Measuring conductivity - cell constant, TDS and temperature
Electrical conductivity reveals in seconds how many dissolved salts sit in a water or process medium. This guide explains the cell constant, converting to TDS and why every reading is referenced to 25 °C.
View conductivity metersWhat is the cell constant and why does it matter?
A conductivity cell measures the resistance between two electrodes. The cell constant K describes their geometry as the ratio of electrode spacing to electrode area, expressed in cm⁻¹. Only by multiplying the measured conductance by the cell constant do you obtain the specific conductivity in µS/cm or mS/cm.
The cell constant must match the range. Pure or low-salt water conducts weakly and needs a small constant near 0.1 cm⁻¹, while concentrated process media call for a larger one around 1 or 10 cm⁻¹. A calibration standard, usually 1413 µS/cm KCl, establishes the cell's real value.
How do you convert conductivity to TDS?
TDS stands for Total Dissolved Solids, the sum of dissolved matter in mg/l. Because charged salts create the conductivity, TDS can be estimated from it: TDS equals conductivity in µS/cm multiplied by a factor between 0.5 and 0.7.
The factor depends on composition. Natural waters rich in chlorides often use 0.5, waters high in sulfate or carbonate rather 0.65 to 0.7. For example, 800 µS/cm with a factor of 0.5 gives about 400 mg/l TDS.
- TDS is an estimate - gravimetric drying at 180 °C remains the reference method.
- Uncharged dissolved substances such as sugar or silica are not captured by conductivity.
- Check which factor a meter uses before comparing readings.
- For reverse osmosis and ion exchange the TDS trend is usually enough for process control.
Why is everything referenced to 25 °C?
Conductivity is strongly temperature dependent: for salt solutions it rises by roughly 2 % per kelvin. Without a temperature reference, readings would not be comparable. Meters therefore convert the value to the reference temperature of 25 °C per EN 27888 (ISO 7888).
Linear compensation with a fixed coefficient of about 2.0 %/K suits natural waters near 25 °C. Ultrapure water needs a nonlinear compensation, because there the self-dissociation of water dominates. The coefficient set must match the medium.
Frequently asked questions
Which cell constant do I need for drinking water?
For drinking and process water in the range of a few hundred µS/cm, a cell constant of 1 cm⁻¹ is standard. It covers about 1 µS/cm to 20 mS/cm and suits most everyday measurements.
Is TDS the same as the amount of salt?
TDS is an estimate of dissolved salts in mg/l derived from conductivity. Uncharged substances are not captured, and the factor depends on the water composition. The precise reference remains the gravimetric determination.
Why does my meter read lower in cold water?
Because conductivity falls as temperature drops. With automatic temperature compensation active, the meter converts to 25 °C and gives a comparable value. Without compensation the readings diverge.
How often should I calibrate?
Weekly for frequent use, and at least before important measurement series. Use a KCl standard close to your range, for example 1413 µS/cm, and check the cell for coating.
Looking for the right conductivity meter?
From ultrapure cells with K 0.1 to rugged process sensors, we supply instruments with temperature compensation and matching calibration standards.
Standards based
Measurement and compensation per EN 27888 / ISO 7888.
Matched cell
Cell constants from 0.1 to 10 cm⁻¹ for every range.
Calibration ready
Standard solutions and documentable values included.
Expert advice
Specialists help with cell choice and compensation.