To convert conductivity to Total Dissolved Solids (TDS), multiply the electrical conductivity (EC) value by a conversion factor. This factor typically ranges from 0.5 to 1.0, but is often around 0.64.
Here's a breakdown of the process:
Understanding Conductivity and TDS
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Conductivity (EC): Measures the ability of a solution to conduct an electrical current. It's primarily determined by the concentration of dissolved ions in the water. It is typically measured in microsiemens per centimeter (µS/cm).
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Total Dissolved Solids (TDS): Represents the total amount of dissolved minerals, salts, metals, cations or anions dissolved in water. It's usually expressed in parts per million (ppm) or milligrams per liter (mg/L).
The Conversion Formula
The general formula for converting conductivity to TDS is:
*TDS (ppm or mg/L) = k EC (µS/cm)**
Where:
- TDS is the Total Dissolved Solids.
- EC is the Electrical Conductivity.
- k is the conversion factor.
Determining the Conversion Factor (k)
The conversion factor (k) depends on the water source and the specific ions present. It's not a universal constant. Here's a general guideline:
- Most Natural Waters: k ≈ 0.55 to 0.8
- Typical Value: k ≈ 0.64. This is often a good starting point if you don't have more specific information.
- Highly Saline Water: k can approach 1.0
Importance of Water Source: Different water sources (e.g., rivers, lakes, groundwater) have different ionic compositions. This variation influences the relationship between conductivity and TDS, hence the range of 'k' values.
Determining 'k' accurately requires laboratory analysis of the water sample to identify and quantify the specific dissolved ions.
Example Calculation
Let's say you have a water sample with an electrical conductivity of 300 µS/cm, and you decide to use a k factor of 0.64.
TDS = 0.64 * 300 µS/cm = 192 ppm (or mg/L)
Important Considerations
- Temperature: Conductivity is temperature-dependent. Most conductivity meters automatically compensate for temperature, but if not, you'll need to apply a correction factor. Usually conductivity readings are normalized to 25°C.
- Water Composition: As mentioned earlier, the ionic composition of the water greatly influences the conversion factor. If you need precise TDS measurements, relying solely on conductivity conversion might not be sufficient.
- Calibration: Ensure that your conductivity meter is properly calibrated for accurate readings.
Summary
Converting conductivity to TDS provides an estimate of the dissolved solids in water. While using a conversion factor is convenient, remember that it's an approximation. For accurate TDS measurements, especially when dealing with critical applications like drinking water quality assessment, laboratory analysis is recommended.
