8
rate increases. If the alkalinity values are sufficiently high it will not be
difficult to control the pH. Most pools managers do prefer to keep the pH
between 7.2 and 7.4, that does ensure low corrosion rates and a sufficient
activity of chlorine.
Langelier Index (LI)
The Langelier Index is a powerful tool to calculate the water balance,
and to predict corrosion or scaling problems. Theoretically, a LI of zero
indicates perfect water condition for swimming pools. If LI>0, scaling
and staining of the water is present, and if LI<0 the water is corrosive
and highly irritating. A tolerance of ±0.4 is normally acceptable.
The Langelier formula is expressed as:
LI = pH + TF + HF + AF – 12.5
where:
LI
= Langelier Index (also called Saturation Index)
pH
= pH of the water
TF
= temperature factor
HF
= hardness factor, log(Ca Hardness, ppm as CaCO
3
)
AF
= alkalinity factor, log(Alkalinity, ppm as CaCO
3
)
To calculate the exact Langelier Index of your water please use the WATER
INDEX reference tables at the end of this chapter to find the Temperature,
Hardness and Alkalinity factors.
Recommendations
For most pools, water is balanced if:
• The pH value is maintained within the recommended ranges of
pH 7.2 - 7.6
• Ideally the Alkalinity should be maintained within a range of
80 - 125 ppm
• The Calcium Hardness should be maintained within a range of
200 - 400 ppm
.
To calculate your water balance three tests are required, measure the
Calcium Hardness, the Alkalinity and the pH of the pool water. Find the
Hardness
and
Alkalinity Factor
in the WATER INDEX reference tables
below.
The water temperature is in general controlled between 24
o
C (76
o
F) and
34
o
C (94
o
F) to ensure pleasant bather comfort. The
Temperature Factor
in this temperature range has minor importance; therefore an average
value of 0.7 may be used
.....
A simple calculation classifies your water in corrosive, scaling, acceptable
or ideal balanced, with treatment recommendations:
Water Balance = pH + TF + HF + AF
