What is TDS?

TDS is really a gravimetric measurement.

TDS (Total Dissolved Solids), is a method used to determine solid content in a solution. To determine TDS, the solution whose volume is known is evaporated and the residue weighed. A conductivity measurement is commonly used to estimate TDS (Total Dissolved Solids) based on the assumption that the solids are predominantly ionic in nature and the relationship between the dissolved ions and conductivity is known. TDS uses units of mg/L (ppm), or g/L (ppt). On some meters, the user can input the TDS factor for the conversion. On more basic units the factor is automatically set to 0.50.