Water chemistry

Saturation Index Explained With a Worked LSI Example

Calculate the Langelier Saturation Index with factor tables, see what moves it and fix a negative reading on a plaster pool.

The saturation index tells you whether pool water tends to dissolve calcium from surfaces or drop it as scale. Plaster pools feel it first, and heaters and salt cells show it too. The formula is simple, and the tables below let you run it on your phone.

The formula

LSI = pH + TF + CF + AF - TDSF

• TF is the temperature factor.
• CF is the calcium hardness factor.
• AF is the alkalinity factor, taken from carbonate alkalinity.
• TDSF is the total dissolved solids factor.

Aim for -0.3 to +0.3. Below -0.3 the water leans aggressive. It can etch plaster and attack metal parts. Above +0.3 it leans scale-forming. You see white crust at the waterline, rough plaster and scaled heaters or cells.

Factor tables

Round your readings to the nearest row.

Carbonate alkalinity = TA - (CYA / 3). Use TDSF 12.1 for most pools. High-TDS pools, such as salt pools with several thousand ppm total, use about 12.2. The difference is 0.1.

Want it done for you? The saturation index tool runs the same math.

Worked example 1: a balanced pool in summer

Readings: pH 7.6, water 84 F, CH 300, TA 100, CYA 40.

1. TF at 84 F is 0.7.
2. CF at 300 ppm is 2.1.
3. Carbonate alkalinity: 100 - 40 / 3 = about 87. Between the 75 and 100 rows, AF rounds to 1.9.
4. TDSF is 12.1.

LSI = 7.6 + 0.7 + 2.1 + 1.9 - 12.1 = +0.2. In range.

Worked example 2: the same water in winter

Water drops to 60 F, and nothing else changes. TF falls from 0.7 to 0.4.

LSI = 7.6 + 0.4 + 2.1 + 1.9 - 12.1 = -0.1. Still in range, but it moved 0.3 from temperature alone. A pool that sits at +0.3 in summer can slide toward aggressive in winter.

Worked example 3: fixing a negative reading

A plaster pool at 76 F reads pH 7.4, CH 200, TA 70, CYA 30.

1. TF at 76 F is 0.6.
2. CF at 200 ppm is 1.9.
3. Carbonate alkalinity: 70 - 30 / 3 = 60. AF is 1.8.

LSI = 7.4 + 0.6 + 1.9 + 1.8 - 12.1 = -0.4. That is below -0.3, so the plaster is at risk.

You have three levers. Each has a cost.

For an 18,000 gallon pool, the multiplier is 1.8:

• pH to 7.6: 6 oz x 1.8 = about 11 oz of soda ash. LSI becomes about -0.2.
• CH to 300: 12 lb x 1.8 = about 22 lb of calcium chloride. LSI becomes about -0.2.

The pH move takes one small dose. The calcium move takes bags, and a customer who pays for chemicals will see it. Start with pH, and add calcium over several weeks if the pool keeps drifting low. Soda ash also raises alkalinity a little, so retest TA afterward. See how to raise alkalinity with baking soda for the TA side.

When to run the numbers

• A new account with etching, rough spots or scale.
• A plaster pool in its first months after resurfacing. If the builder or surface maker gives a start-up program, follow that first.
• A pool with a scaled heater or salt cell.
• Before a large change, such as a partial drain, which dilutes calcium and alkalinity together.
• After a big seasonal temperature swing.

For vinyl and fiberglass, the same -0.3 to +0.3 target applies, and calcium targets are lower at 150 to 250 ppm. Those surfaces etch less than plaster. Scale still forms on them, and aggressive water can attack heater parts, ladders and other metal. See calcium hardness and scale for the raise and lower sides.

Mistakes

• Using TA instead of carbonate alkalinity on a pool with high CYA.
• Using the winter temperature in July. Use the water temperature at the time of the test.
• Fixing a negative LSI by raising CH when a small pH adjustment would do it.
• Chasing the index to exactly 0.0. Anything in the -0.3 to +0.3 band is fine.
• Ignoring surface type. Plaster is the surface most sensitive to a low reading.

Safety

Follow the product label and SDS. Never mix chemicals. Add chemical to water, never water to chemical.

LSI also helps explain stain behavior when metals are in the water. See metal stains in pools.