinder bottom of the reagent cylinder is pressed towards the interior bottom surface of the connector part and creating a fluid tight sealing between the connector part and the reagent cylinder. - fastening means adapted to be fastened to the upper fastening means of the connector part, so that when the locking ring is tightened, the cylinder bottom of the reagent cylinder is pressed towards the interior bottom surface of the connector part and creating a fluid tight sealing between the connector part and the reagent cylinder.