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Electrical Conductivity and TDS

TDS or Total Dissolved Solids is a measure of the total ions in solution. EC is actually a measure of the ionic activity of a solution in term of its capacity to transmit current. In dilute solution, TDS and EC are reasonably comparable. The TDS of a water sample based on the measured EC value can be calculated using the following equation:

TDS (mg/l) = 0.5 x EC (dS/m or mmho/cm) or = 0.5 * 1000 x EC (mS/cm)

The above relationship can also be used to check the acceptability of water chemical analyses. It does not apply to wastewater.
As the solution becomes more concentrated (TDS > 1000 mg/l, EC > 2000 ms/cm), the proximity of the solution ions to each other depresses their activity and consequently their ability to transmit current, although the physical amount of dissolved solids is not affected. At high TDS values, the ratio TDS/EC increases and the relationship tends toward TDS = 0.9 x EC.
In these cases the above-mentioned relationship should not be used and each sample should be characterized separately.
For water for agricultural and irrigation purpose the values for EC and TDS are related to each other and can be converted with an accuracy of about 10% using the following equation:

TDS (mg/l) = 640 x EC (ds/m or mmho/cm).

With the reverse osmosis process, water is forced in a semi-impermeable membrane leaving the impurities behind. This process is capable of removing 95-99 % of TDS, providing pure or ultra-pure water.

Use Lenntech calculators to calculate the TDS content from water analysis and to convert TDS in EC ou visa versa.


Read more: http://www.lenntech.com/applications/ultrapure/conductivity/water-conductivity.htm#ixzz209rCqgre