A TDS Meter measures 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 which is then converted to a TDS reading.

TDS is directly related to quality of water i.e., the lower a TDS figure, the more pure the water. As an example, reverse osmosis purified water will have a TDS between 0 and 10, whereas tap water will vary between 20 and 300.