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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 both water clarity and water quality. Fluids can contain suspended solid matter consisting of particles of many different sizes.
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 [7] [8])
Turbidimetry (the name being derived from turbidity) is the process of measuring the loss of intensity of transmitted light due to the scattering effect of particles suspended in it. Light is passed through a filter creating a light of known wavelength which is then passed through a cuvette containing a solution.
Different kinds of Secchi disks. A marine style on the left and the freshwater version on the right. The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. 1.7 divided into this depth in metres yields an attenuation coefficient (also called an extinction coefficient), for the available light averaged over the Secchi disk depth.
Three glass vials used as turbidity standards for 5, 50, and 500 nephelometric turbidity units (NTU). Turbidity is a measure of the cloudiness of water based on light scattering by particles at a 90-degree angle to the detector. A turbidity sensor is placed in water with a light source and a detector at a 90-degree angle to one another.
A digital sensor is an electronic or electrochemical sensor, where data is digitally converted and transmitted. Sensors are often used for analytical measurements, e.g. the measurement of chemical and physical properties of liquids. Examples of measured parameters are pH value, conductivity, oxygen, redox potentials.
The water color spectrum as seen by a satellite sensor is defined as an apparent optical property (AOP) of the water. This means that the color of the water is influenced by the angular distribution of the light field and by the nature and quantity of the substances in the medium, in this case, water. [2]
Coulter principle — the transient current drop is proportional to the particle volume The tip of the Coulter counter in a buffer solution, counting cells in solution.. A Coulter counter [1] [2] is an apparatus for counting and sizing particles suspended in electrolytes.