NOVELTY - The device comprises a pipeline system (116) for transporting a fluid such as mineral oil product, two metering system (128) for metering of two marker material (114) into the fluid and two optical detectors (140) for detecting the marker material in the fluid. The detector is connected to the metering system by way of a control device (142), which is adapted to influence the metering of the marker material. The concentration of the marker material in the fluid is adjusted to a given desired-value. The control device comprises a controlled system having a two-point-control.    USE - Device for marking a forgery protection product such as mineral oil product, biodiesel and/or bioethanol with a marker material and for marking and differentiation of different product streams in a plant such as production plant, filling plant, pipeline and a reactor (all claimed).    ADVANTAGE - The device ensures safe and efficient marking of a fluid with a marker material in qualitative and quantitative manner.    DETAILED DESCRIPTION - The device comprises a pipeline system (116) for transporting a fluid such as mineral oil product, two metering system (128) for metering of two marker material (114) into the fluid and two optical detectors (140) for detecting the marker material in the fluid. The detector is connected to the metering system by way of a control device (142), which is adapted to influence the metering of the marker material. The concentration of the marker material in the fluid is adjusted to a given desired value. The control device comprises a controlled system having a two-point-control by which the metering of the marker material is interrupted. The pipeline system comprises a main pipeline to parallel switched bypass stretch in which the detector is arranged. The metering system comprises elements such as membrane pump, piston pump, a hose pump or a gear pump or metering valve. The optical detector comprises detectors such as fluorescence detector, phosphorescence detector, Raman detector, scattered light detector, a transmission light detector and an absorption detector. The optical detector comprises a laser light source, which produces a laser light beam, which partially permeates the fluid. The detector comprises an optical evaluation detector for evaluating an emitted, transmitted or controlled light components by the fluid and/or the marker material, an excitation light source for producing an exciting light beam, a reflexion device for partial reflexion of the exciting light beam, and a lock-in detector. The exciting light beam is partially pulsed and/or modulately formed. The pipeline system comprises a filling station for filling the fluid in a transporting device. An INDEPENDENT CLAIM is included for a method for marking a fluid such as mineral oil product with a marker material.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a device for marking a fluid with marker material.    Marker material (114)    Pipeline system (116)    Metering system (128)    Detector (140)    Control device. (142)