the invention relates to a method comprising passing a first variable intensity light beam across a first test - tube wherein the substance to be controlled is circulating . subsequently , a second fixed - intensity light beam is passed across a second test - tube 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 .