ORP (Oxidation-Reduction Potential) controllers automate chemical feed on commercial pools. They measure the pool water's disinfection activity in millivolts and turn feeders on and off to hold a setpoint. Understood correctly, ORP is a powerful tool. Misunderstood, it's a source of false security.
What ORP actually measures
ORP is the voltage potential of oxidizing vs. reducing species in water, measured by a platinum electrode reference-compared to a silver/silver-chloride reference electrode. In a pool, the primary oxidizer is hypochlorous acid (HOCl), so ORP reading correlates with HOCl concentration.
ORP is NOT a ppm meter. It tells you the effective oxidation potential, which is affected by:
- Free chlorine concentration
- pH (HOCl vs OCl⁻ ratio)
- Temperature
- CYA level (bound chlorine doesn't register the same)
- Contaminant load (reducers in the water pull ORP down)
Typical ORP setpoints
| Facility type | ORP setpoint |
|---|---|
| Residential / small pool | 650–700 mV |
| Commercial pool | 700–750 mV |
| Spa / hot tub | 700–750 mV |
| WHO guidance | ≥ 650 mV |
| Many state codes | ≥ 700 mV |
A pool at 750 mV ORP but CYA of 80 ppm may have lower actual kill-power than a pool at 650 mV with zero CYA. ORP targets assume low/moderate CYA.
pH probes and their limitations
pH probes use a glass electrode to measure H⁺ concentration. They're accurate when fresh and properly maintained, but they drift:
- Calibration: Required monthly minimum; weekly is better. Use pH 7.0 and pH 10.0 buffer.
- Cleaning: Rinse with distilled water, wipe with a soft cloth. Never touch the glass bulb.
- Reference solution: The fill solution inside the probe can leak out or degrade. Replace probe every 1–2 years.
- Storage: Never let a pH probe dry out. Keep wet in storage solution.
Amperometric free-chlorine probes
For more precise disinfection monitoring, some controllers use amperometric (electron- flow-based) free-chlorine probes. These read FC directly in ppm, not indirectly via oxidation potential. More expensive, more precise, more maintenance.
Controller logic: on/off vs. proportional
- On/off control: Feeder runs at full rate until setpoint is reached, then shuts off. Simple, but overshoots and undershoots.
- Proportional control: Feeder runs slower as the reading approaches setpoint. Holds more steadily but requires modulating-capable feeders.
- PID control: Adds integral and derivative terms for fastest settling time. Used on high-demand facilities.
The most common controller failure modes
- Drifted pH probe — controller thinks pH is 7.5 when it's really 8.2. Feeder thinks it's balanced while the pool is drifting.
- ORP probe fouling — oils and minerals coat the platinum. Cleans up with a probe-clean solution.
- Setpoint set wrong — too often, setpoints get changed during commissioning and never checked again.
- Feeder output drift — controller commands 100% but feeder delivers 60%. Proportional control masks the problem.
- Reference solution depletion on pH probe — requires probe replacement.
Checks every operator should do
- Daily: visually verify ORP and pH readings match manual tests within tolerance (e.g., ±0.2 pH, ±50 mV)
- Weekly: calibrate pH with fresh buffer
- Monthly: clean probes, verify feeder calibration
- Quarterly: replace pH buffer kits, inspect tubing, pressure-test injection points
- Annually: replace pH probe, inspect ORP electrode, commission-level feeder calibration