The saturation index is the single number that tells you whether your pool water is going to damage the surface. It pulls pH, temperature, calcium hardness and alkalinity into one figure.
For a marbelite or fibreglass pool in Cape Town, it is worth understanding.
What Is the Pool Saturation Index?
The Langelier Saturation Index, usually shortened to LSI, measures whether water is balanced, corrosive or scale forming.
- LSI between minus 0.3 and plus 0.3 is balanced
- Below minus 0.3 the water is corrosive and will attack the pool surface
- Above plus 0.3 the water is scale forming and will deposit calcium
Individually correct readings can still produce bad water. The saturation index is what catches that.
How to Calculate the Pool Saturation Index
The formula adds four factors to pH and subtracts a constant.
LSI = pH + temperature factor + calcium hardness factor + alkalinity factor minus 12.1
| Reading | Value | Factor |
|---|---|---|
| Water temperature | 16 C / 21 C / 27 C | 0.4 / 0.5 / 0.6 |
| Calcium hardness | 150 / 200 / 300 / 400 ppm | 1.8 / 1.9 / 2.1 / 2.2 |
| Total alkalinity | 80 / 100 / 150 / 200 ppm | 1.9 / 2.0 / 2.2 / 2.3 |
Worked example: pH 7.5, water 21 C, calcium 300 ppm, alkalinity 100 ppm gives 7.5 + 0.5 + 2.1 + 2.0 minus 12.1 = 0.0. Perfectly balanced.
Why the Saturation Index Matters for Marbelite Pools
Marbelite is a calcium based surface. Water that is low on the saturation index will dissolve it.
This is the mechanism behind a pool that goes rough and chalky a few years after being finished. The chlorine was fine. The balance was not.
A freshly relined or re-marbelited pool is at its most vulnerable in the first few months, which is when the saturation index is most worth watching.
How to Correct a Low Pool Saturation Index
Low means corrosive. Raise the easiest factor you can move.
- Raise calcium hardness with a hardness increaser, usually the most effective lever
- Raise total alkalinity with sodium bicarbonate
- Bring pH up toward 7.6 with soda ash or aeration
How to Correct a High Pool Saturation Index
High means scale forming. Scale will clog the filter, fur up the chlorinator cell and roughen the surface.
- Drop pH toward 7.2 with pool acid, the quickest lever
- Lower total alkalinity with acid and aeration
- Dilute calcium hardness by draining and refilling part of the pool
Pool Saturation Index and Cape Town Water
Municipal supply here is fairly soft and low in calcium. Pools topped up from the tap tend to drift to the corrosive side.
Wellpoint and borehole water is usually harder, pushing the saturation index the other way, toward scaling.
Knowing which source fills your pool tells you which direction to watch.
Readings You Need Before Using a Saturation Index Calculator
- pH
- Total alkalinity in ppm
- Calcium hardness in ppm
- Water temperature in degrees Celsius
All four need to be current. A saturation index built on an old calcium reading is not worth calculating.
Get a Full Pool Saturation Index Reading in Cape Town
We test all four values on site and tell you whether your water is protecting or eating the surface.
Our team tests and balances pool water across Cape Town and the Winelands. We work from a real reading, not a guess. Request a free quote or see our pool maintenance services.
