Pooli CSI Methodology

1. Why Pooli uses CSI

The Calcite Saturation Index (CSI) estimates whether pool water tends to deposit calcium carbonate (scale) or dissolve it (which can be harder on plaster, tile, stone, and pebble). Pooli leads with CSI for everyday balance guidance, including a slightly negative target band for salt-water generator (SWG) pools to reduce cell scaling risk.

CSI is a scaling tendency index, not a corrosion guarantee for every metal or finish. Recommendations still depend on good test data and normal pool operation.

2. Historical background

• Langelier Saturation Index (LSI). Wilfred F. Langelier’s 1936 work introduced a practical saturation index for calcium carbonate in water systems, originally motivated by municipal distribution concerns.
• Calcite / calcium carbonate saturation. Pool and spa practice adapted saturation ideas to recreational water, often under the names LSI, SI, or CSI.
• Pool industry adaptations. Outdoor pools commonly use cyanuric acid (stabilizer) and sometimes borates. Those species contribute to measured total alkalinity, so pool-oriented calculators typically convert toward carbonate alkalinity before judging saturation. Naming varies by vendor; the corrections matter more than the label.

3. Published sources considered

Pooli’s CSI work is informed by established swimming-pool chemistry practice and public discussion of CSI/LSI, including:

• Trouble Free Pool: CSI and LSI
• Trouble Free Pool: Calcium Saturation Index (CSI)
• Langelier’s original saturation-index framing for calcium carbonate (1936 lineage)
• Conventional factor-based LSI tables used in operator training and MAHC-style water-balance practice (Pooli’s optional LSI comparison path)

Pooli does not claim that its primary CSI is a numbered national or international standard method (for example ASTM/ISO “thermodynamic CSI”). Optional LSI is provided as a conventional factor-based reference, not as a claim of regulatory certification.

4. Goals of Pooli’s implementation

• Practical recommendations for residential and professional pool care
• Pool-specific treatment of cyanurate and borate alkalinity
• Robustness when a single test omits slow-changing parameters (lookback from prior tests)
• Transparent confidence when values are measured, carried forward, estimated, or defaulted
• Clear separation between Pooli’s primary CSI and optional conventional LSI

5. Inputs used

Essentials: pH, total alkalinity (or KH), calcium hardness (or total hardness).

Strongly recommended: water temperature.

Optional (improve accuracy when available): cyanuric acid (CYA), borates, magnesium hardness, measured TDS, salt (especially for SWG pools).

When a current test omits a visible parameter, Pooli may use a recent prior measurement within a type-specific lookback window. Lookback is a real prior measurement, not a numeric default.

6. Carbonate alkalinity treatment

Yes. Pooli converts total alkalinity toward carbonate alkalinity before using alkalinity in CSI. Non-carbonate alkalinity contributions from cyanurate and borate are removed using pH-dependent treatment appropriate to swimming pool water. Exact functional forms are proprietary; the intent matches accepted pool practice: do not treat raw total alkalinity as pure carbonate alkalinity when CYA or borates are present.

7. Calcium hardness treatment

Calcium enters CSI as calcium hardness (ppm as CaCO₃), not as free Ca²⁺ activity after full ion-pairing. Prefer a measured calcium hardness reading whenever possible.

8. CYA adjustment

When CYA is provided (including via lookback), Pooli subtracts cyanurate alkalinity from total alkalinity as part of forming carbonate alkalinity. The correction is pH-dependent. If CYA is omitted, that correction is not applied (treated as zero contribution).

9. Borate adjustment

When borates are provided, Pooli subtracts borate alkalinity contribution using a pH-dependent treatment. Borates are optional and often untested; omitting them leaves that correction at zero.

10. Temperature compensation

Temperature is incorporated in Pooli’s CSI expression. If temperature is missing, Pooli uses a documented default typical swim-season temperature and lowers input confidence accordingly. Optional LSI uses conventional temperature factor tables.

This is not presented as a full thermodynamic K_sp(T) free-ion speciation model.

11. Ionic strength assumptions (TDS vs salt)

Pooli applies a practical ionic-strength correction inside its CSI expression. It does not compute separate thermodynamic activity coefficients γ_Ca and γ_CO₃ from a named free-ion model.

