Editing Turbidity (section)

==Measurement==
[[File:Runoff torbidity.jpg|thumb|right|Turbid creek water caused by heavy rains]]

The measurement of turbidity is a key test of both water clarity and water quality.{{citation needed|date=January 2023}} There are two standard units for reporting turbidity: Formazin Nephelometric Units (FNU) from [[ISO 7027]] and Nephelometric Turbidity Units (NTU) from USEPA Method 180.1. ISO 7027 and FNU is mostly widely used in Europe, whereas NTU is mostly widely used in the U.S. ISO 7027 provides the method in water quality for the determination of turbidity. It is used to determine the [[concentration]] of suspended particles in a sample of water by measuring the incident light scattered at right angles from the sample. The scattered light is captured by a [[photodiode]], which produces an electronic signal that is converted to a turbidity. [[Open source hardware]] has been developed following the ISO 7027 method to measure turbidity reliably using an [[Arduino]] microcontroller and inexpensive [[LED]]s.<ref>{{cite journal |last1=Wijnen |first1=Bas |last2=Anzalone |first2=G.C. |last3=Pearce |first3=Joshua M. |title=Open-source mobile water quality testing platform |journal=Journal of Water, Sanitation and Hygiene for Development |publisher=IWA Publishing |location=London |volume=4 |issue=3 |pages=532–537 |date=2014-05-09 |doi=10.2166/washdev.2014.137|bibcode=2014JWSHD...4..532W |s2cid=111183462 |url=https://hal.science/hal-02119690/document }}</ref>

There are several practical ways of checking water quality, the most direct being some measure of [[attenuation]] (that is, reduction in strength) of light as it passes through a sample column of water.<ref name="fondriest" /> The alternatively used Jackson Candle method (units: Jackson Turbidity Unit or '''JTU''') is essentially the inverse measure of the length of a column of water needed to completely obscure a candle flame viewed through it. The more water needed (the longer the water column), the clearer the water. Of course water alone produces some attenuation, and any substances dissolved in the water that produce color can attenuate some wavelengths. Modern instruments do not use candles, but this approach of attenuation of a light beam through a column of water should be calibrated and reported in JTUs.<ref name="fondriest" />

The propensity of particles to scatter a light beam focused on them is now considered a more meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called a [[nephelometer]] with the detector set up to the side of the light beam. More light reaches the detector if there are many small particles scattering the source beam than if there are few. The units of turbidity from a calibrated nephelometer can be either NTU or FTU, depending on the standard method used. To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly and do not contribute to a turbidity reading), a correlation between turbidity and [[total suspended solids]] (TSS) is somewhat unusual for each location or situation.<ref name="fondriest" >{{cite web |url=https://www.fondriest.com/environmental-measurements/measurements/measuring-water-quality/turbidity-sensors-meters-and-methods/ |title=Measuring Turbidity, TSS, and Water Clarity |publisher=Fondriest Environmental, Inc. |location=Fairborn, OH |access-date=1 February 2021}}</ref>

Turbidity in lakes, reservoirs, channels, and the ocean can be measured using a [[Secchi disk]]. This black and white disk is lowered into the water until it can no longer be seen; the depth (Secchi depth) is then recorded as a measure of the transparency of the water (inversely related to turbidity). The Secchi disk has the advantages of integrating turbidity over depth (where variable turbidity layers are present), being quick and easy to use, and inexpensive. It can provide a rough indication of the depth of the [[euphotic zone]] with a 3-fold division of the ''Secchi depth'', however this cannot be used in shallow waters where the disk can still be seen on the bottom.<ref name="Myre and Shaw" />

Cameras and [[computer vision]] have also been used to measured turbidity.<ref>{{Cite journal |last1=Antonini |first1=Giorgio |last2=Pearce |first2=Joshua M. |last3=Berruti |first3=Franco |last4=Santoro |first4=Domenico |date=2024-08-16 |title=A novel camera-based sensor for real-time wastewater quality monitoring |url=https://iwaponline.com/wpt/article/19/9/3778/104171/A-novel-camera-based-sensor-for-real-time |journal=Water Practice and Technology |volume=19 |issue=9 |pages=3778–3793 |doi=10.2166/wpt.2024.211 |bibcode=2024WatPT..19.3778A |issn=1751-231X|doi-access=free }}</ref> Such monitoring can make use of [[machine learning]] to identify problems in sewage.<ref>{{Cite journal |last1=Ji |first1=Hyon |last2=Yoo |first2=Sung |last3=Lee |first3=Bong-Jae |last4=Koo |first4=Dan |last5=Kang |first5=Jeong-Hee |date=2020-06-22 |title=Measurement of Wastewater Discharge in Sewer Pipes Using Image Analysis |journal=Water |language=en |volume=12 |issue=6 |pages=1771 |doi=10.3390/w12061771 |doi-access=free |bibcode=2020Water..12.1771J |issn=2073-4441}}</ref> 

An additional device, which may help measuring turbidity in shallow waters is the turbidity tube.<ref>Waterwatch Australia, Module 4 - physical and chemical parameters [https://web.archive.org/web/20060513024523/http://www.waterwatch.org.au/publications/module4/turbidity.html "Methods Turbidity"]</ref><ref name="Myre and Shaw">Myre, E, Shaw, R. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field. [http://watsanmissionassistant.wikispaces.com/file/view/Turbidity-Myre_Shaw.pdf/360203097/Turbidity-Myre_Shaw.pdf "The Turbidity Tube"] {{Webarchive|url=https://web.archive.org/web/20151017234221/http://watsanmissionassistant.wikispaces.com/file/view/Turbidity-Myre_Shaw.pdf/360203097/Turbidity-Myre_Shaw.pdf |date=2015-10-17 }}<!-- https://www2.virginia.edu/blandy/blandy_web/education/Bay/TurbidityTubeConstruction&Use_Myre_Shaw.pdf may be the same --></ref> The turbidity tube condenses water in a graded tube which allows determination of turbidity based on a contrast disk in its bottom, being analogous to the Secchi disk.

Turbidity in air, which causes solar attenuation, is used as a measure of pollution. To model the attenuation of beam irradiance, several turbidity parameters have been introduced, including the Linke turbidity factor (T<sub>L</sub>).<ref>HelioClim (Center for Energy and Processes). Paris, France. [http://www.helioclim.net/linke/index.html "Linke Turbidity Factor."] {{webarchive|url=https://web.archive.org/web/20110723143234/http://www.helioclim.net/linke/index.html |date=2011-07-23 }}</ref><ref>{{cite journal |last=Kasten |first=F. |title=The linke turbidity factor based on improved values of the integral Rayleigh optical thickness |date=March 1996 |journal=Solar Energy |publisher=Elsevier |volume=56 |issue=3 |pages=239–244 |doi=10.1016/0038-092X(95)00114-7|bibcode=1996SoEn...56..239K }}.</ref>