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 particles 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. In the United States environmental monitoring the turbidity standard unit is called Nephelometric Turbidity Units (NTU), while the international standard unit is called Formazin Nephelometric Unit (FNU). The most generally applicable unit is Formazin Turbidity Unit (FTU), although different measurement methods can give quite different values as reported in FTU (see below).
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.
Turbidity Units
- Because optical properties depend on suspended particle size, a stable synthetic material called "Formazin" with uniform particle size is often used as a standard for calibration and reproducibility.2 The unit is called Formazin Turbidity Units (FTU).
- Nephelometric Turbidity Units (NTU) specified by United States Environmental Protection Agency is a special case of FTU, where a white light source and certain geometrical properties of the measurement apparatus are specified. (Sometimes the alternate form "nephelos turbidity units" is used34)
- Formazin Nephelometric Units (FNU), prescribed for 9 measurements of turbidity in water treatment by ISO 7027, another special case of FTU with near infrared light (NIR) and 90º scatter.
- Formazin Attenuation Units (FAU) specified by ISO 7027 for water treatment standards for turbidity measurements at 0°, also a special case of FTU.
- Formazin Backscatter Units (FBU), not part of a standard, is the unit of optical backscatter detectors (OBS), measured at ca 180º, also a special case of FTU.
- European Brewery Convention (EBC) turbidity units
- 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
Notes
- ^ Nephelometer comes from the Greek word for cloud, nephos, cf. "nepheloid layer".
- ^ Formazin was first used for standardizing turbidity measurements in 1926.
- ^ 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
See also
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Earth-based meteorological equipment and instrumentation |
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