page_title: Nephelometer
section_title: Turbidity units
hierarchical_section_title: Nephelometer / Turbidity units
caption_attribution_description: A nephelometer installation at Acadia National Park. The nephelometer is being used to monitor air quality in the park.
context_page_description: A nephelometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam and a light detector set to one side 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. Nephelometers are calibrated to a known particulate, then use environmental factors to compensate lighter or darker colored dusts accordingly. K-factor is determined by the user by running the nephelometer next to an air sampling pump and comparing results. There are a wide variety of research-grade nephelometers on the market as well as open source varieties.
context_section_description: 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. The unit is called Formazin Turbidity Unit (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 used)
  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 90° 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 c. 180°, 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
  Brantner Haze Scale (BHS) and Brantner Haze Units (BHU) for purposefully hazy beer
  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
  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.
caption: A nephelometer installation at Acadia National Park
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