Two measurements describing different properties
Conductivity describes a solution’s ability to carry electrical current. It depends on the ions present, their concentration and temperature. TDS describes dissolved solids concentration according to the selected analytical method. Weakly ionised organic molecules can contribute dissolved mass without producing an electrical response equivalent to that of a salt.
The cited mine-water study demonstrates variation between water compositions. Its measured factors are not a conversion table for a food-processing effluent. The practical purchasing question is how the reported TDS value was obtained, rather than assuming that an electronic estimate is identical to a gravimetric determination.
Check units and temperature before converting
One useful convention is estimated TDS in mg/L = k × EC at 25 °C in µS/cm. The numerical factor incorporates those units and must not be applied directly to a reading in mS/cm. One mS/cm equals 1,000 µS/cm. Record whether the instrument displays temperature-compensated conductivity or an uncompensated reading at process temperature.
Calibrate using standards suitable for the range and check the temperature compensation setting. Two instruments can show different TDS values for the same conductivity when their factors differ. Changing the factor changes the displayed estimate; it does not change water composition or improve water quality.
Establish a relationship for the actual feed
Collect paired conductivity and laboratory TDS measurements from the same samples, covering normal production, mixtures and concentrations. Calculate an initial k = TDS/EC and examine its variability. If composition changes during CIP or when a new water supply is introduced, assess those water families separately before fitting a relationship.
Illustrative example: at EC25 of 1,500 µS/cm, a factor of 0.60 estimates 900 mg/L and a factor of 0.75 estimates 1,125 mg/L. The 225 mg/L difference comes from the calculation assumption. It does not prove that the second instrument has detected additional contamination.
Use conductivity for monitoring, with supporting analysis
Conductivity trends help identify salinity changes and monitor RO. Nevertheless, selection requires relevant ionic species, hardness, alkalinity, silica, temperature and other feed characteristics. Equal conductivity does not guarantee equal scaling risk, osmotic pressure or permeate quality. Retain the original measurements and analytical methods so that the design projection can be reviewed when the source changes.
Common questions
Are TDS and salinity interchangeable?
Not exactly. They depend on different definitions and methods. An instrument may calculate both values from the same electrical signal using different models and assumptions.
Can I use a fixed factor of 0.65?
It may be an explicitly identified assumption for a preliminary estimate. Check the error against your water before using it for sizing, mass-balance comparisons or contractual acceptance.