What is the relationship between TDS and EC?

TDS (total dissolved solids) is related to related to EC (electrical conductivity), but they are not the same thing.

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.

The TDS factor for strong ionic solutions is 0.5, while for weak ionic solutions (e.g. fertilizers) it is 0.7.

TDS = factor x EC₂₅

For example: 100 μS/cm conductivity is a TDS of 50 ppm when the factor is 0.5.