TDS and conductivity are typically used to determine the purity of a solution. These parameters are often used to assess samples for checking a water treatment system's efficiency and quality of treated water. TDS and conductivity are convertible by simple calculation, involving a conversion factor. The value of the factor depends upon the type and level of dissolved minerals in the water or the solution.
Total dissolved solids or TDS of a solution is actually the total amount of dissolved mineral salts elements -- mainly sodium, potassium, magnesium, calcium, iron, manganese and aluminium. The total dissolved solids are also responsible for the hardness in water due to formation of the salts of the alkali elements (mainly sodium, potassium, calcium and magnesium) with halogens such as chlorine, bromine and fluorine. TDS is usually expressed in parts per million (ppm).
Conductivity of a solution is the ability of the solution to conduct electricity. This phenomenon is observed when free ions -- the result of dissolved solids in the solution -- are able to carry electrical charge. Therefore, an increase in the TDS means the conductivity also increases. Conductivity is generally expressed in Siemens per centimetre, S/cm or millisiemen/centimeter, mS/cm.
TDS and conductivity can be converted into each other. The conversion can be done by a simple calculation involving a conversion factor that depends upon the chemical composition of the solution. Different solutions contain dissolved solids in different amounts, resulting in a net TDS of the solution. To convert the TDS of the sample into electric conductivity, the concentration of total dissolved solids is divided by the conversion factor. The factor usually varies between 0.54 to 0.96 -- a value of 0.67 is commonly used as an approximation if the actual factor is not known. For a value of TDS recorded by a TDS meter, the conductivity will be: Conductivity (µS/cm) = TDS (ppm) / (0.67).
High TDS levels can cause "mineral tastes" in drinking water and corrosion or encrustation of metallic surfaces by water used in industrial equipment, boilers, domestic plumbing, hot water heaters, toilet flushing mechanisms, faucets, washing machines and dishwashers. Excessive dissolved solids in water can also cause the elimination of desirable food plants and habitat-forming plant species. Sometimes low TDS levels may be unhealthy for plants and fish. The measurement of dissolved solids is extremely important for certain pharmaceutical, manufacturing, industrial, medical and agricultural applications. TDS and conductivity values are used in a number of processes. TDS is used to determine the overall ionic effect in a water source because certain physiological effects on plants and animals are often affected by the number of available ions in the water. Conductivity estimation is used to estimate the variation or changes in natural water and wastewater and also for estimating the sample size necessary for other chemical analyses. TDS can be used to determine amounts of chemical reagents or treatment chemicals to be added to a water sample.