Nephelometer

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A nephelometer at the Kosan, Cheju Island, South Korea NOAA facility.

A nephelometer ^[1] 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 the most common units of turbidity in the US are called Nephelometric Turbidity Units (NTU).

A nephelometer installation at Acadia National Park.

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.

1 Turbidity Units
2 Notes
3 See also
4 Exterior links

[edit] Turbidity Units

Nephelometric turbidity units (NTU) specified by US Environmental Protection Agency.
Sometimes the alternate unit "nephelos turbidity units" is used^[2]^[3]
Because optical properties can depend on suspended particle size, a stable synthetic material called "Formazin" with uniform particle size is used as a standard for calibration and reproducibility.^[4] This leads to Formazin Turbidity Units (FTU's), and typically 1 NTU = 1 FTU.
European Brewery Convention (EBC) turbidity units
Formazine Nephelometric Units (FNU), prescribed for 9 measurements of turbidity in water treatment by ISO 7027
Formazine Attenuation Units (FAU) specified by ISO 7027 for water treatment standards for turbidity measurements at 0°
Concentration Units (C.U.)
Optical Density (O.D.)
Jackson "Candle" Turbidity Units (JTU; an early measure)
Helms Units
American Society of Brewing Chemists (ASBC-FTU) turbidity units
Parts Per Million of standard substance, such as PPM/DE (Kieselguhr)
"Trübungseinheit/Formazin" (TE/F) a German standard, now replaced by the FNU unit.
diatomaceous earth ("ppm SiO2") an older standard, now obsolete

[edit] Notes

^ Nephelometer comes from the Greek word for cloud, nephos, cf. "nepheloid layer".
^ Reducing turbidity in chromic acid solutions, Zeller, III, Robert L.; Morgan, Russell J.; Rabbe, Gilbert D.; Fiscus, Donna R.; Wilkes, Jr., Richard L.; United States Patent 5034211, Filing date 1990-10-29, Publication date 1991-07-23
^ Florida Department of Agriculture and Consumer Services 2006 Codebook Chapter 5L-1: The Comprehensive Shellfish Control code
^ Formazin was first used for standardizing turbidity measurements in 1926.