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Converting uS/cm to ppm for TDS of zinc phosphate lines

I am trying to determine the amount of total dissolved solids (TDS) in water for zinc phosphating lines. I am doing this because for phosphating, it is better to have minimal (TDS) in all water because the bath solution and rinsewater will have a longer life and it reduces the chemicals used and discharged. The conductivity measurements I am taking are in microSiemens per centimeter (µS/cm). How can I convert uS/cm to parts per million (ppm) to determine TDS for the zinc phosphate lines.

Sarah Kamer
- Louisville, Kentucky, USA

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I would guess that the conductivity of your water with a specific PPM of one chemical species will be different to the conductivity with a different chemical species at the same PPM.

Your idea may well work if you always have the same contaminants in your water and the ratios remain the same. In this case you could calibrate your system by directly measuring the actual dissolved solids of a sample by evaporation. If you were to construct a three point graph with a low, mid point and high PPM of dissolved solids this should give you a good curve from which you could get a fairly good indication of TDS at any level.

Remember that conductivity is very temperature sensitive so make sure you have a properly temperature compensated conductivity meter or always take your readings at the same temperature. I also suggest that you use a four electrode meter as the two electrode types are very error prone.

May your TDS always remain optimal.

John Holroyd
- Elkhorn, WI

In the lower ranges ppm is approx. 1/2 of us/cm so if reading conductivity just divide by 2 to get ppm... At the higher ranges it changes, on a graph it is curved not linear.

Mike Ruffner
- Fort Lauderdale, Florida