Does the Redox probe measure chlorine directly?

No. It measures the oxidation-reduction potential in millivolts (mV), which indirectly indicates the water's real disinfecting capacity — not the amount of chlorine, but how effective it is being.

Why do you need a pH probe if there is already a Redox one?

Because the Redox reading depends heavily on pH. With the pH out of balance, the Redox can give a misleading reading even when chlorine is at a reasonable level — which is why one is almost never installed without the other.

How often do the probes need calibrating?

There is no fixed universal rule, but it is a periodic task — a poorly calibrated probe can give stable but incorrect readings, which is worse than having no probe at all, because it creates a false sense of control.

Redox and pH Probes: What Each Is For

If your pool has automatic dosing, these two probes are what tell the system how much product to add at any given moment. They work together, but they measure different things, and confusing what each one does is the most common mistake when talking about this subject.

Redox probe: it measures effectiveness, not quantity

Redox (oxidation-reduction potential) is measured in millivolts (mV), not in ppm. It doesn't tell you "how much chlorine there is", it tells you "how well that chlorine is disinfecting right now". It's the difference between quantity and effectiveness: you can have chlorine "within range" and still have poor disinfecting capacity if other factors (high pH, excess stabiliser) are neutralising it.

Redox (ORP)

250-900 mV · parametric value (RD 742/2013)

0 (no disinfection)5001000 mV

It is a broad legal range, not an exact value, and the regulation only requires measuring it when the disinfectant isn't chlorine or bromine; with chlorine, the Redox probe is a control aid, not a requirement. As a practical guide, water is considered well disinfected when the Redox exceeds 650 mV.

pH probe: the one that makes the Redox reading meaningful

This measures exactly the same as any pH measurement, acidity or alkalinity, on the 0-14 scale. Its role here is to stop the Redox from "lying": with the pH out of range, chlorine loses part of its disinfecting power even though its quantity doesn't change, so the system needs to correct the pH first for the Redox reading to be reliable.

The real advantages of having both

Dosing matched to actual conditions at the time, not to an estimated average
Fewer day-to-day manual measurements
Lower product consumption, the system only adds what is needed, not a fixed dose

The automatic dosing loop

The probes measure Redox and pH continuously, the controller compares the reading with the set-point values, and if a correction is needed it activates the dosing pump, which adds just enough product until the probes confirm that the water is back within range.

The real drawback hardly anyone mentionsThe Redox probe is very sensitive: a variation of just ±5 mV in the reading can represent an error of up to ±30% in the actual chlorine estimate. It isn't a system fault, it's the nature of this measurement, and that is why periodic calibration is not optional.

Why they don't completely replace manual testing

Although they automate day-to-day dosing, a poorly calibrated probe gives stable but incorrect readings, it is still worth checking against a manual reference measurement from time to time, especially if you notice something odd in the water that the probes aren't picking up.

Redox probe and pH probe: what each one is for