Saturation Index (LSI)
One number for whether the water will scale your equipment or dissolve your pool. Calculated from five readings you already take.
- Safe range
- -0.3 – 0.3 index (The aquatic health code's balanced range (5.7.4.6), which is the narrower of the two published figures — the industry operator handbooks allow slightly more on the scaling side and agree exactly on the corrosive end. The narrower one is used here because it errs toward a second look rather than toward plaster damage.)
- Measured in
- index
What it is
The Langelier Saturation Index is the single answer to a question none of the individual readings can settle on their own: is this water balanced with respect to calcium carbonate. It is calculated from pH, temperature, calcium hardness, alkalinity and total dissolved solids — the first used directly, the other four through factor tables. A negative result means the water is short of what it wants and will take it from the pool; a positive one means there is a surplus and it will be deposited.
Why it matters
This is the number that explains damage. Plaster etching, a rough surface underfoot, grout washing out of tile — all of that is aggressive water, and it happens with every individual reading sitting inside its own range. Scale is the same story in reverse, and it costs most where you cannot see it, inside a heater or on a salt cell's plates. Pool operators are taught to balance to this index rather than to any one reading, because the readings trade against each other and the index is where the trade is settled.
How to measure it
There is no kit for this. It is calculated from five readings, so what you actually do is take those five accurately on one sample — and then let a calculator, a pool shop printout or an app do the arithmetic. WiseWaterGauge records all five inputs; the calculation itself is not built yet.
Drop-count titration kit
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A few minutes, and it rewards being unhurried
Add reagent one drop at a time until the sample changes colour, and count the drops. The count is the answer, so there is no colour to judge by eye and no lighting to get wrong.
The four chemistry inputs — pH, calcium hardness, alkalinity and total dissolved solids — are worth taking with a drop-count kit if you intend to act on an index. The factor tables amplify a careless reading: a hardness test overshot by a few drops moves the answer more than it looks like it should.
Where to find it: Pool shops and online, usually as part of a multi-test kit. Aquarium shops sell the same chemistry in smaller bottles.
Digital meter or pen
- How trustworthy
- Dependable
- Cost
- Mid range
- Effort
- Seconds to read, minutes to calibrate
A probe on a handheld body that reads the water electrically and prints a number. No colour to judge, which removes the commonest source of error and introduces a new one: a meter that has drifted looks exactly as confident as one that has not.
pH and temperature are the two inputs where a pen earns its place, and both go straight into the formula — pH directly and temperature through its own factor. A pH read off a faded strip is not a basis for an index.
Where to find it: Online and in pool and aquarium shops. Buy the calibration solution at the same time; a meter without it is a guess with a decimal point.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
The simplest way to get the number today. Most pool shop printouts calculate a saturation index from the panel they just ran and print it at the bottom, which is also a useful check on your own inputs.
Watch out: Ask which index the printout uses. Some software reports a calcite saturation index instead, which is a different calculation with a different balanced range — the figure this product judges against is the Langelier index the aquatic health code names.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • All five inputs from one sample, on one day. An index built from this week's pH and last month's hardness is a number about no particular pool.
- • Take the temperature in the water rather than off the heater's display. It is one of the five, and the equipment pad is not where the water is.
Further reading: CDC: the Model Aquatic Health Code, the guidance US pool codes are written from
If it is too low
The water is aggressive. It will dissolve calcium from wherever it can reach it: plaster, grout, tile and the metal in a heater. This is the more expensive direction, because the damage is permanent and the surface does not grow back.
- • Raise whichever input is furthest below its own range, rather than reaching for pH first. Calcium hardness and alkalinity are usually the ones that have drifted, and both are cheap to correct.
- • In a plaster pool, treat this as urgent rather than as housekeeping — aggressive water works continuously, and a season of it is visible.
- • In a vinyl or fiberglass pool there is far less for the water to attack, but the heater is still metal.
If it is too high
The water is scaling. Expect cloudiness that chlorine does not clear, deposits at the waterline, and — the costly part — a coating inside the heater and on a salt cell, both of which lose efficiency long before anything is visible.
- • Bring pH down first. It is the input the index responds to most directly and the easiest one to move without touching anything else.
- • Check calcium hardness. If it is the input that is high, the only real route down is a partial drain and refill, and a sequestrant holds the scale off meanwhile.
- • Heat pushes the index up, so a hot tub run warm sits nearer the scaling end all the time and needs the other inputs kept lower to compensate.
Common questions
Every reading is in range but my plaster is etching. How?
That is exactly what this index is for. The individual ranges are wide, and a pool can sit legally at the bottom of several of them at once — low pH, low hardness, low alkalinity, cool water — while the combination is aggressive. No single reading looks wrong; the index is where the combination shows up.
Which way is worse, scaling or corrosive?
Corrosive, for most pools. Scale is a maintenance cost and can usually be cleaned off; aggressive water removes material that does not come back, and it works on plaster, grout and metal at the same time. The two published ranges agree exactly on the corrosive end and differ only on the scaling side, which says the same thing.
Does WiseWaterGauge calculate this for me?
Not yet. It records all five inputs and holds the published balanced range, so the guide and the band are ready for the calculation — but today the arithmetic is done by a pool shop printout, a calculator or an app. Combined chlorine is the derived value that is calculated, because it is a subtraction; this one needs the code's factor tables.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.