Automatic pH and chlorine dosing: how it works and when it is worth having
An automatic system measures pH and chlorine continuously and uses dosing pumps to add whatever chemical is missing. It keeps the water stable while you are away or when you have many bathers, but it needs calibration and checking.
What it is made of
Systems vary from one manufacturer to another, but the main parts are the same:
- Measuring cell. A small amount of water from the circuit flows through a chamber in which the sensors are immersed.
- Sensors. A pH electrode and a chlorine sensor.
- Controller. It compares the reading with the level you have set and decides how much to dose.
- Dosing pumps. Small pumps, usually peristaltic, that draw from the pH− and liquid chlorine containers and inject them into the return line.
- Chemical containers in a ventilated spot, kept apart from each other.
How pH and chlorine are measured
| Measurement | How it works | What to watch |
|---|---|---|
| pH | Glass pH electrode | Calibration with buffer solutions, a clean electrode |
| Chlorine via Redox (ORP) | Measures in mV how “active” the disinfectant in the water is | An indirect measurement: it is affected by pH and stabilizer, so the target level is set from a DPD test |
| Free chlorine, amperometric | A sensor that measures free chlorine directly in mg/l | A more direct reading, more demanding to maintain; common in commercial pools |
pH is adjusted first, because it determines how well chlorine performs. That is why the right pH is the foundation of any automation. Automatic dosing usually uses liquid chlorine without stabilizer. If the stabilizer in the water is already high, the Redox reading drops and the system may add more chlorine than needed.
Calibration and regular checks
Automation is only as reliable as its sensors. Electrodes get dirty and age, and their readings slowly drift away from reality. So:
- Test by hand, with a photometer or drops, and compare with the display. If they differ, trust the manual test and calibrate. See how to test the water correctly.
- Calibrate the pH electrode with buffer solutions at the interval the manufacturer specifies, and always at the start of the season.
- Clean the measuring cell and check that water is flowing through it.
- Check the level in the containers, and check the tubes for cracks or crystals.
- In winter, do not let the sensors dry out; store them wet, as the manufacturer specifies.
Alkalinity, hardness and stabilizer are not measured by these systems and are checked by hand from time to time.
Who it is worth it for
- Guesthouses and holiday rentals, where the number of bathers changes from day to day and the water needs to stay stable. See also pools for small hotels.
- Holiday homes and island properties, where the owner is away for weeks. There, automation is combined with a local caretaker, as we explain in remote pool maintenance.
- Large overflow pools, with a lot of water and heavy use.
In a small family pool, where someone tests twice a week, the benefit is smaller. We discuss at the site survey whether there is room in the plant room and which system suits.
Dosing must only operate when water is circulating. Chlorine and acid injected into a pipe with no flow can react with each other and release a dangerous gas. Never mix the two chemicals, either in the containers or in the tubes.
Questions
Do I stop testing with a kit once I have an automatic system?
No. Manual testing is your check on the automation; without it you will not notice when a sensor starts reading wrong. In a home pool, one comparison a week is usually enough.
Does automatic control work with salt chlorination?
Yes. The electrolysis unit produces the chlorine, so no liquid chlorine container is needed, and it is often combined with automatic pH control, because salt chlorination tends to push the pH up.
See also
Water testing
Testing pool water properly: strips, DPD drops or a photometer, how to take a sample, what to test, how often…
Remote maintenance
Remote pool maintenance for a holiday home or island house: helpful automation, a local caretaker with a…
Chemical storage
Storing pool chemicals safely: why chlorine and acid must never go together, where to keep them, how to…