Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1984-08-14
1987-02-10
Rosenberger, R. A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356342, G01N 2100
Patent
active
046419691
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for measuring the concentration of suspended substances in a flowing medium by illuminating the medium with light from a light source to detect light reflections which are indicative of particle concentration. The reflected light is detected in at least two detectors; the other for measuring directly reflected light and one for measuring multiply reflected light. Directly reflected light [refers to] [the] light received from an area of the medium that is directly illuminated by the light source and multiply reflected light is light received from an area of the medium that is rather not directly illuminated, but is illuminated by light that has been scattered from the direct beam. The apparatus is preferably intended for use in so-called consistency measurements, i.e. measurements of the content of fibres in fibre suspensions, such as is found in the pulp and paper industry. The consistency is defined as the weight of fibres per volumetric unit. It is therefore a definition of concentration of suspended solids, and will be so referred to, since the present invention may be used to measure any material that scatters light in the flowing medium.
A number of different methods have been developed for measuring the concentration of fibres in fibre suspensions. The primary methods are based on mechanical principles. In particular, a shear force principle particularly has come into use during the last 50 years. In shear force measurements, the network strength of the fibre suspension is measured first with the aid of a blade, a rotating sensoror the like. The relationship between the concentration and the network strength is then calculated by a special calibration method. More recently, however, optical methods have emerged. There are several reasons for of optically based methods. Firstly, the mechanical meters of the shear force or viscosity type normally require a great deal of maintenance. Secondly, the accuracy, particularly with stationary measurement, is relatively poor with mechanical apparatus. Thirdly, great technical advances were made in the optical and electronic fields, particularly in the latter, which have facilitated and reduced the cost of meters based on optical principles. Also, an accuracy may be obtained with electro-optical techniques which is at least as good as, and in some cases better than, that obtained with corresponding mechanical apparatus.
The disadvantage of electro-optical methods has, however, been the difficulty of measuring higher concentrations. One has therefore normally been limited to measurements in the range under 10 g/liter (1%). This is due to the fact that the primary choice has been to use transmission measurement, i.e. light transmitted through the fibre suspension is detected, rather than light reflected from the suspension. Since the intensity of the light passing through the suspension declines according to an exponential function, the signal that can be generated from the light is close to the signal noise threshold for even relatively low concentrations. Devices based on polarized transmitted, light also suffer from the same limitation.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus of the type mentioned above, but are distinguished in that the detector for the directly reflected light is arranged at such a distance from the light source that a linear relationship exists between the output signal from the detector and the concentration parameter. The measured concentration parameter of the flowing medium vary linearly up to a maximum value of concentration. Beyond the maximum value, the signal from the detector does not increase even though the concentration is higher than second. A detector for detecting the multiply reflected light is disposed at a different location in relation to the light source such that its output signal will be independent of the concentration of the flowing medium within a large interval. An output is then obtain
REFERENCES:
patent: 3714444 (1973-01-01), Carr et al.
patent: 3787703 (1974-01-01), Topol
patent: 3906241 (1975-09-01), Thompson
patent: 4193692 (1980-03-01), Wynn
patent: 4290695 (1981-09-01), Schmitt
Lundberg Krister
Pope Daniel F.
Tidstam Goran
AB Bonnierforetagen
Cooper Crystal D.
Rosenberger R. A.
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