TDS meters in focus

TDS meters are easy to order over the internet and seem to be a user-friendly tool to analyse water quality even for hobby chemists. How useful are they? And what exactly are they measuring?

What is being measured?

The TDS value (Total Dissolved Solids) gives the sum of dissolved solids in water. These solids include for example salts, minerals and metals. The value is also called the conductivity of the water. Because the more such solids or ions are in the water, the better it conducts electricity. TDS meters typically quantify this conductivity in microsiemens or ppm. The latter stands for parts per pillion, i.e. the number of solid particles per one million water mixture particles. A value of 40 ppm means that from one million particles there are 40 dissolved ions and the rest (=999960) are water molecules.

What does it tell about the water quality?

Anyone who assumes that water with a ppm value of zero, is automatically free from any pesticides and drug residues, is far from it. All the TDS meter measures, are merely the number of ions in the water. Most of the pesticides, hormone-disturbing substances and drug residues are usually no ions!

In short: a water with a low ppm value is virtually free from salts and minerals, but can hide a large cocktail of non-ionic chemicals in itself. The result of a TDS meter is therefore not very meaningful with regard to the actual water quality. Other analytical methods, or a water analysis by a reputable laboratory, may be better suited to find out about the real water quality.

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