Saturation Index Field Reference

Saturation indexes help answer one narrow but important question: under the stated conditions, is the water undersaturated, approximately saturated, or oversaturated with respect to calcium carbonate?

They do not certify sanitation, comfort, clarity, electrical safety, metal-corrosion protection, stain identity, or overall water quality.

Thirty-second calculation check

Before accepting any LSI or CSI number, verify:

• Exact pool or spa and actual bulk-water temperature
• Current direct pH, measured TA, CH, CYA, salt or TDS, and borate when present
• Named calculator or formula and version
• Whether the method expects measured TA or already corrected carbonate alkalinity
• No CYA, borate, or other correction applied twice
• Surface and startup plan plus heater, cell, sanitizer, and product limits
• Current result and a predicted result at the expected higher pH or temperature
• Every individual parameter independently acceptable

If the record says only “LSI 0.0” or shows only an app screenshot, it is incomplete.

LSI, CSI, and calculator names

Pool software may label its result LSI, CSI, water balance, or another proprietary term. Different tools can use different:

• temperature factors;
• TDS or ionic-strength corrections;
• CYA and borate corrections;
• constants and rounding;
• input assumptions;
• recommended bands.

Do not assume two tools produce interchangeable numbers because both center around zero. Your company uses one approved method for a property record, identifies its version, and compares trends through that same method unless a documented migration recalculates the history.

Do not build a hybrid formula by taking a temperature factor from one chart, alkalinity factor from another app, TDS constant from a third source, and a target band from a manufacturer manual.

What the sign means

Negative result

The model estimates calcium-carbonate undersaturation under the entered conditions. Prolonged or substantial undersaturation can support dissolution of calcium-bearing cementitious material.

Do not translate the result automatically into “corrosive water.” Corrosion of metal also depends on material, oxygen, chloride, sanitizer, galvanic contact, protective films, bonding, temperature, flow, and other chemistry.

Near-zero result

The model estimates approximate calcium-carbonate equilibrium. It does not mean every input is ideal or that the water will remain there as pH and temperature change.

Positive result

The model estimates increasing calcium-carbonate precipitation tendency. Scale may be more likely on hotter, higher-pH, high-flow or high-field-strength locations such as heaters and salt cells even when the bulk pool has not developed visible scale.

An index does not prove that a white deposit is calcium carbonate. Confirm material and mechanism.

Planning band

For ordinary established service, the common approved planning aim is -0.3 to +0.3 using the approved documented method.

That is a planning band, not a universal release rule:

• a cementitious finish, new startup, repair, or manufacturer may require a narrower or more positive plan;
• a heater or salt cell may impose its own water-chemistry and saturation requirements;
• a vinyl, fiberglass, acrylic, tile, grout, or coated pool or spa has different material constraints;
• every individual pH, sanitizer, TA, CH, CYA, salt, temperature, and product limit still applies;
• local scale risk can exist at a hot exchanger or cell when the bulk-water result looks acceptable.

Use the most protective applicable property, surface, equipment, startup, and product requirement.

Input discipline

pH

pH is both logarithmic and usually the fastest-moving index input. In traditional forms of the saturation relationship, a 0.1 pH change shifts the index by approximately 0.1 when all other inputs remain fixed.

Use a valid direct pH result. Do not use:

• last visit's pH;
• controller setpoint instead of measured water;
• an off-scale comparator color;
• a reading taken before a recent chemical addition mixed;
• an unverified digital result with a contaminated or dry probe.

Calculate the likely result at the property's normal upper pH before the next service, not only immediately after acid treatment.

Measured and corrected alkalinity

Preserve measured total alkalinity exactly as tested. The index needs the carbonate contribution defined by its method, while measured TA can also include cyanurate, borate, and other acid-neutralizing species.

