Why Pool Chemistry Defaults Are Set This Way

Every pool account starts with the same target ranges, dose rates, and saturation-index math. This page is the "why" behind those numbers — the public sources, the stoichiometry, and the few places that are a documented judgment call — so a customer, a health inspector, or a new chemist can see what we based the engine on.

How to change a row, what Custom vs Default means, and how the Dose Plan uses volume live in Water Chemistry and Treatment Protocols FAQ. How to attach a rate to a jug is in Dosing Rules and Application Rates.

Open the live numbers on SettingsWater Chemistry. Every row you have not edited reads Default.

Target Ranges

Three readings are captured on a visit and have no range row: Total Chlorine (to compute combined chloramine), Water Temp (LSI and seasonal chlorine), and TDS (LSI).

Why the Recommended Chlorine Is Not Always 1–3 ppm

Cyanuric acid protects chlorine from the sun and also binds it. A flat 1–3 ppm band ignores that, which is how a salt pool held at the recommended CYA 70 used to be dosed chlorine on every visit and then flagged High.

So the working target is a fraction of the stabilizer reading taken that day, then clamped by the Free Chlorine row:

The engine takes the larger of the flat band and the CYA-derived value at each of min / ideal / max, then never targets below 1 ppm or above 10 ppm. A spa still gets its 3 ppm floor at low CYA. An explicit Custom Free Chlorine row is a hard clamp.

That ratio is industry practice, not a CDC table. CDC still publishes the 1 ppm (2 ppm with stabilizer) and 3 ppm spa floors we use as floors. The 7.5% rule is what keeps a stabilized pool sanitary without fighting its own High badge.

If cyanuric acid was not tested, the chlorine target cannot be scaled and LSI is computed as if stabilizer were 0, which reads less corrosive than reality. The Dose Plan says so.

Langelier Saturation Index

LSI answers a different question from the target bands: will this water dissolve the shell and the heater, or drop scale in them?

LSI = pH + TF + CF + AF − TDSF

The cyanurate share of alkalinity follows the Wojtowicz curve (the trade rule of thumb is one-third of CYA at pH 7.5; the curve is shallower at low pH so a sour pool is not over-called corrosive).

Worked anchor, pinned in tests: pH 7.5, TA 100, CH 300, CYA 40, 80 °F, TDS 500 → LSI ≈ 0.06.

Taylor Technologies publishes the same ±0.3 balanced band and ±0.5 action threshold. Taylor's Watergram write-up is the field form of the same formula.

How the Dose Plan Balances LSI

The index is not a badge only. planLsiBalance solves pH, then alkalinity, then calcium against the real LSI function (bisection, because pH enters twice). Target LSI is 0.

• Scaling (LSI > 0.3) — lower pH first, then alkalinity. Calcium is never raised, even if calcium sits below its own band. Adding calcium to already-scaling water is how heaters and cells fail.
• Corrosive (LSI < −0.3) — raise pH, then alkalinity, then calcium. In winter (below the Winter threshold, default 60 °F) calcium may be carried above its normal band, on purpose, to hold the index.

Where a band and the index disagree, the index wins, because the water is actively etching or scaling. The FAQ lists the on-plan wording.

Default Dose Rates

Rates are "amount per 10,000 gallons to move the reading by one unit." One unit is 1 ppm everywhere except pH, whose unit is 0.1 pH, matching how acid and soda-ash charts are always printed. Per-visit caps are safety limits, not bugs; a capped line names how much of the reading is still short.

10,000 gallons of water weighs about 83,450 lb. The chlorine rates below were checked against that mass and the product's available chlorine; liquid chlorine's 10.7 fl oz (vs a stoichiometric ~10.2) leaves a little room for a jug that has sat in the sun.

A 10% chlorine is not 12.5%: attaching the default rescales the rate (and the cap) to the strength in the product name. The rule is always stored in the product's own stock unit.

Combined chloramine above 0.5 ppm, or an algae / cloudy / green-water symptom, converts the visit to breakpoint chlorination: free chlorine must reach 10× the combined chloramine (or 10 ppm on an algae chip). A partial shock re-forms chloramines, so the plan does not scale it down.

Acid and bicarbonate are never recommended on the same visit. Cal-hypo is a poor choice on a pool already at the top of its hardness band. Shocking with dichlor is how pools end up stabilizer-locked.

Season and Demand

Account for the season when calculating doses is on by default under Season & Demand. Chlorine demand roughly doubles for every 18 °F of warming, so a weekly pool needs far more chlorine in July than in October. With the setting on, chlorine is sized to still be in range when the tech next visits, using water temperature, sun, bather load, stabilizer UV cover, and the gap to the next visit — not to the band at the moment of the test.

A hot weekly liquid-chlorine pool often cannot hold enough residual to cover seven days. The honest output is the one that matters: the plan doses to the safe ceiling and reports the day the water will fall below minimum, which is the argument for a mid-week visit, a floater, more stabilizer, or more cell output.

Turn the setting off to dose to the band exactly as read.

What Is a Judgment Call

Not every number has a CDC paragraph. These are flagged in the engine as such:

• Dichlor and trichlor CYA-per-FC factors, taken from published granular charts.
• Muriatic acid's TA-band rows (60 / 100 / 140) and soda-ash's 6 oz per 0.1 pH, taken from published acid / soda-ash charts.
• Salt-cell alkalinity 60–80 ppm (manufacturer TA tables are still 80–120).
• Controller-run CYA 20–50 ppm (probe-life practice).
• Vinyl / fiberglass calcium sitting below the plaster band.
• Phosphate action at 100 ppb and the extra chlorine-demand bump above 300 ppb.
• The 7.5% FC:CYA ratio itself — industry practice layered on CDC floors, not a CDC table.

If your company uses different numbers, type them into the row and save. It reads Custom. Reset to Defaults at the bottom left puts every row back after a confirmation.

Verify it worked

1. Open SettingsWater Chemistry. Rows you have not edited read Default and match the table above.
2. Hand this page to anyone who asks "why is chlorine 1–3?" — the short answer is that 1–3 is the floor and ceiling; the working target follows stabilizer, per the ratio table.
3. On a visit, a water test with pH, TA, calcium, and temperature produces an LSI verdict. Missing temperature leaves LSI blank; missing CYA warns that the index is biased high.