Swimming Pool Saturation Index!

saturation index or Langelier Stability Index was developed by Wilfred F. Langelier, a professor of Civil Engineering at the University of California-Berkeley. The Langelier index is a method of coating the pipes used for water distribution with a thin layer of scale.

For years now the Langelier index, has been adapted by many industries. The use of this formula has spread to industrial water treatment as well as domestic drinking water treatment worldwide.

However in the early 1970s, the swimming pool and spa industry, developed a modified version of the Langelier Saturation Index.

Because of this modified version of the Langelier saturation index formula, we are able to determine whether our pool and spa water is balanced, in regards to calcium carbonate equilibrium.

Determining the adequacy of your pool water chemistry!

By now you may have gathered that your pool saturation index, is a calculating method that is used by pool operators, to determine the adequacy of the pool or spa water chemistry. This method is in relation to corrosion and scale formation in your pool water.

This is necessary because in swimming pool waters, Calcium bicarbonate is very soluble. However when this chemical breaks down, it becomes insoluble in the form of calcium carbonate. This my friend - is the major scale developer in our pools and spas. Scale usually appears as white or lightly colored rough blotches on your pool walls. It can also attach itself to other objects that are apart of your swimming pool equipment.

In case you don't know already, calcium carbonate scale in your pool plumbing system, as well as your swimming pool filters, can restrict your water flow significantly. It can also cause calcification of your filter bed. This will reduce the efficiency of your pool filtration system and shorten its turn over rate.

Factors that influence the formation of scales in your pool

When you have scale deposits in your swimming pools, it usually means that you have an excess of calcium and magnesium in your pool water. And the reason you have scale forming, is due to the fact that these minerals become insoluble in water. The major contributors to scale formation in our pools or spas include:

  • Calcium hardness.
  • Total alkalinity.
  • pH Level.
  • Water temperature.
  • Total dissolved solids.

The above mentioned factors are apart of the calculating formula that is used when we are calculating the saturation index.

Preventing scale forming through good swimming pool care!

By imputing the major contributors to scale formation into the Langelier saturation index equation, will help you make the necessary correction/s to your swimming pool. For the total dissolved solids, an average factor is used in the formula. The table below is used to obtain the numerical values for three of the remaining factors.

The Langelier Conversion Chart!

TemperatureTf ValueCalcium HardnessCf ValueTotal AlkalinityAf Value
FTfppmCfppmAf
320.0251.0251.4
370.1501.3501.7
460.2751.5751.9
530.31001.61002.0
600.41251.71252.1
660.51501.81502.2
760.62001.92002.3
840.72502.02502.4
940.93002.13002.5
1050.94002.24002.6
8002.58002.9

Factors of Total Dissolved Solids

Less Than 1,000 ppm1000 ppm or Greater
12.112.2

By simply using the figures from the table and following the simplified version of the Langelier saturation index formula, you can achieve proper alkalinity balancing of your pool's water. This process will prevent your swimming pool from becoming a scale forming or corrosive environment.

There are some suspecting agents that are used to prevent staining and discoloration in pool water. Studies show that they may also improve the solubility characteristics of your calcium carbonate. This can help in reducing formation of scale in your swimming pool. As a result of this, the balancing point for pool saturation index which is zero, can shift to a positive number.

What is the ideal range for the Langelier stability index?

In the swimming pool care, the ideal range for the Langelier stability index is -0.5 to 0.5. When you have calculated results that are below -0.5, your pool or spa water is considered corrosive. When this happens, you need to take the required steps to adjust your pH, total alkalinity or calcium hardness. By taking action immediately, you can avoid the serious affects of corrosion.

Now, if your calculated results are higher than +0.5, this is an indication that your pool or spa water is scale forming.

Therefore, you have to again take the necessary steps to adjust the balancing factors of your swimming pool chemistry.

This procedure is necessary because it will help you avoid any scale formation and cloudiness of your pool water.

Determining whether your spa or pool water is balanced!

In order for you to determine whether your pool water is properly balanced, you have to conduct a complete water chemistry analysis. The saturation index conversion chart above, indicates the recommended factors needed for your water chemistry analysis.

Calculating your swimming pool saturation index!

Before you can calculate the Langelier saturation index for your swimming pool, you need the following information!

  • pH level. This is obtained when you carry out a routine pH test of your pool or spa water.
  • The TF - Temperature factor. This represents the appropriate temperature factor that you will use from the conversion chart. The TF value corresponds with the temperature that you get from your water testing.
  • The CF - Calcium Hardness factor value. First you will determine the ppm of your calcium hardness (CH) in the water sample. Once the results of your calcium hardness test is obtained, you will use your Langelier saturation index conversion chart to find the correct CF value.
  • The AF - Total Alkalinity factor. To get the correct AF value for this one, you have to determine the ppm of your pool's total alkalinity in your water sample. Using your SI index conversion chart will give you the correct AF value.

Langelier saturation index calculator!

The above mentioned factors would then be used in a calculating formula that will determine your swimming pool's Langelier saturation index. Lets look at a couple of examples that will help you to understand the formula a little better. However before we write down our examples, lets first set up our calculating formula which looks like this:

SI = pH + TF + CF + AF - TDSF.

Now we use the formula above and plug in the required numerical values for calculating. Using some examples, our calculating formula will now look like this:

Example 1 as it relates to your pool water test readings!. Langelier index:

SI = pH + TF + CF + AF - TDSF

SI = 7.2 + 0.7 + 1.9 + 2.0 - 12.2

SI = - 0.4.

A result like this means that your pool water is slightly corrosive. Remember you want your result to be between - 0.3 and + 0.3.

Example 2 as it relates to your spa water test sample!

SI = pH + TF + CF + AF - TDSF

SI = 7.7 + 0.9 + 2.3 + 2.0 + -12.2

SI = + 0.7.

This result means the your spa water is scale forming. Now that you have a couple of examples to go by, why don't you try some of your own?

You may have noticed that there is no conversion numbers on the conversion chart, for the pH levels.

That's because your pH value that you get from your test sample, is plugged directly into the pool saturation index formula.

Remember that the water for your swimming pool or spa, should always be balanced. Therefore you should know how to use the Langelier saturation index formula.

The fact of the matter is that - as a spa or swimming pool operator, knowledge of it will be helpful in preventing your pool or spa from becoming either corrosive or scale forming. Once your water balancing factors are maintained, there will be very little concern for the ideal range of your Langelier or pool Saturation index

Your pool saturation index aids pool water chemistry!

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