There are two common calculator workflows:

1. Automatic correction: enter measured TA, CYA, borate, and other requested inputs; the calculator derives its carbonate-alkalinity value.
2. Manual corrected input: calculate corrected alkalinity by that exact method and enter it in the field explicitly designated for corrected or carbonate alkalinity.

Never manually subtract a CYA fraction and then enter that value into a calculator that corrects CYA again. Never overwrite the direct TA result with the correction.

The familiar “subtract one-third of CYA” shortcut is not universal across pH, borate, temperature, or algorithms. Use the approved method.

Calcium hardness

Use a calcium-hardness test reported as ppm calcium carbonate. Do not substitute total hardness unless the approved method explicitly supports and converts it.

Confirm a surprising value before using it to justify calcium addition or water replacement. High sanitizer, metals, endpoint error, sample dilution, old reagents, and method limitations can affect a result.

Temperature

Use actual water temperature in the correct unit. Warm water is generally more scale-prone in the calcium-carbonate model.

For a heated pool or spa, calculate:

• current bulk temperature;
• normal operating temperature;
• maximum authorized setpoint or realistically expected temperature when consequential.

Air temperature, heater setpoint, and app weather are not water temperature.

Salt and TDS

Use the field the method requests. Saltwater pools intentionally contain a large salt component; entering a generic TDS default or adding salt to TDS inconsistently can distort the result.

Record whether the value is:

• independently tested salt;
• controller-estimated salinity;
• measured conductivity converted by a stated method;
• TDS estimated from startup value and additions;
• calculator default.

An unknown default reduces confidence. TDS is not a “dirty water” score.

CYA, borate, and other buffers

CYA and borate can contribute to measured alkalinity and pH behavior. Enter them as the selected calculator requires. Do not assume zero merely because the weekly service form lacks a field.

If the property uses an unmodeled buffer or specialty treatment and the effect is consequential, use a suitable approved method or qualified review rather than forcing the water into a calculator that cannot represent it.

Calculation workflow

1. Validate direct tests. Repeat or confirm any decision-changing, off-scale, inconsistent, or interference-prone input.
2. Name the method. Record calculator or formula and version before entering data.
3. Declare the alkalinity convention. Measured TA with automatic correction, or separately calculated carbonate alkalinity, never both.
4. Enter current conditions. Preserve exact units and distinguish measured from default or inferred values.
5. Record current result. Include the applicable property decision band.
6. Check individual limits. Stop if the index disguises unacceptable pH, sanitizer, CYA, salt, equipment, or surface conditions.
7. Identify the driver. Change one proposed input at a time in planning mode to see which factor meaningfully moves the result.
8. Predict future state. Expected pH ceiling, heating, source water, evaporation, refill, and planned chemical side effects.
9. Choose a stable plan. Prefer a maintainable combination over one extreme parameter used to offset another.
10. Apply and verify. Follow the exact product SOP, retest direct measurements after mixing, and recalculate with the same method.

Current versus predicted calculation

A complete service decision includes at least two states.

Current state

Uses tests from the representative sample at the current temperature and water condition.

Predicted state

Uses a realistic next condition, such as:

• expected upper pH before the next visit;
• heated spa operating temperature;
• effect of an authorized acid, bicarbonate, calcium, CYA, salt, or water-replacement plan;
• known hard-source-water refill and evaporation trend;
• seasonal temperature change.

Predicted inputs must be labeled predictions. Do not copy them into direct-test fields.

An adjustment that produces 0.00 today but is expected to reach strongly positive saturation after normal pH rise is not a stable plan.

Individual limits override compensation

The index is a sum or relationship, so one extreme can numerically offset another. That does not make the extremes acceptable.

Reject plans such as:

• keeping pH too low to compensate for high calcium;
• keeping calcium excessive while relying on low alkalinity;
• raising calcium in a vinyl pool solely to force an index number without checking equipment and source water;
• accepting high CYA because corrected alkalinity makes the index look acceptable;
• accepting inadequate sanitizer because the water is “balanced”;
• allowing high salt outside the installed generator range because another input offsets it.

