How To

Calcium Saturation Index (CSI)

In general, most TFP members who follow the TFP Pool School – Recommended Levels page have no need to be concerned with CSI. Those parameters help to keep the CSI well balanced.

In addition, vinyl lined and most of today’s fiberglass pools contain no plaster that could scale/erode.  At times however, there are some conditions that may warrant monitoring your CSI when reviewing your numbers in the PoolMath tool.  The following are just examples and not all inclusive:

– Pools who tend to have extremely hard water or excessively high alkalinity (potential for scale)

– Pools that tend to have naturally soft water or low alkalinity (potential for erosion-plaster only)

– Salt Water Generator (SWG) owners (scale on cell plates)

– Pools with waterline tile (potential for scale or erosion)

– Newly plastered pools; especially those recently completed and closed before winter (potential for dramatic pH increases)

Simply put, CSI is nothing more than a measure of how over-saturated or under-saturated your water is with calcium carbonate. If your CSI is excessively positive, then there is a chance calcium scale could form under the right conditions.  If your CSI is excessively negative, the water could dissolve (etch/erosion) calcium from any plaster source it can find, eg, your pool’s plaster and/or waterline grout.  This is not an overnight process and can take several months to occur. 

How is CSI calculated?  Very simple – by using the PoolMath tool.  Since CSI is primarily calculated by combining pH, TA, CH, and water temperate (among other items), simply enter your test results in PoolMath and refer to the CSI row.  The tool will tell you if your water is balanced or if there is a potential for scale or erosion.

Again, for most pool owners, CSI is not an area that warrants much concern.  For those in the scenarios listed above, pool owners may benefit from learning more and monitoring their CSI.  Practice with the PoolMath tool by changing the pH and other water test parameters so you can see how to manipulate CSI to your advantage.  For example, those with SWGs and very hard water may elect to keep their CSI slightly negative (0.0 to -0.30) to help prevent scale build-up on cell plates.  In most cases, the adjustment of pH has the most dramatic effect on manipulating CSI.

Users who do decide to manage their CSI shouldn’t focus on achieving a “perfect CSI” as no such number exists. Users shouldn’t stress if their CSI falls out of the “safe range” as it takes a long time for any problems to occur.  Instead, always refer to the Pool School – Recommended Levels page first.  When conditions beyond your control subject the pool’s CSI to fall above/below the balanced range, use the PoolMath to help correct the CSI to your advantage.

Lastly, All users should test their CSI levels when closing their pools as cold water temperatures increases the risk of plaster damage during the Winter. To help counteract this users should increase their pH levels to 7.8-8.0 and make sure the CSI is balanced according to colder water temperatures.