Estimate free chlorine from ORP and pH, solve ORP or pH with a Nernst-based model, or use two-point calibration for local water conditions.
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ORP to Chlorine Level Formula
The formula to convert ORP to chlorine level is not straightforward as it depends on various factors such as pH and temperature. However, for the purpose of this calculator, a simplified linear approximation is used:
Variables:
- ORP is the Oxidation-Reduction Potential value (mV)
- Chlorine is the chlorine level (ppm)
| pH | Free Chlorine (ppm) | ORP (mV) |
|---|---|---|
| 7.0 | 0.5 | 715 |
| 7.0 | 1 | 724 |
| 7.0 | 2 | 733 |
| 7.0 | 3 | 738 |
| 7.2 | 0.5 | 703 |
| 7.2 | 1 | 712 |
| 7.2 | 2 | 721 |
| 7.2 | 3 | 726 |
| 7.4 | 0.5 | 691 |
| 7.4 | 1 | 700 |
| 7.4 | 2 | 709 |
| 7.4 | 3 | 714 |
| 7.6 | 0.5 | 679 |
| 7.6 | 1 | 688 |
| 7.6 | 2 | 697 |
| 7.6 | 3 | 702 |
| 7.8 | 0.5 | 667 |
| 7.8 | 1 | 676 |
| 7.8 | 2 | 685 |
| 7.8 | 3 | 690 |
| * Approximate, Nernst-based estimates rounded to nearest mV. Assumes 25°C, no cyanuric acid, Ag/AgCl reference electrode. ORP rises ~9–10 mV per doubling of free chlorine and falls ~60 mV per pH unit. | ||
What is ORP?
ORP stands for Oxidation-Reduction Potential, which is a measure of the cleanliness of the water and its ability to break down contaminants. ORP levels are measured in millivolts (mV) using an ORP meter. A higher ORP value indicates a greater ability to oxidize and thus disinfect contaminants, which is often associated with higher levels of chlorine.
How to Calculate Chlorine Level from ORP?
The following steps outline how to estimate the chlorine level from an ORP measurement.
- First, measure the ORP value of the water using an ORP meter.
- Input the ORP value (in mV) into the calculator.
- Use the simplified formula provided above to estimate the chlorine level in ppm.
- Click the “Calculate” button to get the estimated chlorine level.
- Use the result as a guideline for understanding the disinfection potential of the water.
Example Problem:
Use the following variables as an example problem to test your knowledge.
ORP value (mV) = 750 mV
Using the simplified formula, the estimated chlorine level would be:
Chlorine (ppm) = (750 – 650) / 200 = 0.5 ppm