Real-life case: the morning after a pool party (20 guests, water KO'd)
- The situation: a birthday with ~20 swimmers; the morning after: hazy water, a strong chlorine smell, stinging eyes.
- The test reveals the great classic: collapsed free chlorine + high combined chlorine (chloramines).
- Recovery: cleaning, calculated "breakpoint" shock chlorination, continuous filtration — swimmable again that same evening.
- The chlorine smell was the sign there wasn't ENOUGH of it: the classic misconception, finally understood.
Saturday: the 10th birthday party, twenty swimmers aged 8 to 70, six hours of cannonballs. A total success. Sunday, 9am: the bill. Hazy water, a waterline greasy to the touch, and above all that chlorine smell stinging your nose from the terrace — plus the red eyes of the two brave souls who swam at 9:30. The old-school reflex would have been “it smells of chlorine, whatever we do, let’s not add more”. Here’s why that would have been exactly the wrong call — and the real recovery, hour by hour.
9:30: the test that sets the record straight
Strip, photo, verdict in 30 seconds: free chlorine collapsed, pH slightly high, and — the key — a total chlorine well above the free. That gap is combined chlorine: the chloramines, manufactured all day yesterday as the chlorine threw itself at the sweat and sunscreen of twenty people. They’re what smells strong and stings the eyes. In other words: it smells of chlorine because there’s none left. The classic misconception, dismantled by two numbers.
The app delivers the diagnosis without flinching: sanitisation flat, record organic load, a short recovery window before the heat turns it all into green water. Plan: cleaning, pH, breakpoint shock chlorination, continuous filtration.
10am-12pm: the protocol, in order
10:00. Mechanical cleaning: net across the surface (the greasy film shows up against the light), skimmers emptied — two cups recovered —, waterline brushed where the sunscreen settled, pump basket rinsed.
10:30. pH correction, dose calculated: a shock in high-pH water would lose most of its strength, and this one needs to be precise.
11:30. The centrepiece: breakpoint chlorination. The principle: raise the free chlorine high enough, in one go, to completely oxidise the chloramines. It’s a threshold calculation — underdosed, the chlorination creates new chloramines instead of destroying them, and you make the smell worse while believing you’re helping. Dose calculated on the volume, the measured combined chlorine and the product to hand. Poured in, filtration running continuously, swimming “best avoided” with a countdown on the lock screen.
The afternoon: the comedown, measured not guessed
3pm. The smell has already dropped noticeably — the sign the chloramines are burning off. Photo re-test: combined falling, free still above the swimming range. The app keeps the swim break in place and refines its estimate: this evening.
6:30pm. Re-test: free back in range, combined down to a trace, water crystal clear again — the afternoon sun helped the comedown. Indicator: “swimming ideal”. The kids who’d been circling the pool since 5pm get their answer — measured, not guessed.
What this case teaches
A chlorine smell is a shortage alarm, not an excess alarm — the “no more chlorine, whatever you do” reflex only delays the fix. Breakpoint is a threshold business: it’s the textbook case where an approximate dose isn’t “roughly right” but actively counterproductive. And the window matters: treated on Sunday morning, the water was perfect by evening; ignored until Wednesday, it would have been a full green-water rescue. The next party is already in the app: pool party mode on, water prepped the day before, recovery plan ready before the first dive.
Frequently asked questions
Why did the water smell strongly of chlorine when there was almost none left?
Because what smells isn't active chlorine (nearly odourless) but chloramines — chlorine "spent" on sweat and cosmetics. Strong smell = lots of combined chlorine = sanitisation flat on its back. It's THE classic pool misconception.
A single day to recover after 20 swimmers — is that normal?
With an immediate test and a properly dosed shock chlorination, yes: chloramines are destroyed in one cycle, and filtration clears things up straight after. The delay balloons mainly when you treat late or at random.
And if the water had been left like that for a few days?
Flat chlorine + organic matter + summer heat = a near-guaranteed flip to green water within 48-72 h. The easy recovery window is short — that's exactly what this case illustrates.