Pool School 101 - The Langelier Saturation Index

The Langelier Saturation Index

Stable water balance quality is based on the proper use of this saturation index and formula. Various factors are used to calculate saturation levels (pH, total alkalinity, total hardness, total dissolved solids and temperature). When these factors are placed into the formula, the result provides the information needed to balance the pool water and correct its condition.

First all parameters of the formula must be tested with the appropriate test kits, then the applicable “factors” (see Table below) area placed into the following formula:

Saturation Index = pH + TF + CF + AF – 12.1

pH: measured from test kit
TF: temperature factor; measured at pool
CF: calcium hardness factor, measured from test kit
AF: alkalinity factor, measured at pool

Formula Result:
Between –0.5 and 0.5 = water is balanced
Above 0.5 = water is over-saturated (scale-forming)
Below –0.5 = water is under-saturated (corrosive)


Temperature F ° = TF


Calcium Hardness = CF

Total Alkalinity = AF
32° = 0.0
5 = 0.3
5 = 0.7
37° = 0.1
25 = 1.0
25 = 1.4
46° = 0.2
50 = 1.3
50 = 1.7
53° = 0.3
75 = 1.5
75 = 1.9
60° = 0.4
100 = 1.6
100 = 2.0
66° = 0.5
150 = 1.8
150 = 2.2
76° = 0.6
200 = 1.9
200 = 2.3
84° = 0.7
300 = 2.1
300 = 2.5
94° = 0.8
400 = 2.2
400 = 2.6
105° = 0.9
800 = 2.5
800 = 2.9
128° = 1.0
1000 = 2.6
1000 = 3.0

Saturation Index = pH + TF + CF + AF – 12.1


Example:

Test results:
pH reading is 7.0
Water Temperature is 84° (TF is .7 from chart)
Calcium Hardness reading is 300 ppm (CF is 2.1 from chart)
Alkalinity reading is 25 ppm (AF is 1.4 from chart)

Saturation Index =
(pH) 7.0 + (TF) 0.7 + (CF) 2.1 + (AF) 1.4 – 12.1 = -0.9

Since the equation solution equals –0.9, this indicates a corrosive water condition.


Ideal readings for proper water balance:
- pH 7.2 to 7.6
- Total Alkalinity 80 to 125 ppm (parts per million)
- Calcium Hardness 200 to 400 ppm (parts per million)

In the next sections, we will discuss each individual components of water balance in detail. Click here for a printable copy of the Langelier Saturation Index table shown above.

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