Dissolved ionized solids, such as salts and minerals, increase the electrical conductivity (EC) of a solution. Because it is a volume measure of ionized solids, EC can be used to estimate TDS. Dissolved organic solids, such as sugar, and microscopic solid particles, such as colloids, do not significantly affect the conductivity of a solution, and are not taken into account.
The most accurate way to measure all TDS in water in a laboratory is to evaporate the water leaving behind dissolved solutes as residue, and then weighing the residue.
Units of TDS
Many TDS meters display the TDS in parts per million (ppm); 1 ppm indicates 1 milligram of dissolved solids per kilogram of water.
TDS vs. EC
The terms EC and TDS are both often used to quantify the amount of dissolved solids in water. Pure H2O has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels.
The only accurate way of measuring TDS is to evaporate the water and weigh the solid residue. The TDS level can be estimated by measuring the EC of the water with a meter and converting. A TDS meter is actually an EC meter calibrated in TDS, estimating the TDS from the EC. Some meters can be set to display either value.
All elements dissolved in water have some electrical charge. Therefore, it is possible to estimate the quantity of TDS by determining the electrical conductivity of the water by passing a small current through it. A function similar to an Ohmmeter is performed where the voltage and resultant current are measured and conductivity calculated.
The conversion from EC to TDS can create significant errors as the conversion factor is different for different dissolved solids. For example, to convert the EC from Potassium Chloride to TDS, the conversion factor is 0.50-0.57. Sodium chloride has a factor of 0.47-0.50 and some typically found dissolved minerals can run as high as 0.85. The conversion factor also varies with temperature and only the more sophisticated meters measure the solution temperature and compensate.
- "Frequently Asked Questions". Retrieved 23 May 2017.
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