Dissolved solids rule (v1.0):

• If measured TDS is available, TDS drives the dissolved-solids contribution.
• Else if salt is available, salt is used as a proxy (common for SWG pools).
• Salt and TDS are never added together.
• Salt remains useful SWG context even when TDS takes precedence for dissolved solids.

12. Total hardness vs calcium hardness vs magnesium

1. Prefer measured calcium hardness.
2. Else if total hardness and magnesium hardness are known: calcium hardness ≈ total hardness − magnesium hardness (both as CaCO₃ hardness).
3. Else if only total hardness is known: estimate calcium hardness as about 2/3 of total hardness (common pool heuristic).
4. Else use a documented default and mark confidence Low.

Magnesium is optional and rarely provided. When present, it also contributes to the practical ionic-strength estimate. Impact on CSI is usually small compared with pH, alkalinity, calcium, and temperature.

13. Practical limitations

• Not a free-ion thermodynamic CSI of the form log₁₀[(aCa²⁺ × aCO₃²⁻) / K_sp].
• Not a full dissolved inorganic carbon speciation to free carbonate concentration.
• Not a corrosion index for every material; it indicates calcium carbonate scaling tendency.
• Quality tracks input quality. Strip error, stale lookback, or missing CYA can shift results.
• Sulfate and complete major-ion suites are not required inputs; ionic strength remains a practical estimate.

14. Confidence, defaults, and lookback

Pooli surfaces input confidence in the CSI calculator:

• High: essentials from the current test; calcium hardness measured (not estimated).
• Medium: essentials from prior-test lookback, hardness estimated from TH or TH−Mg, or default temperature.
• Low: numeric defaults for missing essentials (for example pH, TA, or CH).

Lookback is intentional. Slow-changing parameters such as CYA, calcium, salt, TDS, borates, and magnesium are often omitted from a single strip test. Carrying forward a recent measured value is preferred over inventing a default, and is labeled as “from a prior test,” not as an assumption pulled from thin air.

15. Why results may differ from Taylor, Orenda, Pentair, MAHC-style LSI

Different tools can disagree for legitimate reasons:

• Factor-based LSI tables versus pool-adapted CSI expressions
• Whether and how CYA and borates are removed from alkalinity
• Whether dissolved solids use TDS, salt, or fixed constants
• Hardness treatment (CH vs TH heuristics)
• Target bands and product advice layered on top of the index

Pooli shows optional LSI for comparison. CSI remains the primary index for Pooli’s water-balance guidance.

Direct answers (technical FAQ)

1. Is Pooli CSI the thermodynamic definition CSI = log₁₀[(aCa²⁺ × aCO₃²⁻) / K_sp]? No.
2. Which activity coefficient model for γ_Ca and γ_CO₃? Pooli does not use a separate free-ion γ_Ca/γ_CO₃ speciation model. It uses a practical ionic-strength correction inside a pool-adapted SI.
3. Is carbonate concentration from full DIC speciation? No. Pooli uses carbonate alkalinity derived from total alkalinity after CYA/borate adjustments.
4. How is calcium represented? Calcium hardness concentration (ppm as CaCO₃), not free Ca²⁺ activity.
5. How is temperature incorporated? Temperature dependence is included in Pooli’s CSI expression. This is not a full thermodynamic K_sp(T) free-ion model.
6. How are ionic strength / TDS treated? Practical ionic-strength correction; measured TDS preferred; salt as proxy when TDS is absent.
7. TA, CYA, borates? Yes. Total alkalinity is converted toward carbonate alkalinity with CYA and borate adjustments when those inputs exist.
8. TH vs CH? Prefer CH; else TH−Mg if magnesium is known; else about 2/3 of TH.
9. Empirical tables? Primary CSI is a continuous pool-adapted expression. Optional LSI uses conventional interpolated factor tables.
10. National/international standard? Pooli does not claim a numbered CSI standard method. The implementation follows established swimming-pool CSI methodology (see sources above), with optional conventional factor-based LSI for comparison.

Methodology version & changelog

Current version: v1.0

• v1.0 — Initial public methodology. Documents practical pool CSI (not free-ion thermodynamic CSI). Optional magnesium hardness and measured TDS (TDS preferred over salt for dissolved solids). Input confidence and lookback labeling. Optional conventional LSI for reference.

Disclaimer: CSI guidance is a decision aid based on user-provided or looked-back water tests. It does not replace professional inspection, local code, or manufacturer instructions for equipment and surfaces.