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PREPARING THE POOL WATER
8
Saturation Index (SI)
The Saturation Index is a formula used to predict the calcium carbonate saturation of water, that is, whether your water will
precipitate, dissolve, or be in equilibrium with calcium carbonate. Your water is properly balanced if the SI is 0 ± 0.3. If your SI
is greater than 0.3, scaling and staining will occur. If your SI is less than -0.3, then the water is corrosive to metallic fixtures and
aggressive to plaster surfaces and vinyl liners. A high or low SI can cause premature damage to the cell, equipment or pool
finish. As a general rule, higher concentrations of calcium, total dissolved solids, pH, and alkalinity all promote a greater
tendency for scale. Scaling potential also increases with increasing temperature.
Use the chart below to determine your overall water balance. Test your water for pH, water temperature, Calcium Hardness,
Total Alkalinity and Salt Level and use the equivalent Factors (TF, CF, AF, Constant) from the chart below to determine your
Saturation Index. Adjust your chemicals to maintain balanced water.
pH + TF + CF + AF – SC = SI
Temperature TF
Calcium
Hardness
CF
Total
Alkalinity
AF
Salt Level SC
60
o
F 15.6
o
C 0.4 150 ppm 1.8 75 ppm 1.9 0 – 1000 ppm 12.1
66
o
F 18.9
o
C 0.5 200 ppm 1.9 100 ppm 2. 0 1001 - 2000 pp m 12.2
76
o
F 24.4
o
C 0.6 250 ppm 2.0 125 ppm 2. 1 2001 - 3000 pp m 12.3
84
o
F 28.9
o
C 0.7 300 ppm 2.1 150 ppm 2. 2 3001 - 4000 pp m 12.4
94
o
F 34.4
o
C 0.8 400 ppm 2.2 200 ppm 2. 3 4001 - 5000 pp m 12.5
103
o
F 39.4
o
C 0.9 600 ppm 2.4 250 ppm 2. 4 5001 - 6000 pp m 12.6
-.3 -.2 -.1 0 .1 .2 .3
Corrosive to
metals, etches
plaster
finishes, and
irritates skin
OK
Scaling,
staining, and
cloudy water
conditions
Examples
Water test results #1:
Total Alkalinity = 125 ppm => AF = 2.1
Calcium Hardness = 400 ppm => CF = 2.2
pH = 7.4 => pH = 7.4
Water Temperature = 84 F => TF = 0.7
Salt Level = 3000 ppm => SC = 12.4
7.4 + 0.7 + 2.2 + 2.1 – 12.4 = 0 (Water is perfect l y balance d )
Water test results #2:
Total Alkalinity = 200 ppm => AF = 2.3
Calcium Hardness = 600 ppm => CF = 2.4
pH = 7.8 => pH = 7.8
Water Temperature = 84 F => TF = 0.7
Salt Level = 3000 ppm => SC = 12.4
7.8 + 0.7 + 2.4 + 2.3 – 12.4 = 0.8 (Water is scale forming)