Concept: Measure of the degree at which water looses its transparency due to the presence of suspended particulates is termed as turbidity. Nephelometer ● Water containing 1 milligram of finely divided silica per liter has a turbidity of 1 NTU. ● It is based on color matching technique. ●The NTU is measured with an electronic instrument called a nephelometer. ● The water to be measured is placed in a standard container. A light beam passes through the water and strikes a sensor on the other side of the container. A second sensor is mounted at right angles to the beam, measuring light scattered by particles in the water. From the ratio between the light intensities at the two sensors the turbidity in NTU can be calculated. Jackson turbidity method ● Jackson turbidity unit (JTU) is equal to turbidity produced by 1 mg in 1 liter of distilled water. ● This method is based on the Absorption principle. Baylis Turbidometer ● This turbidimeter is based on color-matching techniques. ● Light intensity is measured in the direction of incident light. ● Formazin is a chemical compound that produces more reproducible standards than SiO2 and has replaced it as a reference. The turbidity unit is also sometimes called FTU