How water balance is calculated

Langelier Saturation Index (LSI)

The LSI is a predictive model for calcium carbonate (CaCO₃) saturation in water. Originally developed by Wilfred Langelier in 1936 for municipal water treatment, it has become the gold standard for pool water balance assessment. The index predicts whether water will tend to deposit calcium scale (positive LSI) or dissolve existing calcium surfaces (negative LSI). A balanced pool usually targets an LSI between -0.3 and +0.3.

• pH measurement against the saturation pH (pHs) at current temperature
• Temperature correction factor derived from thermodynamic solubility constants
• Total dissolved solids (TDS) correction for ionic strength effects
• Calcium hardness and total alkalinity as primary mineral contributors

Cyanuric Acid Correction

Cyanuric acid (CYA) forms a reversible complex with hypochlorous acid (HOCl), the active sanitiser species. This complexation reduces effective free chlorine concentration in a non-linear relationship that most simple calculators ignore. The calculator allows for the relationship between CYA and chlorine, which changes with pH and temperature. This gives a more useful estimate than looking at free chlorine alone.

Dosage Calculations

Chemical dosing recommendations are derived from stoichiometric relationships between the target parameter change and the chemical's molecular contribution. The system accounts for: • Reagent purity (commercial-grade chemicals are not 100% pure) • Volume-proportional scaling with your exact pool capacity • Sequential dosing order to prevent precipitation reactions • Buffer capacity effects - alkalinity changes affect pH, and vice versa

• Sodium bicarbonate raises alkalinity at approximately 1.5 ppm per 10g per 10,000L
• Hydrochloric acid reduces pH with alkalinity-dependent buffering resistance
• Calcium chloride additions account for dihydrate vs anhydrous forms
• Stabiliser (CYA) dosing uses dissolution kinetics for granular vs liquid forms

Temperature & Seasonal Modelling

Water chemistry is temperature-dependent. Higher temperatures increase calcium carbonate solubility product (Ksp), shift carbonate equilibria, and accelerate chlorine degradation. The calculator uses Melbourne seasonal temperature assumptions to give more relevant guidance across the year.