The conversion of total dissolved solids (TDS), expressed in parts per million (ppm), to electrical conductivity (µS/cm) mainly depends on the nature and concentration of dissolved ions in the water, as each ionic solution exhibits specific conductivity properties. However, a general rule is commonly used to estimate this relationship.
General Relationship between TDS (ppm) and Conductivity (µS/cm) :
TDS (ppm) ≈ Conductivity (µS/cm) x k
where k is a conversion factor that depends on the chemical composition of the water. For industrial steam boilers, this factor generally varies between 0.55 and 0.70, with a commonly adopted value of 0.65, resulting in:
TDS (ppm) ≈ Conductivity (µS/cm) x 0,65
Factors Influencing the Conversion
The exact value of k depends on several parameters, including:
- Chemical Composition of the Water: Water rich in chlorides (NaCl) will have a factor close to 0.5, whereas water containing primarily sulfates or carbonates will be closer to 0.7.
- Water Temperature: Conductivity increases with temperature, so it is recommended to standardize measurements at 25 °C.
- Type of Chemical Treatment Used
Practical Recommendations for Steam Boilers
To ensure optimal operation and avoid issues such as scaling or corrosion, it is crucial to adhere to the boiler manufacturer’s recommendations regarding the maximum water conductivity. In the absence of specific guidelines, a TDS value of 3,500 ppm is often adopted as the reference limit for boilers operating at pressures below 300 psig.