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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.

Compare with a variation

Result

σ Conductivity 390.6 µS/cm

What a conductivity meter would read on the same water.

With your numbers
390.6 µS/cm=250 ppm0.64
LaTeX 
390.6\,\mathrm{µS/cm} = \frac{250\,\mathrm{ppm}}{0.64}

This result is a link — the address bar holds your numbers, so it can be pasted into a post and opened to the same answer.

What this looks like

Conductivity against total dissolved solids Total dissolved solids swept from 125 ppm to 375 ppm with everything else held at your numbers. The dashed lines cross where you are.
Conductivity against total dissolved solidsConductivity rises from 195 µS/cm to 586 µS/cm as total dissolved solids rises from 125 ppm to 375 ppm. At your total dissolved solids of 250 ppm it is 391 µS/cm.100200300400500600100200300400250 ppm391 µS/cmTotal dissolved solids (ppm)Conductivity (µS/cm)
The formula behind the curve
σ=tr
σ
Conductivity, µS/cm
t
Total dissolved solids, ppm
r
Conversion factor
LaTeX 
σ = \frac{t}{r}
What moves the answer Each input moved 10% either way, with the others held still, and the effect on conductivity.
What moves the answerConductivity is most sensitive to Conversion factor, which moves it by about 11.1% for a 10% change. It is least sensitive to Total dissolved solids, at about 10%.Change in the answer when each input moves by 10%-20%-10%10%20%Conversion factor±11.1Total dissolved solids±10
The formula behind the curve
σ=tr
σ
Conductivity, µS/cm
t
Total dissolved solids, ppm
r
Conversion factor
LaTeX 
σ = \frac{t}{r}

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.