Reading the result
- −0.3 to +0.3: balanced. Many service techs aim for about 0 to +0.3 on plaster.
- Below −0.3: corrosive to plaster, grout and metal over time.
- Above +0.3: scale-forming, especially on heaters and salt cells where water is hottest.
How it is calculated
We use the standard Langelier formula: LSI = pH − pHs, where pHs = (9.3 + A + B) − (C + D), with A from TDS, B from water temperature, C from calcium hardness and D from carbonate alkalinity. Carbonate alkalinity is your total alkalinity minus the share contributed by cyanurate and borate at your pH, which matters in stabilized pools.
Water temperature changes the result: check the saturation index at your warmest (hot tubs, heaters) and coldest (winter) temperatures.
Not professional advice. Results are estimates for residential pools and hot tubs. Read and follow product labels: add chemicals separately, to water (never water to chemicals), with the pump running, and retest before adding more. Commercial and public pools must follow local health codes.