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CELL
6
11. THE LANGLIER INDEX
The Langlier Index calculation table (below) can be used to check the pool water
balance.
Readings of pH, water temperature, calcium hardness and total alkalinity are needed. These
readings are used to obtain the corresponding factor readings from the table below and then, to
perform the Langlier Index calculation. Add the pH, TF, HF and AF together. Then subtract 12.1 to
give the Langlier Index reading.
For example, if pool water had the following values; pH 7.4, temperature 24 o C or 75.2 o F, calcium
hardness 100 ppm, and total alkalinity 200 ppm, then the Langlier Index calculation would be
worked out as follows:
Start with pH
+7.4
* If the result is between -0.2 and +0.2, then the pool water is
Add TF
+0.6
in balance.
Add HF
+1.6
Add AF
+2.3
* If the result is lower than -0.2, then the pool water is corrosive.
Subtract ( K=12.1)
-12.1
* If the result is higher than +0.2, then the pool water is scale-
Langlier Index =
-0.2
forming.
To rectify corrosive water, the total alkalinity will need to be raised by adding sodium bicarbonate
(table B, next page).
To rectify scale-forming water, the total alkalinity will need to be lowered by adding hydrochloric
acid (table A, next page).
Please note: the use of sodium bicarbonate in inert-surfaced pools (fibreglass, fibreglassed
concrete, painted concrete or vinyl-lined) is simply a WASTE of chemicals. The unchanging calcium
level means that the total alkalinity becomes a function of pH and water temperature and will then
find its own equilibrium.
The ideal water balance is:
* Water temperature of 15 o C - 30 o C (59 o F - 86 o F)
*Calcium level of 60 - 120 ppm
*Magnesium level of greater than 20 ppm
*Iso-cyanuric acid level 30 - 50 ppm
*Total Alkalinity >150 ppm
* pH 7.4 - 7.8 for Concrete, Quartzon, Pebble or Tiled Pools and 7.0 - 7.6 for Fibreglass or
Painted Pools
To maintain water in balance for alkaline surfaced pools (such as marblesheen, pebbled, quartzon
and tiled) the total alkalinity should be greater than 150 ppm if the pH is to be kept
below 7.8. Consult tables A and B (next page) to achieve the correct total alkalinity.
LANGLIER INDEX CALCULATION TABLE
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Temp
Pool Water
Temperature
Factor
o
o
(
C)
(
F)
(TF)
0
0.0
32.0
3
37.4
0.1
8
46.4
0.2
12
53.6
0.3
15
0.4
59.0
19
0.5
66.2
24
75.2
0.6
29
84.2
0.7
34
93.2
0.8
40
104.0
0.9
53
1.0
127.4
Hardness
Total
Calcium
Factor
Alkalinity
Hardness
(HF)
as CaCO
as CO
3
3
5
5
0.3
25
1.0
25
50
50
1.3
75
75
1.5
100
100
1.6
150
150
1.8
200
1.9
200
300
300
2.1
400
400
2.2
800
800
2.5
1000
1000
2.6
15
Alkalinity
Factor
(AF)
0.7
1.4
1.7
1.9
2.0
2.2
2.3
2.5
2.6
2.9
3.0
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