A nephelometer is an instrument for measuring 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 is a wide variety of research-grade nephelometers
on the market as well as open source 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 chart to the left shows the types and sizes of various particulate
contaminants. This information is helpful toward understanding the
character of particulate pollution inside a building or in the ambient air.
It is also useful for understanding 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 through dust
and particulate matter. This reflected light influences the amount of radiation
reaching the earth's lower atmosphere and warming the planet. A simple
demonstration of this is found in www.aurora-nephelometer.com as seen
below. = 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 suppression= 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 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. The unit is called Formazin Turbidity Unit.
Nephelometric Turbidity Units 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.
Formazin Nephelometric Units, prescribed for 9 measurements of turbidity in water
treatment by ISO 7027, another special case of FTU with near infrared light and
90° scatter. Formazin Attenuation Units specified by
ISO 7027 for water treatment standards for turbidity measurements at 0°, also a
special case of FTU. Formazin Backscatter Units, not part of
a standard, is the unit of optical backscatter detectors, measured at c.
180°, also a special case of FTU. European Brewery Convention turbidity
units Concentration Units
Optical Density Jackson "Candle" Turbidity Units
Helms Units American Society of Brewing Chemists
turbidity units Parts Per Million of standard substance,
such as PPM/DE "Trübungseinheit/Formazin" a German
standard, now replaced by the FNU unit. diatomaceous earth 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 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, while the international
standard unit is called Formazin Nephelometric Unit. The most generally
applicable unit is Formazin Turbidity Unit, although different measurement
methods can give quite different values as reported in FTU.
Gas-phase nephelometers are also used to study the atmosphere. These can provide
information on visibility and atmospheric albedo.
See also ISO 7027
Water purification Notes