I'm working on retrieval of turbidity from remote sensing approach and for validation want to compare in-situ data with satellite derived product so need conversion of NTU to mg/l.
I think the national turbidity unit and FTU are the only way to describe the turbidity of the solution
You can calculate TSS ( total suspended solids) instead of turbidity
The units used for these measurements are nephelometric turbidity units (NTU). The conversion between various scales requires calibration of the instrument. Each application of turbidity measurements is slightly different and instrument response will vary. The standardized solution used to calibrate the instrument is a Formazin solution. 1 NTU is defined as the response to a 1 mg/l solution of Formazin.
Please check
http://www.transportation.alberta.ca/Content/docType245/Production/The%20conversion%20of%20Nephelometric%20Turbidity%20Units.pdf
also, I think that may be useful
Question What is the difference between the turbidity units NTU, FNU, FTU, and FAU? What is a JTU?
Summary Turbidity units NTU, FNU, FTU, FAU, and JTU
Answer NTU stands for Nephelometric Turbidity Unit and signifies that the instrument is measuring scattered light from the sample at a 90-degree angle from the incident light. FNU stands for Formazin Nephelometric Units and also signifies that the instrument is measuring scattered light from the sample at a 90-degree angle from the incident light. FNU is most often used when referencing the ISO 7027 (European) turbidity method. NTU is most often used when referencing the USEPA Method 180.1 or Standard Methods For the Examination of Water and Wastewater. When formazin was initially adopted as the primary reference standard for turbidity, units of FTU or Formazin Turbidity Units were used. These units, however, do not specify how the instrument measures the sample.
FAU or Formazin Attenuation Units signify that the instrument is measuring the decrease in transmitted light through the sample at an angle of 180 degrees to the incident light. This type of measurement is often made in a spectrophotometer or colorimeter and is not considered a valid turbidity measurement by most regulatory agencies.
A JTU or Jackson Turbidity Unit is a historical unit used when measurements were made visually using a Jackson Candle Turbidimeter. Water was poured into a tube until a flame underneath the tube could no longer be distinguished.
The turbidity units NTU, FNU, FTU, AND FAU are all based on calibrations using the same formazin primary standards. Therefore when a formazin standard is measured, the value for each of these units will be the same, however the value on samples may differ significantly.
link
http://hachcompany.custhelp.com/app/answers/answer_view/a_id/1000336/~/what-is-the-difference-between-the-turbidity-units-ntu%2C-fnu%2C-ftu%2C-and-fau%3F-what
There is no fixed relationship between NTU and sediment concentration. It depends on the size distribution, and to a lesser amount on density, then on particle absorption. NTU is determined by cross-sectional area, and closely related to backscatter. mg/L as a weight is a volumetric measurement. You should use remote sensing to go after NTU, like the Dogliotti et al (RSE 2015). Then use local field data to determine the NTU to sediment relationship. In some regions, the water may have fairly constant optical grain size (NTU/sed). I have seen a difference of 2x in tributaries of Chesapeake Bay. And tropical carbonate muds, which are highly reflective, tend to have high NTU/sed.
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