Pool Chemical Balance Calculator

Both the Calcium Factor and Alkalinity Factor here are computed as `log10(reading) - constant` rather than the reading itself, because their effect on calcium carbonate saturation (the chemistry Wilfred Langelier's 1936 Saturation Index actually measures) follows a logarithmic relationship, not a straight-line one — doubling calcium hardness from 100 to 200 ppm shifts the saturation balance by the same amount as doubling it again from 200 to 400. This is standard LSI chemistry, not a simplification specific to this calculator.

The calculator looks up TDSF from fixed bands — 12.1 below 1,000 ppm, rising to 12.5 above 4,000 — rather than computing it continuously, because that's how the standard published LSI reference tables actually present this factor. It's a deliberate match to industry convention rather than an approximation the calculator introduces on its own.

Target pH is the pH value that would bring LSI to exactly 0 if your current temperature, calcium hardness, alkalinity, and TDS all stayed fixed — it's solved directly from the LSI formula by isolating pH. In practice, adding acid to reach that pH also nudges alkalinity down somewhat depending on your buffering capacity, so treat it as the correct target under today's readings, and retest after adjusting rather than assuming the other four factors held perfectly constant.

When LSI already falls within that balanced band, `targetAlkalinity` is simply left equal to your current reading — the calculator only nudges it when water is corrosive (raising alkalinity, scaled by how far negative the LSI is) or scale-forming (lowering it, scaled by how far positive). This keeps the suggestion proportional to the actual imbalance rather than always pushing toward some fixed alkalinity number regardless of whether an adjustment is even needed.