nephelometric turbidity unit

A unit measuring the lack of clarity of water, used by water and sewage treatment plants, in marine studies, and so on. Symbol, NTU, or ntu.   Water containing 1 milligram of finely divided silica per liter has a turbidity of 1 NTU. The NTU replaced the Jackson turbidity unit.

The NTU is measured with an electronic instrument called a nephelometer. The water to be measured is placed in a standard container. A light beam passes through the water and strikes a sensor on the other side of the container. A second sensor is mounted at right angles to the beam, measuring light scattered by particles in the water. From the ratio between the light intensities at the two sensors the turbidity in NTU can be calculated.

To calibrate the device a standard solution is needed. An early approach was to suspend a measured mass of silica in a measured volume of distilled water. In another approach, defined quantities of reagents were mixed to produce a fine precipitate. The current technique employs microspheres of polymer, which led to a new turbidity unit, the Formazin turbidity unit (FTU).  Measurements in NTU and FTU are roughly equivalent.

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