A TDS Meter measures the amount of Total Dissolved Solids (TDS) in a solution.
This measurement is used to determine the purity of the water produced by your RO unit and will give an indication of when your DI resin or membrane life is coming to an end. The inline version measures the TDS of the feed water before the RO Unit and the TDS of the pure water from the unit.
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.