The Electrical Conductivity of Water

The electrical conductivity of water estimates the total amount of solids dissolved in water -TDS, which stands for Total Dissolved Solids. TDS is measured in ppm (parts per million) or in mg/l.    

Factors Affecting the Electrical Conductivity of Water

The electrical conductivity of the water depends on the water temperature: the higher the temperature, the higher the electrical conductivity would be. The electrical conductivity of water increases by 2-3% for an increase of 1 degree Celsius of water temperature. Many EC meters nowadays automatically standardize the readings to 25^oC.

 
While the electrical conductivity is a good indicator of the total salinity, it still does not provide any information about the ion composition in the water.

The same electrical conductivity values can be measured in low quality water (e.g. water rich with Sodium, Boron and Fluorides) as well as in high quality irrigation water (e.g. adequately fertilized water with appropriate nutrient concentrations and ratios).

Units of measurement of the Electrical Conductivity of Water

The commonly used units for measuring electrical conductivity of water are:

μS/cm (microSiemens/cm)   
            
or

dS/m   (deciSiemens/m)

Where: 1000 μs/cm = 1 dS/m 

TDS and Electrical Conductivity

Since the electrical conductivity is a measure to the capacity of water to conduct electrical current, it is directly related to the concentration of salts dissolved in water, and therefore to the Total Dissolved Solids (TDS). Salts dissolve into positively charged ions and negatively charged ions, which conduct electricity. 

Since it is difficult to measure TDS in the field, the electrical conductivity of the water is used as a measure.
  
The electrical conductivity of the water can be determined in a quick and inexpensive way, using portable meters. 

Distilled water does not contain dissolved salts and, as a result, it does not conduct electricity and has an electrical conductivity of zero.

Nevertheless, when the salt concentration reaches a certain level, electrical conductivity is no longer directly related to salts concentration. This is because ion pairs are formed. Ion pairs weaken each other's charge, so that above this level, higher TDS will not result in equally higher electrical conductivity.


  
EC can be converted to TDS using the following calculation:

TDS (ppm) = 0.64 X EC (μS/cm) = 640 X EC (dS/m)

This relation provides an estimate only.