TDS meter:      

TDS meter indicates the total dissolved solids (TDS) of a solution, or the concentration of dissolved solid particles. Since dissolved ionized solids, such as salts and minerals, increase the conductivity of a solution, a TDS meter measures the conductivity of the solution and estimates the TDS from that reading.TDS can measure Water, Salt solution Mineral solution, dissolved organic solids, Sugar, Microscopic solid particles Colloids.