The LSI cheat sheet every pool tech should tape to their truck dashboard

You can't see LSI. That's the problem. Free chlorine you can smell. Algae you can see. A bad pH reading lights up the test strip. But the slow grind of corrosive or scaling water, the kind that quietly chews through a heat exchanger or fogs the tile line over an entire season, only shows up in the math. By then your customer is staring at a $1,800 heater bill and asking why their pool guy didn't catch it.

Every working tech we've trained falls into the same trap at some point: they treat pH and alkalinity as the whole story. They are not. Water is a saturated solution, and it is constantly deciding whether to dissolve calcium out of the plaster and equipment, or deposit it onto the surfaces. LSI is the score for which way it's tipping right now.

This is the cheat sheet we wish someone had taped to our dashboard on the first day of route work. Five inputs, one quick formula you can do in your head, three scenarios you'll see every week, and a printable summary at the bottom.

What LSI actually measures

Langelier Saturation Index is a saturation score for calcium carbonate. Positive number, the water has more dissolved calcium than it can hold and starts dropping it out as scale. Negative number, the water is under-saturated and will pull calcium out of whatever surface it can: plaster, tile grout, copper heat exchangers, gunite.

The common mistake is confusing "balanced" with "in range." You can have every individual test in spec (pH 7.4, alkalinity 90 ppm, calcium hardness 250 ppm) and still have water that is aggressively corrosive at 50°F, or actively scaling at 95°F. Each input is a knob. LSI is the dashboard light.

The five inputs that drive LSI

Five things move the index. You measure four of them on every service call. The fifth (TDS) you only need to recheck a few times a year unless something dramatic is happening.

• pH: biggest swinger, easiest to fix. Each tenth of a pH point moves LSI by 0.1. A pool at 7.2 is one full 0.4 below a pool at 7.6.
• Water temperature: colder water is more aggressive. A spa at 102°F and a pool at 58°F can have identical chemistry and behave totally differently. Don't ignore the thermometer.
• Calcium hardness: the calcium that's either being deposited or pulled out. Most residential pools want this between 200 and 400 ppm. Below 150 ppm and your water is hungry.
• Total alkalinity: pH's buffer. Healthy range is 80 to 120 ppm for plaster pools, 100 to 150 for vinyl/fiberglass. Low alkalinity = pH bounce + corrosive drift.
• Total dissolved solids (TDS): everything dissolved in the water. Salt pools live around 3,000–4,500 ppm by design. The TDS factor in the formula is small but matters at the extremes.

The field formula you can do in your head

Here's the version that fits on a dashboard sticker:

LSI = pH + TF + CF + AF − TDS factor

TF is the temperature factor. CF is the calcium hardness factor. AF is the alkalinity factor. Each factor is a one-number lookup from this table. Memorize the row your area lives in and the whole thing becomes a 15-second mental calc.

Quick lookup table

TDS factor is 12.1 for TDS under 1,000 ppm, 12.2 for 1,000–2,000, and 12.3 for 2,000–4,000. Salt pools running 3,500 ppm use 12.3. That's the whole TDS story for routine work.

Worked example. A 78°F plaster pool reading pH 7.5, alkalinity 110, calcium 250, TDS 800:

LSI = 7.5 + 0.6 + 2.0 + 2.1 − 12.1 = 0.1

Balanced. Walk to the next stop.

Reading the result

• −0.3 to +0.3: balanced. Where you want every account. Small adjustments only.
• Below −0.3: corrosive. Water is eating something. Copper heater coils, plaster, tile grout, vinyl seams. The lower it goes, the faster it goes.
• Above +0.3: scaling. Calcium is dropping out as scale. Cloudy water that won't clear, white crust on the tile line, plugged heater tubes that overheat and shut down on the high-limit switch.

The damage is asymmetric. Scaling is ugly but reversible: drain and acid wash, replace the heater tubes if it's gone too far. Corrosion is permanent. You can't un-eat plaster or un-pit a copper coil. Aim slightly positive (+0.1 to +0.2) if you have to err in a direction.

Three scenarios you'll see every week

Scenario 1: high alkalinity, low calcium

The textbook "aggressive water" pool. New construction or a recent refill on city water. Calcium might be 80 ppm. Alkalinity is fine at 120, pH locked at 7.5. LSI lands around −0.6. Customer sees nothing wrong because everything "tests in range."

Fix: add calcium chloride. About 1.5 lb per 10,000 gallons raises calcium ~10 ppm. Get to 250 ppm minimum. Don't chase pH down to compensate. That just makes it worse and bounces alkalinity around. Calcium is the lever here.

Scenario 2: cold-water heater corrosion

Late October in the northeast. Customer wants the pool open for the grandkids' weekend visit. Water is 58°F, otherwise the chemistry looks fine. LSI lands at −0.4 because the temperature factor cratered. Run the heater for a long weekend at that LSI and you'll start pulling copper into solution. Customer doesn't see it until next summer when the plaster starts to stain.

Fix: bring pH and alkalinity to the high end of range (pH 7.6, TA 120) before they fire the heater. Bump calcium to 350 ppm if it's low. The extra 0.3 in LSI buys you a safe margin while the water's cold.

Scenario 3: hot tub LSI drift

Spas live at 102°F with high bather load and aggressive sanitizer demand. Owners constantly add product, evaporation concentrates calcium, and TDS climbs. LSI drifts positive over a few weeks. Then suddenly the heater is making grinding sounds and the inside of the cover is white-crusted.

Fix: partial drain. 30–40% fresh water exchange every 3–4 months on heavily used residential spas. Don't try to chase scaling with acid alone. You'll spike pH bouncing and crash the spa's sanitizer.

The dashboard cheat sheet

Bottom line

• LSI is the dashboard light, not pH. Run it every visit.
• Five inputs: pH, temperature, calcium, alkalinity, TDS. Memorize your local factor row.
• Target −0.3 to +0.3. Aim slightly positive in cold water.
• Corrosion is permanent; scaling is reversible. Err scaling.
• If your software computes LSI for you on every reading, even better. If it doesn't, tape this to the dashboard.