conversion
Conductivity and TDS converter
Convert between microsiemens and ppm — and see how far two correctly working meters can disagree about the same water.
0.5 for a NaCl-referenced meter, 0.7 for a 442 one. Check the manual.
150 µS/cm = 75 ppm at your factor
Moderately mineralised. Lake Malawi sits in this band, as do many hill streams and most temperate rivers.
The same water reads 75 ppm on a 0.5-factor meter and 105 ppm on a 0.7-factor one — a spread of 30 ppm with nothing wrong with either meter. This is why a TDS figure quoted without its factor is not a measurement anybody else can use.
Working — every step, so you can check it
- Conductivity
- 150 µS/cm
- Your factor
- 150 × 0.5 = 75 ppm
- A 0.5-factor meter would read
- 75 ppm
- A 0.7-factor meter would read
- 105 ppm
- Spread between the two conventions
- 30 ppm on the same water
Formula
- TDS (ppm) = conductivity (µS/cm) × k
- conductivity (µS/cm) = TDS (ppm) ÷ k
- k is the meter's conversion factor, typically 0.5 for NaCl-referenced meters and 0.7 for 442-referenced meters
- Spread between conventions = conductivity × (0.7 − 0.5)
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- The conversion factor is constant across the range, which is approximately true over the range an aquarium occupies and not exactly true anywhere.
- Readings are temperature-compensated to 25 °C. Uncompensated conductivity rises by roughly 2 % per degree, so a warm tank reads high on a meter without compensation.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- A 'TDS meter' does not measure dissolved solids. It measures conductivity and multiplies by a factor the manufacturer chose, and that factor depends on which ions are present. Two meters reading the same water can differ by 40 % while both work correctly.
- Conductivity is blind to anything that does not carry charge. Sugars, most organics, silica and urea are dissolved solids and contribute nothing to the reading.
- It cannot tell you WHICH ions. Water at 300 µS/cm of calcium bicarbonate and water at 300 µS/cm of sodium chloride are the same number and completely different water, and only one of them is what a rift-lake cichlid wants.
- The habitat bands shown are AquariumHQ's habitat-class figures — generalisations about kinds of water body, not measurements of any species.
References
- /water/tds
- /learn/kinds-of-water-fish-come-from