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What is the difference between TDS and conductivity?

Both Conductivity (EC) and total dissolved solids (TDS) are water quality parameters, but there is a difference.

Definition: 

TDS concentration describes the presence of inorganic salts and small amounts of organic matter in water and conductivity is the measure of water capacity to conduct electrical current.


How is it measured?

TDS is expressed in Mg/l or PPM (parts per million) while conductivity is usually expressed in micro siemens (µS/cm). 

What is the difference? 

Conductivity (EC) is looking at how well electricity can pass through the substances that are present in the water. While TDS is looking at the dissolved solids in the water and looks at both conductivity (EC) generating particles as well as particles that don’t conduct electricity which is where the main difference occurs. 

Conductivity (µS/cm) expressed in TDS 

Both conductivity and TDS are often used interchangeably. Most water purification systems use µS/cm to measure conductivity. Therefore, in most instances,  conductivity (EC) will be measured and converted to TDS to get an approximate TDS reading. Converting µS/cm to PPP/Mg/L (TDS) the general conversion factor is 0.64. Meaning that 100 µS/cm = 64 TDS.