TDS meters and EC (Electrical Conductivity) meters measure related but different parameters:TDS Meters: Measure the total concentration of dissolved solids in ppm (parts per million).EC Meters: Measure the electrical conductivity of the water, indicating ion concentration.While they are related (higher conductivity often means higher TDS), they are not directly interchangeable. Some devices, like the HM Digital Pen Style TDS/EC/Temp Meter, offer both functionalities, providing flexibility and comprehensive water quality testing.