A nephelometer in Kosan, Cheju Island, South Korea.
A nephelometer (from Greek νεφέλη, cloud, and μέτρον, measure) is an instrument for measuring particles suspended in a liquid or a gas, using a photocell placed at a 90 ° angle to a light source. The density of particles is a function of the light reflected by the particles towards the photocell. The amount of light reflecting a given density of particles depends on the properties of the particles, such as their shape, color and reflectivity. The calibration of the apparatus with standard samples allows to establish a working correlation between turbidity and suspended solids (which is a more useful measure, but generally more difficult to quantify).
Nephelometers used in water quality tests are commonly called turbidimeters. However, there may be differences between the turbidimeter models, depending on the geometric configuration of the light source with respect to the photocell. A nephelometric turbidimeter always detects the light reflected by the particles and not the attenuation due to turbidity.
The turbidity unit for a calibrated nephelometer is called the Nephelometric Turbidity Unit, UTN or NTU.
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