langelier-saturation index: pool water chemistry - pool & spa water scaling and corrosion

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"Langelier/Saturation Index"
Useful in Preventing Scaling & Corrosion.

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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. Please refer to the Glossary, if there are terms or phrases that require explanation. For an alphabetized listing of the entire contents of the website, use the Table of Contents link, on top of every page.

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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

LANGELIER INDEX CONVERSION CHART

Temp°F / °C

CH PPM

TA PPM

32°F / 0°C

0.0

0.3

0.7

37°F / 3°C

0.1

1.0

1.4

46°F / 8°C

0.2

1.3

1.7

53°F / 12°C

0.3

1.5

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

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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