Saturation Index In Swimming Pool Water

Saturation index or Langelier Stability Index  indicates the degree to which calcium carbonate saturation is present in swimming water. It was developed by Wilfred F. Langelier, a professor of Civil Engineering at the University of California-Berkeley. The Langelier index saturation is a method of coating the pipes used for water distribution with a thin layer of scale.

For years now the Langelier saturation index formula , has been adapted by many industries today. The use of this formula has spread to industrial water treatment plants 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 the water our swimming pools and spas balanced, in regards to calcium carbonate equilibrium.

Determining The Adequacy Of Your Pool Water Chemistry

By now you may have gathered that your swimming 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 the water of your swimming pool.

This is necessary because in swimming pool water, 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 swimming pools and spas.

Scale usually appears as white or lightly colored rough blotches on the walls or surfaces of your swimming pools. 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 swimming 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 greatly 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 amount of calcium and magnesium in the water of your swimming pool. And the reason you have scale formation, 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 a part of the calculating formula that is used when we are calculating the saturation index in swimming pool water.

Preventing Scale Formation In Swimming Pools

By imputing the major contributors to scale formation into the Langelier saturation index equation, it will help you make the necessary corrections 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 Saturation Index Conversion Chart

TemperatureTf ValueCalcium HardnessCf ValueTotal AlkalinityAf Value

Factors Of Total Dissolved Solids

Less Than 1,000 ppm1000 ppm or Greater

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 swimming 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 the formation of scale in your swimming pool.

As a result of this, the balancing point for swimming pool saturation index, which is zero, can shift to a positive number.

The Ideal Range For Langelier Saturation 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 swimming pool or spa water is considered corrosive. When this happens, you need to take the required steps to adjust your pH level, total alkalinity, or calcium hardness.

By acting immediately, you can avoid the serious effects of a corrosive environment.

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 must 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.

How To Tell If Your Swimming Pool Water Is Balanced

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

How To Calculate Your swimming Pool Saturation Index

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

  • Swimming pool 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 are 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 must 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.

How Do I Calculate Saturation Index 

The above-mentioned factors would then be used in a calculating formula that will determine your swimming pool's Langelier saturation index. Let's look at a couple of examples that will help you to fully 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 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 are no conversion numbers on the conversion chart, for the pH levels.

That's because the 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 to help determine this information.

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

Your pool saturation index aids pool water chemistry!

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