The invention relates to a device tier determining a concentration of a chemical substance dissolved in a liquid, which chemical substance absorbs light (10) of an absorption wavelength, wherein the device comprises: at least one flow-through chamber (2) having at least one inlet opening (4), at least one outlet opening (6), and at least one peripheral wall (24); at least one laser (8) for emitting light (10) of the absorption wavelength; and at least one detector (18) for detecting the emitted light (10); wherein the laser (8) and the detector (18) are arranged in such a way that the light (10) emitted by the laser (8) is conducted through the flow-through chamber (2) along at least two different paths before said light hits the detector (18), and wherein the device has two lasers (8) which emit light of different wavelengths. The invention further relates to a method for producing a flow-through chamber for such a device.