TDS Estimated from Conductivity (TDS = k × EC)

Evaporating a litre of water to dryness and weighing the residue is the only true TDS measurement, and it takes a drying oven and several hours. A conductivity meter gives an answer in two seconds, and because dissolved salts are what carry the current the two track each other closely enough for daily work: TDS in mg/L ≈ k × conductivity in µS/cm. The factor k is not a constant of nature — it depends on which ions dominate. Chloride-rich waters run near 0.55, typical fresh surface and well waters around 0.65, and sulphate- or bicarbonate-heavy waters climb toward 0.70. A tower reading 500 µS/cm at k = 0.65 is carrying roughly 325 mg/L of dissolved solids.

Two things will bite you. First, conductivity is strongly temperature-dependent — about 2% per °C — so every meaningful reading is temperature-compensated to 25 °C; a probe without compensation reading warm blowdown will overstate TDS by 15% or more. Second, non-ionic dissolved solids are invisible to the meter. Silica, dissolved organics and sugars contribute real gravimetric TDS but carry almost no current, which is why a high-silica cooling water or a food-plant effluent can gravimetrically test far above what its conductivity predicts. Calibrate k against one oven-dried sample of your own water and it becomes a genuinely reliable daily tool.