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Dissolved solids to conductivity

What conductivity does a TDS reading in ppm correspond to?

The formula
σ=tr
σ
Conductivity, µS/cm
t
Total dissolved solids, ppm
r
Conversion factor
LaTeX 
σ = \frac{t}{r}

Work it out

ppm

What a TDS meter reads. Distilled water is under 1; tap water 50–500; sea water about 35 000.

The ppm-per-(µS/cm) factor the meter was calibrated with. 0.64 unless its manual says otherwise.

or see a worked example with typical numbers

Method

  1. A TDS meter measures conductivity in µS/cm and multiplies by its factor to print ppm. Divide the ppm by the same factor to get the conductivity back.

Assumptions

  • The factor is the meter's, and the meter's factor was chosen for a typical mix of ions — sodium chloride for most. Water dominated by different ions reads differently for the same true conductivity.
  • Temperature-compensated to 25 °C, as meters are. Conductivity rises about 2 % per degree; a reading taken on a cold bench and used for a warm cell is low.
  • This is a convention between two ways of describing the same measurement, and no more. Nothing about the cell depends on it except that the conductivity is what the leak resistance calculation needs.