A TDS Meter indicates the amount of Total Dissolved Solids (TDS) in a solution. Since dissolved ionized solids like salts and minerals contribute to the conductivity of a solution, a TDS meter measures the conductivity of the solution then converts that figure to an estimated TDS reading.
Unit
A TDS meter typically displays the total weight of dissolved solids as a proportion of 1 litre of water. The unit used by most TDS meters is parts per million (ppm).For example, a TDS reading of 1 ppm would indicate there is 1 milligram of dissolved inorganic solids in each 1 litre of water.
Dissolved organic solids such as sugar, and microscopic solid particles such as colloids, do not significantly effect the conductivity of a solution so they are not measurable by a TDS meter.