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"Langelier/Saturation Index"
Useful in Preventing Scaling & Corrosion.
The Pool & Spa Informational Website
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Determining the Scaling or Corrosive Tendencies
of water
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The
Langelier or Saturation Index is a calculation used to determine the adequacy of
the pool or spa water chemistry, as it relates to corrosion and scale-formation.
The adjustment of the pH, total alkalinity or calcium hardness can be used to
adjust the Langelier Index, so that it falls within the ideal range.
Corrosion is a major concern in swimming pools and is caused by acidic
conditions and/or the absence of proper water balance. The factors that
contribute to corrosive conditions include: low pH readings, low total
alkalinity and low calcium hardness (soft water). Corrosive water
conditions are more likely to cause irritation and sensitivity to chlorine.
Scale-forming conditions, associated with cloudy water conditions, are caused by
alkaline water conditions and/or the absence of proper water chemistry.
The factors that contribute to scale-formation include: high pH readings,
high total alkalinity and high calcium hardness (hard water).
Scale-forming tendencies are more likely to be associated with cloudy water
problems and a loss of chlorine efficiency.
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LANGELIER
or SATURATION INDEX |
The Langelier or Saturation
Index is useful in determining the corrosive or scale-forming tendencies of pool
or spa water. In order to calculate this Index, it is necessary to have
the following information:
pH Obtained by testing the pool or spa
water
TF
Temperature Factor - using the chart below, look up the actual pool or spa water
temperature at the time of sampling, in order to obtain the appropriate
TF value.
CF
Calcium Hardness Factor - determine the PPM of calcium hardness (CH) in the
water sample. Using the chart below, use the appropriate calcium hardness
(CH) reading, in order to obtain the appropriate
CF
value.
AF
Total Alkalinity Factor - determine the PPM of total alkalinity (TA) in the
water sample. Using the chart below, use the appropriate total alkalinity
(TA) reading, in order to obtain the appropriate
AF
value.
The ideal range for the
Langelier or Saturation Index is -0.5 to +0.5. Test values more negative
than -0.5 (i.e., -0.6) are considered corrosive and steps should be taken to
adjust the pH, total alkalinity or calcium hardness, in order to avoid the
effects of corrosion. Test values higher than +0.5 (i.e., +0.6) are
indicative of scale-forming tendencies and steps should be taken to adjust the
pH, total alkalinity or calcium hardness, in order to avoid scale formation and
cloudiness.
The following formula is used
to calculate the Langelier/Saturation Index:
Langelier Index = pH
+
TF +
CF
+
AF
- 12.1*
* -12.1 is a
constant used in the calculation of the Langelier/Saturation Index
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LANGELIER INDEX
CONVERSION CHART |
|
Temp°F / °C |
|
TF |
|
|
CH PPM |
|
CF |
|
|
TA PPM |
|
AF |
|
32°F / 0°C |
|
0.0 |
|
|
5 |
|
0.3 |
|
|
5 |
|
0.7 |
|
37°F / 3°C |
|
0.1 |
|
|
25 |
|
1.0 |
|
|
25 |
|
1.4 |
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46°F / 8°C |
|
0.2 |
|
|
50 |
|
1.3 |
|
|
50 |
|
1.7 |
|
53°F / 12°C |
|
0.3 |
|
|
75 |
|
1.5 |
|
|
75 |
|
1.9 |
|
60°F / 16°C |
|
0.4 |
|
|
100 |
|
1.6 |
|
|
100 |
|
2.0 |
|
66°F / 19°C |
|
0.5 |
|
|
150 |
|
1.8 |
|
|
150 |
|
2.2 |
|
76°F / 24°C |
|
0.6 |
|
|
200 |
|
1.9 |
|
|
200 |
|
2.3 |
|
84°F / 29°C |
|
0.7 |
|
|
300 |
|
2.1 |
|
|
300 |
|
2.5 |
|
94°F / 34°C |
|
0.8 |
|
|
400 |
|
2.2 |
|
|
400 |
|
2.6 |
|
105°F / 41°C |
|
0.9 |
|
|
800 |
|
2.5 |
|
|
800 |
|
2.9 |
|
128°F / 53°C |
|
1.0 |
|
|
1000 |
|
2.6 |
|
|
1000 |
|
3.0 |
The following example
illustrates how the Langelier/Saturation Index is calculated:
The actual
pH = 7.4
The actual water temperature = 78°F or 25°C : from the chart use
TF =
0.6
The actual calcium hardness (CH) = 198 PPM ; from the chart
use CF = 1.9
The actual total alkalinity (TA) = 97 PPM : from the chart
use AF = 2.0
-12.1 is a constant used in the calculation of the
Langelier/Saturation Index
Langelier
Index =
pH
+
TF
+
CF
+
AF
-
12.1*
Langelier
Index = 7.4 + 0.6 + 1.9 + 2.0 - 12.1* =
-
0.2 (from above data)
Results:
-0.2
is in the ideal range of -0.5
to +0.5
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LANGELIER INDEX
CORRECTIONS |
The Langelier Index is a tool that is most frequently used by knowledgeable
pool and spa professionals.
For the typical pool or spa owner, that maintains the water balance, within
or close to the recommended ranges for pH, total alkalinity and calcium
hardness, the Langelier Index should not be a concern, inasmuch as it will fall
within the ideal range of
-0.5 to +0.5.
In those instances where the pool or spa water chemistry is
substantially out or balance or problems are in evidence, the Langelier Index
can give insight into the proper course of corrective treatment.
Low Langelier Indexes can be raised, to within
-0.5 to +0.5, by adjusting the
pH to 7.2 to 7.6, the total alkalinity to 80 to 120 PPM and the calcium hardness to
150 to 200 PPM for a masonry pool or spa or 80 to 200 PPM for a vinyl or fiberglass
unit. A low Langelier Index can result in corrosion, bather irritation and
discomfort.
High Langelier Indexes can be lowered, to within
-0.5 to +0.5, by adjusting
the pH to 7.2 to 7.6, the total alkalinity to 80 to 120 PPM and the calcium hardness
to 150 to 200 PPM. While it is always possible to lower the pH, it is not as
simple with the total alkalinity or calcium hardness. Lowering the total
alkalinity usually lowers the pH as well. Lowering the calcium hardness is
not always possible, given the fact that some pools are filled with hard water.
In those situations, where the calcium level is high, attention should be paid
to lowering the pH and/or total alkalinity as a means of improving the Langelier
Index. A high Langelier Index can lead to scale formation, cloudy water,
filtration problems, heater problems, loss of chlorine efficiency and bather
discomfort.
It is not always possible to get the pH and total alkalinity in range, due to
the nature of the dissolved minerals. pH is always more important!
To more correctly determine the total alkalinity, deduct 30% of the cyanuric
acid reading from the total alkalinity test result.
Pool water chemistry is where
the path to proper water quality,
clarity and sanitation starts.
Water testing provides the basis for determining which chemicals are
required to correct and optimize chemical concentrations. The proper
maintenance of the pool water is an ongoing responsibility and not something to
be taken for granted. Having the right water testing equipment will make
the task easier, save money on chemicals and help avoid problems.
Become better informed and take charge of your own
water quality!
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