Turbidy is measured to determine the clarity of the water or, in other words, how many particulates are floating around in the water, such as plant debris, sand, silt, and clay, which affects the amount of sunlight reaching aquatic plants. Such particulates in the water are often referred to as total suspended solids (TSS). Excess turbidity can reduce reproduction rates of aquatic life when spawning areas and eggs are covered with soil. Turbidity measurements are often used to calculate the inputs from erosion and nutrients. Turbidity or TSS sensors are used to measure the clarity of the water.
Turbidity meters ascertain this reading using one of several methods. One technique is to pass a beam of light through the sample, with the amount of light absorbed being proportional to the turbidity. This method does not work well for highly colored or very turbid samples. Most other methods measure light that is reflected either directly from the sample or off its surface. The amount of scattered light, which can be measured at various angles, is proportional to the turbidity.
Turbidity is most commonly measured in Nephelometric Turbidity Units (NTU) but is sometimes measured in Jackson Turbidity Units (JTU).
As with conductivity and TDS, turbidity is an approximation of the amount of TSS in a sample. The relationship depends on several factors including the size and shape of the suspended particles and their density.