A method for measuring the content of a substance in the form of a thin film of a mixture comprising a plurality of substances (material), for example for measuring the content of water in paper

摘要

A method using infrared radiation at different wavelengths for the measurement of the content of a first substance (water) in a film 9 (paper web) comprising at least one other substance (wood fibre) is disclosed. A first measuring signal I lambda 1 is obtained at a measuring wavelength lambda 1 absorbed by chemically uncombined water of the paper and a second measuring signal I lambda 2 is obtained at a comparison wavelength lambda 2 substantially not absorbed by water nor by the other substances of the paper. A third measuring signal I lambda 3 is obtained at a second measuring wavelength lambda 3 absorbed by the wood fibre of the paper. Light of the required wavelengths is obtained by a filter wheel 3 in combination with a radiation transmitter 1. The measurement signals I lambda 1, I lambda 2, I lambda 3 are obtained from a detector 8. The weight per unit area Mc of the paper is measured. The water content of the paper is then calculated according to the formula IMAGE wherein Mw is the weight per unit area of the water contained in the paper, and aw and ac are respectively the absorption coefficient of water and wood fibre, according to Lambert's Law. A further embodiment using an additional wavelength 4 and a different formula. IMAGE