A nephelometer or aerosol photometer is an instrument for measuring the concentration of suspended particulates in a

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...

or gas

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

. A nephelometer measures suspended particulates by employing a light beam (source beam) and a light detector set to one side (often 90°) of the source beam. Particle

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

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 Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...

, and

reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...

. Nephelometers are calibrated to a known particulate, then use environmental factors (k-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 Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized so ...

varieties.

Nephelometer uses

The main uses of nephelometers relate to air quality measurement for pollution monitoring, climate monitoring, and visibility. Airborne particles are commonly either biological contaminants, particulate contaminants, gaseous contaminants, or dust. The accompanying chart shows the types and sizes of various particulate contaminants. This information helps understand the character of particulate pollution inside a building or in the ambient air, as well as the cleanliness level in a controlled environment. Biological contaminants include mold, fungus, bacteria, viruses, animal dander, dust mites, pollen, human skin cells, cockroach parts, or anything alive or living at one time. They are the biggest enemy of indoor air quality specialists because they are contaminants that cause health problems. Levels of biological contamination depend on humidity and temperature that supports the livelihood of micro-organisms. The presence of pets, plants, rodents, and insects will raise the level of biological contamination.

Sheath air

Sheath air is clean filtered air that surrounds the aerosol stream to prevent particulates from circulating or depositing within the optic chamber. Sheath air prevents contamination caused by build-up and deposits, improves response time by containing the sample, and improves maintenance by keeping the optic chamber clean. The nephelometer creates the sheath air by passing air through a zero filter before beginning the sample.

Global radiation balance

Nephelometers are also used in global warming studies, specifically measuring the global radiation balance. Three wavelength nephelometers fitted with a backscatter shutter can determine the amount of solar radiation that is reflected back into space by dust and particulate matter. This reflected light influences the amount of radiation reaching the earth's lower atmosphere and warming the planet.

Visibility

Nephelometers are also used for measurement of visibility with simple one-wavelength nephelometers used throughout the world by many EPAs. Nephelometers, through the measurement of light scattering, can determine visibility in distance through the application of a conversion factor called Koschmieder's formula.

Medicine

In medicine, nephelometry is used to measure immune function.

Fire detection

Gas-phase nephelometers are also used in the detection of smoke and other particles of

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

. 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 computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterprise ...

s and telephone switches.

Turbidity units

*Because optical properties depend on suspended particle size, a stable synthetic material called "

Formazin Formazine (formazin) is a heterocyclic polymer produced by reaction of hexamethylenetetramine with hydrazine sulfate. The hexamethylenetetramine tetrahedral cage-like structure, similar to adamantane, serves as molecular building block to form ...

" 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 The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...

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''Florida Department of Agriculture and Consumer Services 2006 Codebook Chapter 5L-1: The Comprehensive Shellfish Control code''
) *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 Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...

) of the source beam and the detector. A nephelometric turbidimeter always monitors light reflected off the particles and not

attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at var ...

due to cloudiness. In the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

environmental monitoring the

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

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 An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

. These can provide information on

visibility The visibility is the measure of the distance at which an object or light can be clearly discerned. In meteorology it depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time o ...

and atmospheric

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

References

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