The combined result and each consequential direct parameter must both pass.

Treatment hierarchy

Before selecting a product, ask:

1. Is the result real and reproducible?
2. Is an individual value already outside a controlling limit?
3. Which factor drives the current and predicted state?
4. What is naturally changing, pH rise, heating, evaporation, hard fill, rain, overflow, sanitizer, cover, or aeration?
5. Which adjustment improves the system with the fewest unwanted side effects?
6. Can the target remain stable between visits?
7. Is water replacement or source-water management more appropriate than stacking chemicals?

Typical side effects:

• acid lowers pH and consumes alkalinity;
• sodium bicarbonate raises alkalinity and can influence future pH behavior;
• sodium carbonate raises pH and alkalinity and can increase precipitation risk;
• calcium chloride raises CH and releases heat during dissolution;
• stabilized chlorine raises CYA over time;
• liquid chlorine contributes salt after use;
• salt addition changes generator operation and dissolved-solids context;
• water replacement changes several inputs at once according to source-water chemistry.

No side-effect list substitutes for the exact label and procedure.

Pattern interpretation

Negative today, acceptable at expected pH

The result may be a transient post-acid state. Confirm every individual limit and whether the predicted pH rise is supported by the property's trend. Do not intentionally create a harmful low-pH window.

Acceptable pool, positive heated spa

Temperature and different chemistry or aeration can move the spa into scale tendency. Calculate and manage the spa separately when isolated or materially different.

Acceptable bulk water, recurring cell scale

Review cell surface conditions, pH history, temperature, CH, alkalinity, salt, output, polarity or self-cleaning function, flow, and local generation effects. Do not clear scale repeatedly without controlling the mechanism.

Positive index with cloudy water

Calcium-carbonate precipitation is one possibility, particularly after high-pH product or heating. Air, dead growth, debris, filter media, incompatible products, contamination, and filtration failure remain separate possibilities.

Negative index with metal staining

The index does not identify iron, copper, manganese, galvanic corrosion, or a failing heat exchanger. Use stain, metal, equipment, and source-water evidence.

Current etching or scale claim

Today's index cannot date damage, prove past exposure, identify workmanship, or assign responsibility. Preserve history, startup records, chemistry trend, surface observations, source water, and prior treatment.

Data-quality sensitivity

Because pH directly and strongly affects the result, a small pH testing error can change the classification near a boundary. Uncertain TA correction, CH endpoint, temperature unit, salt, or CYA can also matter.

When the result is close to an action threshold:

• repeat the most influential direct tests;
• use a fresh representative sample;
• confirm units and calculator convention;
• calculate a credible low-input and high-input case;
• choose a conservative action that does not rely on the most favorable edge.

Do not report two decimal places as certainty when inputs do not support them.

Common index failures

• using last week's temperature;
• entering measured TA after manual CYA correction into an auto-correcting calculator;
• deleting measured TA and keeping only corrected alkalinity;
• using total hardness as CH;
• leaving salt or borate at a hidden default;
• comparing an Orenda result with a different manufacturer's target without method review;
• calculating the pool and applying the result to an isolated spa;
• chasing exact zero with unstable pH;
• declaring all negative water corrosive to every material;
• declaring a white deposit calcium scale from index alone;
• using a favorable index to excuse bad sanitizer, pH, CYA, salt, or clarity;
• saving a screenshot without direct inputs and calculator version.

Reproducible record

• Direct tests preserved separately
• Pool or spa and actual temperature identified
• Calculator or formula name and version
• Measured TA and corrected-input convention
• CYA, salt or TDS, borate, and defaults visible
• Current result and applicable target band
• Individual parameter gate passed
• Expected-pH and operating-temperature prediction
• Adjustment side effects modeled
• Actual product and amount recorded
• Direct retest and same-method recalculation complete