What Chlorine Does
Free chlorine kills bacteria, viruses and algae. Combined chlorine is free chlorine that has already reacted with sweat, urine and sunscreen — that's the smell. You want free chlorine in range and combined chlorine as close to zero as you can get.
Target free chlorine: 1–3 ppm (up to 4–5 ppm in a heatwave or after a party, then let it drop).
The Types You'll See
- Liquid chlorine — fast, no calcium, no stabiliser. My default for weekly dosing on concrete pools.
- Granular calcium hypochlorite — good for shock. Adds calcium.
- Stabilised tablets (trichlor) — slow dissolve in a floater or skimmer. Convenient, but they add cyanuric acid every time. Don't rely on tablets alone in a hot QLD summer.
- Salt chlorinator — makes liquid chlorine from salt. You still test free chlorine. The cell doesn't replace testing.
How Much to Add
These are starting points for a 50,000 litre pool to raise free chlorine by about 2 ppm. Scale up or down with your volume.
| Product | Approx. dose for +2 ppm |
|---|---|
| Liquid chlorine (~12.5%) | 800 ml – 1.2 L |
| Cal hypo granular (~65%) | 150 – 200 g |
| Trichlor tablet | Not for a one-off raise — they dissolve slowly |
Always dissolve granules first. Add with the pump running. Retest after a few hours.
How Often
- Summer, heavy use: test 2–3 times a week. Dose whenever free chlorine is under 1 ppm.
- Shoulder season: weekly.
- Winter: weekly or fortnightly if the cover is on and nobody is swimming — don't let it hit zero.
Sunlight eats chlorine. That's why cyanuric acid (stabiliser) at 30–50 ppm matters on outdoor pools. Above 80–100 ppm, chlorine gets "locked" and you need a partial drain.
What About Salt Pools?
A salt cell is a chlorine factory, not a set-and-forget machine. If free chlorine is low, turn the cell output up, increase run time, or add a slug of liquid chlorine while you work out why (dirty cell, low salt, low stabiliser, or a heatwave).
Tips That Save Money
- Fix pH before you add chlorine. High pH wastes it.
- Dose at dusk, not midday.
- Don't stack trichlor tablets in the skimmer for weeks — CYA creeps up and pH drops.
- A clean filter and 8 hours of circulation in summer do as much as an extra bottle of chlorine.
