Electrical conductivity and total dissolved solids

This calculator calculates the total dissolved solids of water by its electrical conductivity using the formula
,
where

• TDS is the total dissolved solids of water in mg/L or ppm
• SC is the measured specific conductance of water, usually in μS/cm
• k is the conversion factor. Most household water quality meters (TDS meters) have it equal of 0.5, but this may not always be true.

You can read more about the relationship between the electrical conductivity and the total dissolved solids of water under the calculator.

Electrical conductivity and total dissolved solids

Electrical conductivity

µS/cm

• µS/cm
• mS/cm
• S/cm
• S/m
• EC

Conversion factor

Calculate

Total dissolved solids (ppm, mg/L)

 

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Relationship between electrical conductivity and the total dissolved solids of water

Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular (colloidal sol) suspended form. TDS is usually expressed in milligrams per liter (mg/L or mg/dm³), or parts per million (ppm). For dilute solutions, these units are considered equal, 1 mg/L = 1 ppm.

This measure characterizes the quality of freshwater and is used in the study of water quality for streams, rivers, and lakes. According to the document of the World Health Organization, water with the TDS of less than 300 mg/L is assessed as excellent, from 300 to 600 mg/L, as good, from 600 to 900 mg/L, as fair, from 900 to 1200 mg/L, as poor, and more than 1200 mg/L - as unacceptable¹. At the same time, extremely low TDS values also make the water unacceptable for drinking due to its insipid taste (distilled water). Also, the productivity of farming crops, raising livestock, and poultry depends on the TDS of water.

The standard method for determining the total dissolved solids of water is the gravimetric method. In this method, a filtered (with 2-micrometer or smaller pores) and an accurately measured volume of water is evaporated for a specified time at a specified temperature (for example, 1 to 2 hours at 180°C), and then the dry residue is measured. The quality of the result is influenced by various factors such as pore size and filter thickness, evaporation time, sample volume, etc. In addition, at temperatures above 100°C, organic matter decomposes with the release of CO₂, but since its content is usually low, this effect is neglected. Another method is the chemical analysis of water and the calculation of TDS through the concentration of ions.

All these methods are expensive and time-consuming, therefore, they are inconvenient for use where quick results are needed. For field conditions, a method is used based on establishing the relationship between TDS and the electrical conductivity of water. More correctly, it is the relationship between TDS and specific conductance, however, the term electrical conductivity is more generally used.

Specific conductance is a measure of the ability of a substance to conduct an electric current. In SI units, conductivity is measured in siemens per meter (S/m) or Ohm⁻¹ · m⁻¹. Siemens is the reciprocal of resistance for conductivity. Sometimes you can find the obsolete mho notation, which is the reversed ohm.

In the physical sense, the specific conductance of water is the reciprocal of the electrical resistance of water at a temperature of 25°С, located between two electrodes with a surface of 1 cm², the distance between which is 1 cm. Pure water has poor electrical conductivity, about 0.055 μS/cm at 25°С. Accordingly, the more ions are dissolved in it, the greater its electrical conductivity. This allows us to express the relationship between TDS and electrical conductivity in the form of a linear conversion factor.

The main problem is that this conversion factor is not constant and depends on whether the water is surface water or water from a well, which salt ions prevail in it, etc. For example, when studying the TDS of mine waters in South Africa, the factor varied from 0.25 to 1.34². Studies of natural water sources in the United States gave a range from 0.54 to 0.96, while most of the factors were within the range of 0.55 - 0.75³. And in southern Australia, for example, the recommended value is 0.64 ⁴.

Thus, to apply this method, either a preliminary calculation of the conversion factor based on the value of the TDS obtained by another method and the value of the specific electrical conductance is required, or the use of the conversion factor recommended for the given region. At the same time, when giving the value of the TDS obtained by the method of measuring the electrical conductivity, it is necessary to mention the used conversion factor.