Conductivity & TDS

Conductivity & TDS

The conductivity of water simply refers to its ability for electricity to pass through it. Pure water cannot conduct electricity but water that contains small amounts of impurities can. Conductivity is therefore a very useful measure of the salts and minerals dissolved in the water. The conductivity of water is normally measured in microsiemens per centimetre (µS/cm). Conductivity increases with temperature and therefore conductivity levels are normally quoted at the standard temperature of 25°C.

Water Conductivity µS/cm
Distilled Water 0-5
Melted Snow 2-42
Tap Water 50-800
UK Rivers 300-2,000
Seawater 50,000

Measurment of Conductivity

Conductivity is measured with a probe and a meter. Two electrodes separated by a known distance are immersed into the water sample, the drop in voltage in the detecting circuit is caused by the resistance of the water and is used to calculate the conductivity per centimetre.

High quality deionised water will give a reading of less than 5 µS/cm, whilst typical tap water will give a reading of 50 to 800 µS/cm depending on hardness, and other salts. Sea water might measure as much as 50,000 µS/cm.

Total Dissolved Solids (TDS)

The total dissolved measurement is closely related to conductivity and is another indication of the overall purity of the water. There are two principal methods for determining TDS, they are gravimetric analysis and conductivity. The gravimetric method involves evaporating all of the liquid away and then measuring the mass of the residue left. A far more practical and more commonly used method is to measure TDS using conductivity, TDS and conductivity are highly correlated an TDS is often calculated from conductivity and a correlation factor.