Every pool management decision starts with accurate test results. The four core tests — free chlorine, pH, total alkalinity, and calcium hardness — each reveal a different dimension of water safety and stability.
Key Facts
- Free chlorine (1–3 ppm) is the test most directly linked to swimmer health.
- pH (7.2–7.6) controls how effectively chlorine kills pathogens.
- Total alkalinity (80–120 ppm) buffers pH against sudden changes.
- Calcium hardness (200–400 ppm for plaster pools) protects pool surfaces and equipment.
Test 1: Free Chlorine
Free chlorine (FC) is the most critical safety test. It measures the chlorine that is actively available to sanitise water. Target range is 1–3 ppm for pools and 3–5 ppm for hot tubs. Below 1 ppm, sanitation is inadequate. Above 5 ppm, swimming is uncomfortable and surfaces may bleach. FC should be tested at least twice per week in summer and after every heavy rain or large bather load. Use a DPD-based test kit or test strips rated for free chlorine specifically — OTO-based kits measure total chlorine, which is less useful.
Test 2: pH
pH is the most important factor in chlorine effectiveness. The target is 7.2–7.6 for pools. Test pH every time you test chlorine. pH tends to drift upward in pools as carbon dioxide escapes from the water surface. Aeration from waterfalls, jets, and high temperatures accelerates this. Sodium carbonate (soda ash) raises pH; muriatic acid or sodium bisulfate lowers it. Always adjust alkalinity before pH — an alkalinity adjustment will often bring pH close to target on its own.
Tests 3 & 4: Alkalinity and Hardness
Total alkalinity (80–120 ppm) is tested weekly in new pools or monthly once stable. Alkalinity adjustment is the foundation of pH stability — fix it first and pH becomes much easier to manage. Calcium hardness (200–400 ppm for plaster, 150–250 ppm for fibreglass and vinyl) protects pool surfaces. Low hardness makes water aggressive; high hardness leads to scaling. Test hardness monthly. Adjust with calcium chloride to raise; partial drain and refill to lower.
Examples
A pool owner runs a full test in July: FC 1.0 ppm, pH 7.7, TA 90 ppm, hardness 280 ppm, CYA 45 ppm. The pH at 7.7 is borderline high but not an emergency. FC at 1.0 is acceptable but trending low for mid-summer — adding chlorine tonight and increasing the pump run time will help. Everything else is in range. No urgent adjustments needed except monitoring FC more closely over the next week.
Common Mistakes to Avoid
- Only testing chlorine and pH and ignoring alkalinity until pH becomes impossible to stabilise.
- Using an OTO test kit (which measures total chlorine) and treating the result as if it were free chlorine.
- Collecting the water sample from the surface near a return jet, which gives a falsely favourable reading.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference