pH measurement in pools & swimming pools
Without the right pH, even the best chlorine does not work. Why the zone between 7.0 and 7.4 is decisive - and which electrode reliably feeds the dosing system.
Why 7.0 to 7.4 is the magic zone
In the pool, the pH value is the control knob on which almost everything hangs. Chlorine develops its disinfecting power mainly as hypochlorous acid - and its share is strongly pH-dependent: at pH 7.0 roughly three quarters of the free chlorine is in the effective form, at pH 8.0 only about a quarter. A pH that is too high therefore means more chlorine consumption with worse disinfection. At the same time, the range around 7.2 is skin-friendly (close to the tear film of the eye), prevents limescale deposits and protects liner and equipment. Hence the rule: pH first, everything else second.
Manual testing or dosing system?
Test strips and drop tests are fine for a small above-ground pool. But as soon as an automatic dosing system regulates the pH - the comfort standard for permanently installed pools - a pH electrode takes over the measurement. It sits in a measuring cell in the circulation loop and continuously reports the actual value to the system, which then doses pH-minus or pH-plus. Often a redox electrode measures the disinfecting power right next to it - the two look alike but are not interchangeable.
Exactly how the two quantities differ - and why the millivolt display is no chlorine meter - is explained in the basics article pH and ORP.
Which electrode fits?
- Design: A rugged gel electrode - maintenance-free in continuous operation, insensitive to the mounting position in the measuring cell.
- Pressure rating: In the pressure line behind the pump, the electrode must withstand the system pressure - check the manufacturer specification (often rated up to 1-2 bar) or measure in an unpressurised bypass.
- Connector and mounting: BNC connectors and a 12 mm shaft diameter for common measuring cells and holders are widespread; many dosing systems from different manufacturers use compatible electrodes.
- Saltwater pools: Electrolysis systems get along with standard gel electrodes, but the salty water ages them somewhat faster - keep an eye on the calibration values.
The care rhythm through the season
- Season start: Check the electrode and calibrate with fresh buffer solutions (pH 7 and pH 4) - never with last year's opened bottles.
- During the season: Recalibrate roughly monthly and occasionally cross-check the value with a manual test kit; remove limescale with dilute acid.
- Winterising: Remove the electrode and store it frost-free, moist in its protective cap with KCl solution. An electrode left to dry out or freeze in the empty measuring cell over winter is the most common pool-electrode death.
Rule of thumb for lifespan: Pool electrodes in continuous operation last one to two seasons reliably. If the dosing system refuses calibration in spring or the readings creep sluggishly, replacement is almost always more economical than troubleshooting.
Matching products: Pool electrodes, redox electrodes and calibration solutions are available in our shop aquaundco.com (German-language).