How it works, with the assumptions in plain sight
The picture is real: the sun's position is calculated for your town and every hour of the week, and clouds, rain and temperature come from the Open-Meteo forecast. What happens to the water is decided by a simplified model we wrote to show how a pool behaves, not to replace a test. These are its assumptions:
- Sunlight uses up the most chlorine: the hourly loss depends on the UV index. Without stabiliser, in full summer sun, the model loses about 80 % in two hours (industry sources put it at 75-90 %); with stabiliser, roughly ten times less.
- Warm water uses slightly more chlorine. We estimate its temperature from the air temperature, with a lag: water takes days to warm up and cool down.
- Each swimmer uses about 3 g of chlorine, spread between 11:00 and 21:00 (studies, done in indoor pools, give 2.8 to 4.1 g per swimmer). Rain also brings in dirt that consumes it.
- Algae are always present in tiny amounts. They grow with warmth and light, and free chlorine holds them back. Once chlorine runs out they multiply and also consume any chlorine that comes in.
- The pump filters 8 hours a day (9:00 to 17:00) and gradually clears cloudiness.
- The pool on the right tops chlorine back up to 1.2 ppm as soon as it drops below 1 ppm. We add up what was needed to give you the week's grams.
What it does not simulate
It ignores pH (which changes how well chlorine works), phosphates, shade from trees or buildings, leaves, and salt chlorinators or automatic dosing pumps, which make chlorine on their own. Too much stabiliser also weakens chlorine, and that is not modelled either. Treat it as a way to see what heat does to a pool, not as an exact forecast of yours: that is what a test kit is for.