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The Saturation Index

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Updated: August 2014

The Saturation Index

The Saturation Index is the key to understanding pool water chemistry under a wide variety of conditions.

The Saturation Index sometimes called the Langlier Index or Water Balance Index, is the driving force for determining if a pool is balanced. An unbalanced pool is either acidic(corrosive) or basic(scaling) . The index determines whether the water is balance or not. Water can be balanced, even though it may not meet "traditional" values for being balance. Local water supplies have an influence on the water chemistry.

Saturation Index Equation: SI = pH + TF + CF + AF -TDS

 TF
 CF
 AF
 TDS
= Temperature Factor in degrees Fahrenheit
= Calcium Hardness Factor in ppm Calcium Carbonate
= Alkalinity Factor
= 12.07 for Total Dissolved Solids(TDS) factor for TDS 500ppm
= 12.10 for Total Dissolved Solids(TDS) factor for TDS 1000 ppm
= 12.2 for Total Dissolved Solids(TDS) factor for TDS 1000-2000ppm
= 12.2 for Total Dissolved Solids(TDS) factor for TDS >2000 ppm

To Calculate the Saturation Index, test for pH, temperature, calcium hardness and total alkalinity. Find the corresponding values in the chart below and enter those into the equation. Acceptable values are -0.3 to +0.3, ideally 0.0 to +0.3.

Note: Take your value from each column. Match you temperature to get TF, Your Calcium to get CF and Alkalinity to get AF

Temperature
Calcium Hardness
Total Alkalinity
Temp
TF
ppm
CF
ppm
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
1.7
150
2.2
76
0.6
200
1.9
200
2.3
80
0.7
300
2.1
250
2.4
94
0.8
400
2.2
300
2.5
105
0.9
800
2.5
600
2.8

Sample Results:
   
pH 7.4
   Temperature= 83'F
   Calcium Hardness = 300ppm
   Alkalinity = 100 ppm
   TDS = 800 ppm

The Calculation:    
    SI = 7.4 + 0.7 + 2.1 + 2.0 - 12.1 = 0.1

The SI falls within an acceptable range and the water is balanced.

 

Links to Other Water Chemistry Topics

Calcium Deposits

Chlorine Products

Glossary of Chemical Terms

Potassium Monopersulfate

Saturation Index

Spa Disinfectants

Water Testing