Nephelometer
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A nephelometer is an instrument for measuring suspended particulates in a liquid or gas colloid. It does so by employing a light beam (source beam) and a light detector set to one side (usually 90°) of the source beam. Particle density is then a function of the light reflected into the detector from the particles. To some extent, how much light reflects for a given density of particules is dependent upon properties of the particles such as their shape, color, and reflectivity. Therefore, establishing a working correlation between turbidity and suspended solids (a more useful, but typically more difficult quantification of particulates) must be established independently for each situation.
A more popular term for this instrument in water quality testing is a turbidimeter. However, there can be differences between models of turbidimeters, depending upon the arrangement (geometry) of the source beam and the detector. A nephelometric turbidimeter always monitors light reflected off the particles and not attenuation due to cloudiness. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU).
Gas-phase nephelometers are also used to study the atmosphere. These can provide information on visibility and atmospheric albedo. Gas-phase nephelometers are also used in the detection of smoke & other particles of combustion. In such use, the apparatus is referred to as an aspirated smoke detector. These have the capability to detect extremely low particle concentrations (to 0.005%) and are therefore highly suitable to protecting sensitive or valuable electronic equipment, such as mainframe computers and telephone switches.
See also: turbidity, water purification
