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Sunday, May 3, 2020

Turbidity meters

Very often it is necessary to measure the amount of solids suspended in fluids. In this case the turbidity is measured as the function of the amount of suspensions in fluids. Fig. 7.10 schematically explains an operational principle of a transmission-type turbidity meter. A constant-candlepower lamp 1 provides a lightbeam which passes through the lens 2, glass windows 3, and a sample cell 4.

A fluid with solid suspensions to be measured flows through the sample cell. The light beam is scattered by solids in suspensions. The degree of scattering depends on the amount of solids, and hence on the turbidity of the fluid. This scattering effect is called the Tyndall effect. The light beam with reduced intensity falls on the photo cell 5 (usually photosensors), which converts the measured light intensity to an electrical signal inversely proportional to the turbidity of the fluid (or the amount of suspended solids). The scale of a device 6 for measurement of this electrical output signal is calibrated in ppm of solids suspended in fluid. Since the photosensor is temperature sensitive, a heater and thermostat are employed to maintain its temperature at a constant value. By matching the length of the light path to the level of turbidity, we can vary ranges of turbidity to be measured.

The formation of deposits on the windows of the sample cell reduces the accuracy of turbidity measurements, so frequent maintenance of these windows is required.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Basics of Instrumentation & Control

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