We often see confusion arise when looking at the difference between conductivity (EC) and TDS. What is the difference? Can they be converted? Let’s explore the two in more detail.
What is Electrical Conductivity (EC)?
Under the SI (International Systems of Units), conductivity is referred to as ‘electric conductivity’ as an international standard term but you will often just see the word ‘conductivity’ which is more commonly used. For the purpose of this article, we will refer to conductivity as EC. EC measures the ability for a material to conduct electricity meaning how well can the liquid carry electric current through it. When looking at EC in water or liquid, we will generally find water in its natural state such as rain, lake water, rivers etc. will have a low level of EC (pure water has no EC). When water is subject to pollution, contamination, or impurities, we will see a change in the EC of that water as the dissolved substances increase the level of EC. As such, EC can be a good indicator of pollution in water. It is worth noting, however, that impurities such as salt in seawater, for example, cause very high EC readings as the water contains a large amount of dissolved salt. This is because when salt is dissolved, it separates into ions (electrically charged atoms).
Conductivity is usually measured in microSiemens (µS/cm) and readings generally fall between 30 µS/cm to 2000 µS/cm. Seawater, for example, will have a reading of approximately 50,000 µS/cm. An interesting fact worth mentioning is that conductivity will actually increase when the temperature of the water increases, hence EC is commonly recorded at a 25°C with temperature and EC being recorded simultaneously. Another unit of measurement for EC is mS/cm or milliSiemens per centimetre. 1 mS/cm = 1000 µS/cm and you will often find measurements displayed as mS/cm for higher levels of EC (i.e. 2000 µS/cm or more).
What is TDS?
Total Dissolved Solids (TDS) refers to the number of substances that have been dissolved in the liquid. These substances can include salts, minerals, metals, calcium and other compounds which can be both organic and inorganic. In simple terms, TDS refers to anything present in the water that is not pure water and is not a suspended solid. The most commonly used method of determining TDS is measuring specific conductivity to detect the presence of ions in water (EC). Once the EC is determined, a conversion factor is run (generally by the meter performing the measurement) to determine the TDS. The conversion factor will differ depending on the sample you are testing, conversion factors generally range from 0.4 – 1.0. This method, however, only estimates TDS levels. For a true TDS measurement, you need to take the sample back to the laboratory and carry out an evaporating and weighing procedure. Part of the reason why the conversion from EC is not entirely accurate is that some dissolved solids may not increase the EC readings and won’t be picked up in the measurements, therefore, does not get converted to TDS. It’s worth noting that even though TDS meters only provide an estimate, they are fairly accurate and you will generally find a TDS meter will be satisfactory for most applications. It is worth checking what level of accuracy is required to determine the best measurement method for your individual needs.
TDS is generally measured in parts per million (ppm) but can also be measured in mg/L. Generally, good quality water will fall between 0 and 600 ppm and readings over 1200 ppm are often considered an unsatisfactory level of TDS.
What is the difference between EC and TDS?
While TDS can be determined from EC, the two do differ which is why the conversion is only an estimate. EC is looking at how well electricity can pass through the substances that are present in the water. TDS is looking at the dissolved solids in the water and looks at both EC generating particles as well as particles that don’t conduct electricity which is where the main difference occurs. Therefore, the EC to TDS conversion factor will change depending on the contents of the sample and these conversions are performed automatically by your TDS meter. It is important to get the right meter suitable for your application. Even though it might make sense to choose a meter with the widest measurement range, this will decrease the accuracy of your measurements.
In most instances, EC will be measured and converted to TDS to get an approximate TDS reading. TDS meters will perform this conversion automatically based off a conversion factor. This factor will range depending on the sample being tested and it is important to choose the most appropriate meter for your application. Most modern meters, however, will allow you to adjust the conversion factor to help you get the most accurate conversion when measuring EC.