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Old 08-05-2018, 07:59 AM   #11 (permalink)
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Default Re: TDS meter

The conversion factor from conductivity to TDS is about 0.50 for plain NaCl solution and about 0.67 for freshwater with a "typical" mix of other ions. So, a conductivity measurement of 100 uS corresponds to about 50 mg/L if NaCl is the majority of the dissolved material, or 67 mg/L in a freshwater sample containing an average mix of Ca, Mg, SO4, N, K, Cl, Fe, Mn, NO3, etc.

A chemically "accurate" measure of TDS (by evaporation and weight) is biologically meaningless, since it doesn't distinguish between the mineral ions essential for fish and plant growth and the dissolved organic compounds from wood and leaves. A sample could have high TDS (measured by weight) due to organics but have very few mineral ions. It's like measuring the nutritional content of your grocery shopping cart by weight.
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Old 08-06-2018, 05:53 AM   #12 (permalink)
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Default Re: TDS meter

You don't need accurate measurement of TDS. You need consistent measurement of TDS. TDS meters are essentially conductivity meter. If one brand uses a factor of 0.5, and another brand uses 0.67. Comparison of TDS is no longer meaningful.

Conductivity or TDS is not a solute specific measurement like pH, NO3, NH3, PO4 . . . etc. It is a quick, proxy measurement of many parameters: salinity, hardness, osmotic pressure, level of pollution, or how long you haven't changed water. It's biological meaningful only if you can correlate TDS with the parameters you are interested in, and the measurement is consistent.
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