Abstract: Radiations of two different frequencies are transmitted through a fluid comprising a mixture of materials, wherein a radiation is absorbed in the range of the radiation absorption by one of the materials, and another radiation is not absorbed, and wherein the radiations operate with pulses of identical pulse repetition frequency and identical pulse intervals. The detected electrical signal from the monitored radiations passed through the fluid, the center frequency of which corresponds to the reciprocal value of the pulse interval, is filtered out with the aid of a filter. The concentration of the absorbing material in the fluid is measured from the amplitude changes of the filtered-out signal. By the use of filters, the bandpass ranges of which are respectively adapted to the occurring pulse repetition frequencies, and based on the interpretation of the measuring signal as a modulated signal, a high safety against noise signals is attained.

Abstract: The process for measuring the concentration of materials, wherein a radiation is absorbed in the range of the radiation absorption of the material, and another radiation is not absorbed, operates with pulses of identical pulse repetition frequency and identical pulse intervals. The detected electrical signal, the center frequency of which corresponds to the reciprocal value of the pulse interval, is filtered out with the aid of a filter. The concentration of the absorbing material is determined from the amplitude changes of the filtered-out signal. By the use of filters, the passband ranges of which are respectively adapted to the occuring pulse repetition frequencies, and based on the interpretation of the measuring signal as a modulated signal, a high safety against noise signals is attained.