pH and chlorine in Spanish pools: what the numbers actually mean

The two numbers that matter most

Pool water chemistry involves several parameters, but two govern almost everything: pH and free chlorine (or its proxy measurement, ORP).

pH measures how acidic or alkaline the water is, on a scale from 0 to 14. The ideal range for a swimming pool is 7.2 to 7.6. Below that range, the water becomes acidic — corrosive to pool surfaces and irritating to swimmers' eyes and skin. Above that range, the water becomes increasingly alkaline, which causes scale deposits and, critically, makes chlorine far less effective.

Free chlorine is the active disinfectant in the water. A reading between 1.0 and 3.0 parts per million is the standard target for most pools.

Why pH affects chlorine — and why this matters more than most people realise

Here is the thing that surprises most pool owners: chlorine's effectiveness is almost entirely dependent on pH. At pH 7.0, roughly 70% of chlorine in the water is in its active, disinfecting form. At pH 7.8 — still within what many people would consider an acceptable range — that drops to around 30%. At pH 8.0, less than 20% of your chlorine is doing anything useful.

This means a pool with perfectly adequate chlorine levels and slightly high pH can still fail to disinfect properly. You can be spending money on chlorine that is largely inactive.

In Spanish conditions, pH tends to drift upward. Sun and heat cause water to off-gas carbon dioxide, which raises pH. A pool sitting in direct Andalusian summer sun will typically see its pH rise faster than a shaded pool in northern Europe.

What ORP tells you

ORP stands for oxidation-reduction potential, measured in millivolts. Rather than measuring the quantity of chlorine in the water, ORP measures its activity — how much disinfecting work the chlorine is actually doing. A reading above 650 mV indicates effective disinfection. Below 600 mV, the water's disinfecting capacity is compromised regardless of what the chlorine reading says.

ORP is a more useful real-world measure than free chlorine concentration alone, because it captures the combined effect of chlorine level and pH together. A sensor measuring ORP gives you a direct answer to the question that actually matters: is this water safe to swim in right now?

Why manual testing has limits

Manual testing with a kit or test strips gives you an accurate reading at the moment you test. What it cannot tell you is what happened between tests. In a pool that is checked once a week, the reading on Friday tells you the state of the water on Friday — not what happened on Tuesday when there was a heavy bather load, or on Wednesday afternoon when a summer storm diluted the chemistry.

Continuous monitoring with a floating sensor gives you the full picture — every 15 to 30 minutes, all week, whether or not anyone is at the property. When a reading drifts out of range, an alert fires immediately rather than waiting for the next scheduled visit.

Senzo's pool sensor measures both pH and ORP continuously, alongside water temperature, and sends automatic alerts when any reading moves outside safe parameters.