Calcium Saturation Index

Since the early 1960s, the pool industry has been using a concept called the Calcium Saturation Index (CSI) to predict water’s tendency to be corrosive or scale-forming. And for just as long, it has been both a friend and a nemesis. The index is fraught with errors in its understanding, causing some aquatic professionals to question its application to pools. With a better understanding of the common myths and mistakes associated with the CSI, pool operators will be able to apply this important concept of pool-water balance in a reliable manner.

History Lesson

The original CSI was derived from a theoretical concept of Dr. Wilfred Langelier, Professor Emeritus of Civil Engineering at the University of California, Berkeley. His paper detailing the model was first published in 1936. Since then, Langelier’s index has become widely used and studied, not only in the pool industry, but in industrial and domestic drinking-water treatment as well as other applications. It is interesting to note that his work dealt with enclosed loops of municipal water piping systems, and he never made mention of its use on water loops that are open to the atmosphere, like swimming pools.

Back To Basics

The concept of pool-water balance, as determined using the CSI, is very important to the trained operator. The numerical expression helps predict the water’s aggressiveness or scaling potential. The CSI itself is comprised of five variables:

1. pH

2. Temperature

3. Bicarbonate alkalinity

4. Calcium hardness

5. Total dissolved solids

The classic representation of the CSI is as follows:

CSI = pH + Tf + Af + Cf − 12.1

In this simplified formula, pH is inserted directly, and the appropriate factors for temperature, bicarbonate (total) alkalinity and calcium hardness are taken from a table and entered into the formula (the factor for total dissolved solids is included in the constant). The resulting calculated value is the pool water’s “balance.” Negative values indicate increasingly aggressive water, while positive values indicate the water has an increasing potential to scale. Water that yields a CSI between –0.3 and +0.3 is generally considered “balanced.” The goal is to avoid highly aggressive water that will degrade the pool surface and equipment, while also avoiding scaling water which can deposit scale on pool interiors, pipes, etc.

Flaws And Fallacies

With its readily identifiable benefits in preserving the pool structure and equipment, there are some notable flaws and fallacies with the index. Common errors in understanding the CSI include correct terminology, the definition of “balanced,” CSI parameters, temperature factor correction and the influence of total dissolved solids.

While seemingly trivial, proper terminology is a foundation for understanding what the CSI does and does not do. First, the correct title for this concept is Calcium Saturation Index, not “Langelier Saturation Index,” “Langelier Index” or “Saturation Index.” This is the term Langelier used and the only name that is adequately descriptive of what is saturated.

Second, negative CSI values show water’s tendency to be aggressive, not corrosive. Dr. Langelier’s original work focused on predicting precipitation of calcium scale, and scholars agree there is no correlation between the corrosion of metal and the CSI. Negative values do, however, show the water’s aggressiveness toward plaster and other non-metallic surfaces as it seeks to dissolve calcium in order to reach equilibrium.

Another fallacy is that the CSI predicts scale-forming water. In fact, it does not. The CSI only indicates the water’s scaling potential. Pool water with CSI values of +0.5, or even +1.0 and higher, will not precipitate scale in a well-managed pool. It takes a pH above 8 (around 8.3 to 8.4) for calcium carbonate to fall out of solution. This is actually related to the alkalinity, the buffering capacity of the water. At a typical pool with a pH between 7.2 and 7.6, no scale can form.

When interpreting the CSI, it is important to know that zero is not necessarily perfectly “balanced.” A variety of factors make the wisest choice subjective for any particular pool. Some operators choose to hold a –0.1 to account for “up-scaling” heaters, but many times holding +0.3 or more, with an understanding of the Ryzner Index, is the preferred point while keeping a low pH to maximize chlorine’s effectiveness.

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