Pool water is judged on disinfection effectiveness rather than drinkability. The controlling parameters are free residual chlorine and pH — if either drifts, the disinfectant stops working regardless of how much you add. Bacteriological indicators confirm whether the disinfection is actually holding.
These are widely used operating ranges for public and semi-public pools rather than a single statutory limit. We report your measured values against the range your operator or local authority works to.
| Parameter | Unit | Typical range | Why it matters |
|---|---|---|---|
| Free residual chlorine | mg/L | 1.0 – 3.0 | The active disinfectant |
| pH value | — | 7.2 – 7.8 | Below 7.2 chlorine is harsh; above 7.8 it stops working |
| Total alkalinity | mg/L | 80 – 120 | Buffers pH against swings |
| Calcium hardness | mg/L | 200 – 400 | Low corrodes surfaces, high scales them |
| Turbidity | NTU | Under 0.5 | Lifeguards must see the pool floor |
| Cyanuric acid (stabilised pools) | mg/L | Under 100 | Too much locks up free chlorine |
| Total coliform | per 100 ml | Absent | Disinfection failure indicator |
| E. coli | per 100 ml | Absent | Faecal contamination |
| Pseudomonas aeruginosa | per 100 ml | Absent | Causes ear and skin infections |
| Heterotrophic plate count | CFU/ml | Under 200 | General biofilm load |
If a micro sample is collected in an ordinary bottle, the chlorine in the water keeps killing bacteria on the way to the laboratory and you get a clean result from a dirty pool. The bottle has to contain a neutraliser. This is the single most common reason a pool passes on paper and fails in practice.
Chlorine and pH should be read on site at least daily — those drift hourly with bather load and sunlight. A laboratory panel including bacteriology is a periodic confirmation that the daily routine is working, typically monthly for a busy commercial pool.
Usually pH drift or chloramines from combined chlorine rather than a lack of disinfectant. Test pH, free and total chlorine together — the gap between free and total is what irritates skin and eyes.
Yes, and it needs testing more often. Warm agitated water with a high bather-to-volume ratio grows Pseudomonas far faster than a main pool does.
Daily pool-side check of chlorine and pH for housing-society and hotel pools.
Lab note: Daily checks do not replace periodic microbiological testing of pool water.
Check price on AmazonLow-cost dip-and-read pH check for drinking water, pool water and soil.
Lab note: Colour matching gives an approximate value, not a decimal reading.
Check price on AmazonChecks residual chlorine in municipal supply, overhead tanks after cleaning, and chlorinated well water.
Lab note: Colour comparison; test immediately after collecting the water.
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