There is no standar relation ...

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There is no standar relation between EC / TDS. Conductivity is the facility of electrons to move in a standard distance (1 cm), and it is relative to the dissolved ions and the ionic strengh of these ions. As higher the ion strengh as the conductivity will be higher fr the same ion concentration. So, high ionnic strengh ions like sodium chloride will give much higher conductivity than low ion strengh ions like Calcium carbonate in the same ionic concentration.

For a certain water quality you have to find the relation between EC/TDS by measuring the real TDS by total evaporation of the sample at 105oC.

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I have also to make clear that TDS has a specified clearly value (i.e. x mg/lt), instead of Conductivity, which's value is also temperature dependant, as the ions "speed" is higher as temperature rises. The relation between Conductivity/ΔΤ is also no standard (usually 1-2,5%/°C), depending also from the ions type (MW, ionic strenght etc.). So, concerning Conductivity, you have always to mention the temperature measured, in order to be certain.

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