Calcium Saturation Index

A little-known fact is that the CSI has been tested and shown to work only inside certain parameters for each variable. While most of the parameter limits exceed what is likely in a swimming pool or spa, it should be noted that the reliability of the index is severely compromised if any value falls outside the following parameter limits:

· pH: 6.5 to 9.5

· Temperature: 32 to 212°F

· Bicarbonate alkalinity: 10 to 800 ppm

· Calcium hardness: 50 to 700 ppm (up to 900 ppm)

· Total dissolved solids: 50 to 1000

One largely unknown flaw is in the published tables. The temperature factors have been miscalculated and copied in all major pool-operation texts. Table 1 shows all of the recalculated, corrected factors for each variable. The only notable differences appear in the temperature column, which is only about 0.1 in the typical pool and spa range. It should also be noted that the table complies with the previously mentioned CSI parameters.

Over the years, the total dissolved solids’ (TDS) influence has been greatly exaggerated. The constant–12.1–includes the TDS factor for 1,000 parts per million (ppm) of TDS. This is not an uncommon value in many pools. It is important to know that the CSI has only been shown to work with TDS values up to 1,000 ppm. But, using Langelier’s original equation, the next tenth of negative influence would not occur until 10,000 ppm TDS, not the commonly referenced 1,000 ppm. It can safely be assumed that unless a pool is filled with sea water, the influence of TDS is negligible, and therefore can be ignored altogether in most pools. Even in pools and spas using chlorine generation–commonly called saltwater systems–where the salt content is typically maintained between 3,000 and 6,000 ppm, the TDS influence is small enough to be ignored.

A Better Way?

With all the associated problems with the CSI, the question persists, “Is there a better way?” In short, the answer is no, but other methods are worth discussing. The Ryzner Stability Index (RSI) has received some deserved attention from the industry, and has been applied successfully in some cases. The formula for the RSI is:

RSI = 2 x (12.1 − Tf − Af − Cf) − pH

The midpoint for this index is 6.5, with values below 6.0 indicating an increasing scaling potential, and values well above 7.0 indicating true corrosive tendencies of the water. While the RSI is useful for predicting water’s corrosivity, it becomes apparent that simply maintaining a positive CSI value with high calcium hardness will yield similar results.

Another method that was developed by a pool technician is the Hamilton Index (HI). It has not become widely used–or well known–in the commercial pool sector. Interestingly, there is no theory or science behind the index, but it is claimed that extensive field testing has shown it to work. It is interesting to note that the HI requires the pool water pH to be maintained between 7.8 and 8.2. This range is not even legal in most states, and would not yield an effective work-value of chlorine.

With an understanding of the common errors associated with the Calcium Saturation Index, pool operators will be able to reliably maintain safe pool water that also ensures the structure and equipment lasts for years. The CSI is an important and useful tool, and to date, there has not been a better method developed, but operators must be alert to what the index does and does not do.

Matthew Griffith is the pool operator for Georgia Institute of Technology. He has an extensive aquatics background managing municipal, private, school district and university programs. As an instructor, Griffith has taught CPO courses around the country, and holds numerous other certifications in pool operations and service. He can be reached via e-mail at

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Related posts:

  1. Water Chemistry
  2. Strategies For A Healthy Pool
  3. Troubleshooting Filters
  4. Pool Genetics
  5. Clearing The Water

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