Automatic Chlorine and pH Dosing for Swimming Pools: Installation Guide

Chlorine demand in a pool changes hour by hour with bather load, sunlight and water temperature. Manual dosing cannot follow this fluctuation: excess chlorine in the morning and insufficient disinfection in the afternoon is the typical pattern. Excess chlorine turns into eye and skin irritation and chloramine odour; a shortfall turns into microbiological risk.

In an automatic system, measurement probes read free chlorine and pH continuously and the controller runs the pumps proportionally toward the setpoint. The result is a stable 1.0 to 1.5 mg/L of free chlorine, a pH of 7.2 to 7.6 and chemical savings of up to 20 to 30%. The second gain, as important as the chemical saving, is a lower fresh water demand, since chloramines and total dissolved solids build up far more slowly.

Commercial pools are required by public health regulations to measure and record water quality at regular intervals; systems that measure and log automatically meet this obligation effortlessly and keep the evidence file ready for inspection.

Sample water is taken from the clean line after the filter and passed through a probe holder (flow cell). Free chlorine (amperometric or redox/ORP), pH and temperature probes are fitted to this cell, and a flow switch locks out dosing when circulation stops. An amperometric chlorine probe measures directly in mg/L and is preferred for regulatory records; ORP is more economical but is affected by cyanuric acid and pH changes.

The controller manages two pumps according to the measured values: a chlorine pump for sodium hypochlorite and a second pump for the pH reducer (usually dilute sulphuric or hydrochloric acid). pH is stabilised first, because chlorine effectiveness depends directly on pH: at pH 8.0 the disinfecting power of free chlorine is roughly half of what it is at pH 7.2.

The practical sizing rule: about 1.5 to 2 L/h of chlorine pump and 1 L/h of acid pump per 100 m³ of pool volume is a typical starting point; in commercial pools with high bather load the figure must be verified against daily chlorine consumption. A PVDF head and PTFE diaphragm are standard for pool chemicals, and SEKO's solenoid pump ranges cover this need from 0.4 to 110 L/h.

In deck-level (overflow) pools the balance tank is the heart of the circulation circuit and the hydraulic reference of the dosing system. Sample water is taken from the filter outlet, not from the balance tank, because tank water represents bather load and chemical injection with a delay, whereas post-filter water is identical to what returns to the pool.

The sample line is run in 6 to 10 mm tubing by the shortest route and without air pockets; the flow to the cell is typically 30 to 60 L/h and is fixed with a regulating valve. A fine strainer is fitted at the start of the line and the outlet returns to the balance tank or the pump suction. Pressure fluctuation in the sample water distorts probe readings, so the cell must be kept independent of the filter backwash line.

Injection points must always be downstream of the sample take-off, after the heater and filter, on the return line to the pool. Injection valves are spring-loaded with a check function; a spring preload matched to line pressure (typically 1 to 3 bar) prevents pool water from flowing back into the chemical line when the pump stops.

1) Injecting too close to the measurement point: the chemical fools the probe and the system starts to oscillate. Injection must always be after the sample take-off, preferably downstream of the heater. 2) Skipping the flow switch: if dosing continues when the circulation pump stops, a dangerous concentration builds up inside the pipe.

3) Injecting chlorine and acid side by side: this creates a chlorine gas risk inside the line; leave at least 30 to 50 cm between the two points and, where possible, put another piece of equipment in between. 4) Neglecting probe calibration: the pH probe should be calibrated monthly and the chlorine probe within the interval given by the manufacturer.

5) Placing the chemical tanks in the same bund or spill tray: hypochlorite and acid must be stored in strictly separate compartments. Getting these five points right determines most of the system's reliability; the rest is regular maintenance and calibration.

The first cost driver of a pool dosing system is the measurement technology: simple ORP-based controllers are economical, while an amperometric free chlorine probe, combined chlorine measurement and temperature compensation raise the investment but are needed for regulatory records in commercial pools. The second driver is the pump class: pool volume and bather load determine pump flow and therefore the model.

The third driver is the depth of integration: data logging in the controller, alarm forwarding to the building management system or GSM, management of several pools from one panel and pre-wiring inside the panel all affect both the initial investment and commissioning time. Accessories such as chemical tanks, spill trays, level switches and injection valves should not be considered separately from the quotation.

The information needed for a sound quotation: pool volume (m³), circulation flow and filtration turnover time, whether the pool is indoor or outdoor and heated, average and peak daily bather numbers, the chemicals in use and the panel location. With this data the system is sized correctly the first time; otherwise insufficient pump capacity comes back as a replacement cost in mid-season.

Daily: cross-check controller readings with a DPD test kit and record them; check chemical tank levels. Weekly: clean the probe flow cell and its strainer, inspect the pump suction strainers and the injection valves.

Monthly: calibrate the pH probe with pH 7 and pH 4 buffer solutions, verify the chlorine probe against a DPD reference and check the injection valves for scaling and crystallisation. Seasonally: replace diaphragm and tubing kits, flush tanks and lines with clean water and renew the probe electrolyte. Probe life is typically 1 to 2 years; an ageing probe drifts and will not hold calibration.

SEKO's pool range of measurement, control and dosing equipment delivers this architecture as a single package. As an authorised SEKO dealer, Vanera provides site survey, installation, calibration and season-opening maintenance on site. For a quotation sized to your pool, fill in the Quick Quote form on our dosing pump page; our engineering team will propose a probe, controller and pump configuration for your volume and bather load.