The invention relates to a method comprising passing a first variable intensity light beam across a first test-tube (3) wherein the substance (1) to be controlled is circulating. Subsequently, a second fixed-intensity light beam is passed across a second test-tube (4) with a reference sample. The intensities of both beams are compared once they have crossed over the test tubes and the intensity of the first beam is varied so that said intensities are equal. The parameter of interest in the first test tube is calculated by means of signal processing which causes the first beam to vary.