How PoolKit calculates saturation index
PoolKit calculates the Langelier Saturation Index (LSI), which pool worksheets often shorten to Saturation Index (SI). To calculate it, PoolKit compares the measured pH to saturation pH (pHs): the pH where the water is at equilibrium with calcium carbonate (CaCO3), so dissolving plaster and forming calcium scale are both unlikely.
PoolKit calculates pHs from total dissolved solids, temperature, calcium hardness, and carbonate alkalinity. Because Cyanuric Acid (CYA) contributes to a total alkalinity test, PoolKit estimates the CYA portion at the measured pH and subtracts it before using alkalinity in the saturation pH equation.
Sources: the EPA operator reference for the continuous LSI equation and the CDC Model Aquatic Health Code Annex for aquatic-venue interpretation and the factor-table method.
CYA-adjusted alkalinity
LSI uses carbonate alkalinity, not raw total alkalinity. Because CYA contributes to a total alkalinity test but does not protect calcium carbonate balance the same way carbonate alkalinity does, PoolKit removes the CYA portion first.
- Start with total alkalinity: -.
- At this pH, 1 ppm CYA counts as - of alkalinity.
- CYA alkalinity: -.
- Carbonate alkalinity used for LSI: -.
The MAHC factor method uses CYA-adjusted total alkalinity for the AF lookup and advises deducting 30% of CYA. PoolKit instead estimates the CYA contribution at the measured pH. If a value has already been CYA-adjusted before entry, leave CYA at 0 here to avoid subtracting it twice.
Compared with the MAHC factor table
The MAHC Annex presents LSI = pH + TF + CF + AF - TDSF, with each factor selected from a table. PoolKit calculates the same saturation balance concept with continuous equations instead of table breakpoints.
Continuous equation
It avoids rounding jumps between table rows, shows the CYA correction explicitly, handles edge cases like zero alkalinity, and is easier to link from a logbook with exact readings.
Factor table
It is fast, familiar in operator training, and practical when an operator needs to calculate by hand without a device.
TDS is treated as ppm or mg/L for the saturation pH equation.
If adjusted carbonate alkalinity is below 1 ppm, PoolKit shows the diagnostic work but does not report an LSI.