A complete guide to understanding the difference between electrical conductivity and Total Dissolved Solids (TDS), how they are measured, how TDS is calculated from conductivity, and which parameter to use for your application.
Electrical conductivity measures the ability of water to conduct an electrical current. Pure water (distilled or deionised) conducts electricity very poorly because it contains virtually no ions. As ionic substances — salts, acids, alkalis, and minerals — dissolve in water, they break into positively and negatively charged ions that can carry electrical current. The more ions present, the higher the conductivity.
Conductivity is expressed in Siemens per centimetre (S/cm). For most water quality applications, the values are small enough to use microsiemens (μS/cm) or millisiemens (mS/cm). It is measured using a conductivity cell — two electrodes immersed in the liquid — with an AC signal applied between them.
Total Dissolved Solids (TDS) measures the total concentration of all dissolved materials in water — salts, minerals, metals, organic compounds — expressed in milligrams per litre (mg/L) or parts per million (ppm). True TDS can only be measured by evaporating a known volume of water sample and weighing the solid residue left behind — a time-consuming laboratory process.
In practice, TDS is estimated electronically from conductivity measurement using a conversion factor. This is fast, continuous, and accurate enough for most applications — which is why all TDS meters and controllers actually measure conductivity and calculate TDS from it.
| Parameter | Conductivity | TDS |
|---|---|---|
| What it measures | Electrical conductance of water | Total dissolved solid concentration |
| Unit | μS/cm or mS/cm | ppm or mg/L |
| Direct measurement | Yes — measured directly by electrode | No — calculated from conductivity |
| Accuracy | High — direct measurement | Approximate — depends on conversion factor |
| Temperature effect | ~2%/°C — ATC compensation required | Same — derived from conductivity |
| Typical use | Industrial process control | Drinking water, RO permeate quality |
| Standards reference | Industrial specifications (μS/cm) | Drinking water standards (ppm) |
The conversion formula is: TDS (ppm) = Conductivity (μS/cm) × Conversion Factor
The conversion factor is not universal — it depends on the type of ions dissolved in the water. Different dissolved salts have different relationships between their conductivity contribution and their mass concentration. Common conversion factors:
| Water Type | Typical Conversion Factor |
|---|---|
| Natural groundwater / surface water | 0.55 to 0.70 |
| RO permeate | 0.50 to 0.55 |
| Sodium chloride (NaCl) solutions | 0.50 to 0.55 |
| Potassium chloride (KCl) solutions | 0.50 |
| Mixed industrial effluent | 0.60 to 0.70 |
Most instruments default to a factor of 0.5 or 0.67 and allow the user to adjust it. For critical applications, the factor should be verified against a gravimetric TDS measurement of the actual water being monitored.
| Water Source | Conductivity | TDS (approx.) |
|---|---|---|
| Ultrapure water | < 0.1 μS/cm | < 0.05 ppm |
| RO permeate (pharma) | 1 to 10 μS/cm | 0.5 to 5 ppm |
| RO permeate (industrial) | 10 to 50 μS/cm | 5 to 25 ppm |
| Good drinking water | 100 to 300 μS/cm | 50 to 150 ppm |
| BIS max drinking water | ~1000 μS/cm | 500 ppm |
| Boiler feed water | 50 to 500 μS/cm | 25 to 250 ppm |
| Cooling tower water | 500 to 3000 μS/cm | 250 to 1500 ppm |
| Seawater | ~50,000 μS/cm | ~35,000 ppm |
| Requirement | Recommended Instrument |
|---|---|
| Display conductivity only — no control | 5500 Conductivity Indicator |
| Display both conductivity and TDS | 5500 (front panel TDS/conductivity switch) |
| Automatic conductivity control with relay | 160-COND Conductivity Controller |
| Automatic TDS control in ppm | 160-TDS TDS Controller |
| Lab testing — pH, ORP, conductivity, TDS | 5501 Benchtop Multi-Parameter Meter |
Conductivity measures electrical conductance in μS/cm. TDS measures dissolved solid concentration in ppm. TDS is calculated from conductivity using a conversion factor (typically 0.5–0.7). Conductivity is preferred for industrial control; TDS for drinking water quality reporting.
TDS (ppm) = Conductivity (μS/cm) × conversion factor (typically 0.5–0.7). At 500 μS/cm with factor 0.5: TDS = 250 ppm. The factor depends on the type of dissolved ions — use 0.5 for most RO permeate, 0.55–0.65 for natural water.
BIS standard IS 10500 specifies maximum 500 ppm TDS for drinking water, with a desirable limit of 300 ppm. Water above 1000 ppm is generally considered unacceptable for regular consumption.
Higher temperature makes ions move faster, increasing their ability to carry current. Conductivity increases approximately 2% per °C. Always use ATC (auto temperature compensation) to compare readings taken at different temperatures — all values are corrected to 25°C reference.
For pharmaceutical purified water: conductivity above 1.3 μS/cm at 20°C fails USP standards. For general industrial RO, a sudden rise above your normal permeate baseline indicates membrane deterioration. The 160-COND controller can alarm or divert permeate automatically when conductivity exceeds set limits.
Countronics manufactures conductivity indicators and TDS controllers for RO plants, boiler water, pharma, and industrial water treatment. Contact us for recommendations.