Device for extracorporeal blood treatment with - a dialyzer (10) that is separated by a semipermeable membrane (11) in a first and a second chamber, in which the first chamber (12) is arranged in the path of the dialysis fluid and the second chamber (13) is connectable with the blood circulation of a patient (1) by means of a blood inlet duct (14) and a blood outlet duct (15), - an inlet duct (20) for fresh dialysis fluid, - a drain (30) for used dialysis fluid, - a measuring device (40) disposed in the drain (30) to determine the absorption of the used dialysis fluid that runs down the drain (30), wherein the measuring equipment (40) has at least one radiation source (41) for essentially monochromatic electromagnetic radiation such as a detection system (42) for detecting the intensity of the electromagnetic radiation, characterized by the fact that they are approved means (50) for compensating alterations in the intensity of the electromagnetic radiation of the radiation source (41) that occur and / or in the sensitivity of the detection system (42), in which a regulator is provided as means (50) of temperature (51) by which the temperature of the radiation source (41) can be regulated at predetermined operating temperatures T1 and / or the temperature of the detection system (42) at predetermined operating temperatures TDispositivo para el tratamiento extracorporal de sangre con - un dializador (10) que está separado por una membrana semipermeable (11) en una primera y una segunda cámara, en que la primera cámara (12) está dispuesta en el camino del líquido de diálisis y la segunda cámara (13) es conectable con la circulación sanguínea de un paciente (1) por medio de un conducto de entrada de sangre (14) y un conducto de salida de sangre (15), - un conducto de entrada (20) para líquido de diálisis fresco, - un desagüe (30) para líquido de diálisis usado, - un equipo de medida (40) dispuesto en el desagüe (30) para determinar la absorción del líquido de diálisis u
