Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1992-01-10
1993-09-14
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250574, 250901, 25022725, 356442, 356336, G01N 2100, G01N 1502, H01J 516
Patent
active
052452000
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to apparatus for preventing the blockage of a measuring head intended for effecting measurements on substances suspended in a flowing medium; and a method for registering the state of a moving suspension containing particle fractions of mutually very different sizes.
BACKGROUND OF THE INVENTION
The amounts in which material is suspended in different kinds of aqueous suspensions is an important measuring parameter, not least from the aspect of environmental care and protection. By suspended material is meant here generally such substances as those which can be separated mechanically by filtration and centrifugation. Especially in the forest industries, the suspended material may consist of many different components such as fibers, fiber fragments and various fillers and coating agents. These components may vary widely in size, from a few millimeters in length and some tens of microns in width (fibers) down to a particle diameter of about 1 micron and smaller (filler). The concentration of a suspension can vary within wide limits, from some mg/l to tens of g/l. It is important not only to measure the content of suspended material, but in many cases also to indicate when the size distribution of the suspended material changes, and the extent of this change. One area of use in this respect is to indicate the effect of the addition of floculating chemicals.
The majority of instruments at present available on the market for continuously measuring suspended materials are based on optical measurement principles, e.g., on light absorption, light scattering and the influence of polarized light. The most common method comprises measuring the turbidity of the suspension, in which attenuation or scattering of light is used as a measurement of the suspended-material content. The extent to which light is scattered, however, depend not only on the concentration of the suspended material, but also on the particle size, shape, surface structure and refraction index of the material concerned.
Thus, suspensions in which the particle-size distribution varies considerably can give misleading information with regard to the concentration of the suspension. This is illustrated in FIG. 22, which discloses the result of turbidity measurements in which attenuation of the light transmitted through suspension was measured. The result (the output signal from the measuring apparatus) is shown as a function of the concentration of cellulose fibers (large particles) and for clay (small particles). As will be seen, such an instrument is far more sensitive to clay than to fibers for a given concentration.
U.S. Pat. No. 4,110,044 describes an optical method which, in principle, enables the concentration of suspended material to be determined independently of particle size. This measuring principle is based on recording not only the mean value of the light transmitted, but also on measuring fluctuations in light intensity, in the form of a signal which includes the square of the true effective value of an alternating voltage component of the measurement signal obtained. As a function of particle size, this value has a mirror-reversed behavior in relation to the signal which is based on the direct voltage component of the measurement signal. Consequently, the sum of these two signals will provide a measurement of the amount of material in the suspension independently of particle size.
Furthermore, it is possible to obtain a relative measurement of the particle-size distribution of the suspended material, by forming the quotient of these two signals.
This method is called the TP-method and is quite effective, particularly in the case of low concentrations, where good linearization of both the direct voltage signal and the square of the true effective value can be obtained.
However, in the case of suspensions having a particle-size distribution in which the particles are predominently large particles, the signal formed by the square of the effective value is influenced to a greater extent b
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Messinger Michael
Nelms David C.
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