A hotel pool with clear, odourless water. Guests swimming comfortably. The pool operator adds the same chlorine dose every morning that he has been adding for years. Yet a guest develops a skin infection, and a health department water test shows inadequate disinfection. The chlorine was there. The problem was that it was not working — and the reason it was not working was that the pH was too high.
Chlorine in water exists in two forms — hypochlorous acid (HOCl), the active disinfectant, and the hypochlorite ion (OCl⁻), which has only about 1/80th the disinfecting power. The ratio between these two forms depends entirely on pH:
| Pool pH | Active Chlorine (HOCl) | Disinfection Effectiveness |
|---|---|---|
| 7.0 | ~75% | Excellent |
| 7.2 | ~66% | Very good |
| 7.5 | ~50% | Good |
| 7.8 | ~35% | Moderate |
| 8.0 | ~25% | Poor |
| 8.5 | ~10% | Very poor — unsafe |
A pool that appears to have adequate chlorine but has drifted to pH 8.0 may actually have less than a quarter of the disinfecting power the operator thinks it has — with the same chlorine dose and the same total chlorine reading on a test kit.
ORP (Oxidation Reduction Potential) cuts through the complexity of chlorine chemistry by measuring the actual disinfecting power of the water directly, in millivolts. It does not matter which disinfectant is used, what the pH is, or what the total chlorine level is — ORP measures the net oxidising power at that moment.
The World Health Organization recommends a minimum ORP of 650 mV for safe pool water. Most operators target 650–750 mV. A pool with ORP above 650 mV is effectively disinfected. A pool below 650 mV may not be — regardless of the chlorine dose. ORP is the most reliable indicator of actual disinfection status.
pH and ORP interact — pH affects how well chlorine works, which in turn affects ORP. Monitoring both gives the complete picture:
In a hotel, resort, club, or public pool, manual testing and dosing is inadequate. pH and chlorine levels can change significantly between manual tests as bathers introduce organic load, as sunlight degrades chlorine, or as fresh water dilutes the treatment chemicals. Automatic control using online electrodes and dosing pumps maintains both parameters continuously — without waiting for the next manual test.
| Parameter | Recommended Range | Standard |
|---|---|---|
| pH | 7.2 to 7.8 | BIS IS 3949 / WHO |
| ORP | 650 to 750 mV | WHO |
| Free chlorine | 1.0 to 3.0 ppm | BIS IS 3949 |
| Total dissolved solids | Below 2000 ppm | General guideline |
pH determines how much chlorine is in the active disinfectant form. At pH 7.0, 75% of chlorine is active. At pH 8.0, only 25% is active. The same dose at high pH delivers far less disinfection — creating a safety risk even when chlorine levels appear normal.
WHO recommends a minimum of 650 mV. Most operators target 650–750 mV. Below 650 mV, disinfection may be inadequate. Above 800 mV, skin and eye irritation can occur. ORP is the most reliable real-time indicator of actual disinfection effectiveness.
No. Without pH control, chlorine effectiveness varies dramatically even with a consistent dose. pH must be maintained at 7.2–7.8 for chlorine to work effectively. Monitoring ORP alongside pH gives the complete disinfection picture.
The Model 7215-01 combined pH and ORP controller monitors and controls both parameters simultaneously from one panel-mounted instrument — controlling pH correction and chlorine dosing pumps automatically.
Automated dosing is increasingly required by hotel rating bodies and state health departments for commercial pools. Beyond regulatory requirements, it provides documented proof of water safety — important protection against guest health complaints.
Contact Countronics for controller recommendations, electrode selection, and pricing. We respond within 24 hours.
