A modular system comprising an optical measuring device (1; 1'; 1"; 41; 61) configured for being connected in a flow path, said optical measuring device (1; 1'; 1"; 41; 61) comprising: - a first and a second flow path connector (5, 7; 5', 7'; 5", 7"); - at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) provided in between the first and second flow path connectors (5, 7; 5', 7'; 5", 7") such that a flow in a flow path in which the optical measuring device (1; 1'; 1"; 41; 61) can be arranged will flow through the first flow path connector (5; 5'; 5"), the at least two flow cell parts 3; 3a, 3b; 3a, 3b, 3c, 3d) and through the second flow path connector (7; 7'; 7"); wherein each of the at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) comprises a fixedly mounted light transmitting device and a fixedly mounted light receiving device, wherein an optical path of fixed length is provided between a light transmitting end of the light transmitting device and a light receiving end of the light receiving device and wherein said light transmitting device is configured to be connected to a light source and said light receiving device is configured to be connected to a detector, wherein said optical path is configured to be provided in a flow which will be transferred through the optical measuring device (1; 1'; 1"; 41; 61) when it is connected to a flow path, and wherein the optical flow paths each have different optical path lenghts; characterized in that the optical measuring device (1; 1';1";41; 61) further comprises at least one releasable connection device arranged for releasably connecting the at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) to each other in between the first and the second flow path connectors (5, 7; 5', 7'; 5", 7") to allow exchange of flow cell parts. - a first and a second flow path connector (5, 7; 5', 7'; 5", 7"); - at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) provided in between the first and second flow path connectors (5, 7; 5', 7'; 5", 7") such that a flow in a flow path in which the optical measuring device (1; 1'; 1"; 41; 61) can be arranged will flow through the first flow path connector (5; 5'; 5"), the at least two flow cell parts 3; 3a, 3b; 3a, 3b, 3c, 3d) and through the second flow path connector (7; 7'; 7"); wherein each of the at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) comprises a fixedly mounted light transmitting device and a fixedly mounted light receiving device, wherein an optical path of fixed length is provided between a light transmitting end of the light transmitting device and a light receiving end of the light receiving device and wherein said light transmitting device is configured to be connected to a light source and said light receiving device is configured to be connected to a detector, wherein said optical path is configured to be provided in a flow which will be transferred through the optical measuring device (1; 1'; 1"; 41; 61) when it is connected to a flow path, and wherein the optical flow paths each have different optical path lenghts; characterized in that the optical measuring device (1; 1';1";41; 61) further comprises at least one releasable connection device arranged for releasably connecting the at least two flow cell parts (3; 3a, 3b; 3a, 3b, 3c, 3d) to each other in between the first and the second flow path connectors (5, 7; 5', 7'; 5", 7") to allow exchange of flow cell parts.