What is the relation between electrical conductivity (EC) & dissolved solids (TDS)?
Though there is a close relationship between TDS and Electrical Conductivity, they are not the same thing. Total Dissolved Solids (TDS) and Electrical Conductivity (EC) are two separate parameters. TDS is the combined total of solids dissolved in water. EC is the ability of something to conduct electricity (in this case, water's ability to conduct electricity).
The only true method of measuring TDS is to weigh residue found in water after the water has evaporated. You know those spots you see on a glass after you wash it and let it air dry? That's TDS! That residue has mass, and it's possible to weigh it, but if you're not in a lab, it can be tricky thing to do. Therefore, we can estimate TDS levels based on the conductivity of the water since the hydrogen and oxygen molecules of the H2O carry almost no electrical charge. The EC of most other metals, minerals and salts will carry a charge. A A TDS meter measures that EC level and then converts it to a TDS measurement. Since different metals, minerals and salts will be more or less conductive than others, there are different conversion factors that can be used.
ppm (parts per million) is the most commonly used scale to measure TDS (Total Dissolved Solids).
µS (micro-Siemens) is the most commonly used scale to measure EC (Electrical Conductivity).