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Water Turbidity Sensor (Overview)
Designed by Mark Little
Turbidity analysis is the study of the optical properties of water where particles suspended in the water cause the light passing through it to be absorbed and/or scattered from its original path. Water turbidity (or cloudiness) is caused by suspended solids, such as silt, algae, organic matter and micro-organisms.
There are two common ways to measure turbidity, The first is to use a turbidity tube and the second is to use a turbidity meter. The turbidity tube is simple and easy to use, but does not allow for automated readings.
Absorptiometers are best for measuring low levels of turbidity as the amount of light scattered by the suspended is low and hard to detect. To measure really low levels of turbidity requires a constant light source coupled with transmission sensor that has a low internal noise level so that small changes in level are not masked by the sensor's internal noise. The other issue that affects the ability of the sensor to measure changes in transmission though the water is the ambient light in the water chamber. High levels of ambient light such as sunlight can saturate the sensor, preventing it from detecting small changes in level caused by increasing turbidity. This problem is normally solved by simply putting the water chamber in a light proof container that excludes all external light.
There are two common ways to measure turbidity in water with a turbidity meter. The first is by measuring the absorption (or attenuation) of the light intensity passing through the sample. Meters that use this principle are called Absorptiometers. The other common method is to measure the amount of light that is scattered at an angle of 90° from the incident beam. Meters using this principle are know as Nephelometers.
Neplometers are more suited to higher levels of turbidity where the levels of scattered light are higher, although ultimately the level of the light scattered to the scattering sensor will reduce due to absorption.
To get the widest range of turbidity readings a combination of the Absorptimeter and the Nepllometer provide the best solution. Fortunately, the same light source can be used for both measurement configurations simultaneously.
The turbidty tube is a simple and accurate way to measure water turbidity. A clear tube is marked off in centrimetres from the bottomand target is painted on the bottom of the tube. As an alternative, an approximate turbidity scale can be directly marked on the tube using the following formula (where NTU represents Nephelometric Turbidity Units).
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