A complete guide to conductivity meters — how conductivity is measured, the difference between conductivity and TDS, temperature compensation, and industrial applications.
Pure water (distilled or deionised) has very low conductivity — close to zero — because it contains almost no dissolved ions. As salts, minerals, acids, or alkalis dissolve in water, they break apart into positive and negative ions that can carry electrical current. The more ions present, the higher the conductivity.
This makes conductivity measurement an excellent and fast method of assessing water quality — far simpler than chemical analysis and continuous rather than batch-based. A sudden rise in conductivity in a water treatment system signals increased dissolved solids that may require corrective action.
A conductivity meter uses a conductivity cell — a pair of electrodes with a precisely defined geometry — immersed in the liquid being measured. A small AC voltage is applied between the electrodes. The resulting current flow depends on the resistance of the liquid between the electrodes, which in turn depends on the ion concentration.
The instrument measures this resistance and converts it to conductivity using the cell constant — a factor that depends on the electrode geometry (the distance between and area of the electrodes). The cell constant is either 1.0 or 0.1 for most industrial conductivity cells. Automatic temperature compensation using a PT100 sensor corrects the reading to the standard reference temperature of 25°C.
| Parameter | Conductivity | TDS |
|---|---|---|
| Measures | Electrical conductance of water | Total dissolved solids concentration |
| Unit | μS/cm or mS/cm | ppm or mg/L |
| Direct measurement | Yes — measured directly | No — calculated from conductivity |
| Conversion | — | TDS = conductivity × 0.5 to 0.7 |
| Used in | Industrial process control | Drinking water, RO permeate |
| Water Type | Typical Conductivity |
|---|---|
| Ultrapure water (semiconductor grade) | < 0.1 μS/cm |
| RO permeate (pharma/food) | 1 to 50 μS/cm |
| Drinking water | 50 to 500 μS/cm |
| Boiler feed water | 100 to 2000 μS/cm |
| Cooling tower water | 500 to 3000 μS/cm |
| Industrial process water | Up to 200 mS/cm |
A conductivity meter measures the ability of water to conduct electricity in μS/cm or mS/cm — directly indicating the concentration of dissolved ions. Higher conductivity means more dissolved salts or minerals in the water.
Conductivity is measured directly in μS/cm. TDS is calculated from conductivity using a conversion factor (typically ×0.5 to 0.7) and expressed in ppm. Both indicate water purity — conductivity is used for process control; TDS for drinking water quality.
Conductivity increases approximately 2% per °C. ATC uses a PT100 sensor to correct all readings to 25°C — ensuring consistent, comparable measurements regardless of process liquid temperature.
The cell factor (1.0 or 0.1) depends on the geometry of the conductivity electrode. It is set from the front panel to match your specific conductivity probe for accurate measurements.
A sudden rise in RO permeate conductivity (typically above 50 μS/cm for food/pharma grade) indicates membrane deterioration or bypass. The 160-COND controller can activate an alarm or divert the permeate automatically when conductivity exceeds the set limit.
Countronics manufactures conductivity indicators and controllers in Sahibabad, Ghaziabad. ISO 9001:2015 certified, established 1978. Models: 5500, 5501, 160-COND, 160-TDS. Contact: +91-9810536887.
Tell us your application — RO plant, cooling tower, pharma water, or ETP — and we will recommend the right model with datasheet and pricing within 24 hours.