Have you ever made rock candy before? To make rock candy, you heat water to a boil and add sugar (with a little bit of acid from lemon juice to break the sucrose down into fructose and glucose). You keep adding sugar until you get a pretty good sugar syrup boiling. Remove the solution from the heat, pour it off into a clean glass jar and dip a stick in it. Keep the glass jar covered and undisturbed in a cool part of the kitchen. Let the stick dry for a day. Put the stick back into the sugar solution and let it sit there for a few days.
What happens? Giant sugar crystals grow on the stick!
Now, take lollipop and put it in a glass container with hot water. And let it sit there. What happens? The lollipop slowly dissolves into the water. What you see there is saturation (or lack thereof) in action. When your pool water is unsaturated with calcium, it’s like the lollipop in hot water, the calcium from your plaster will dissolve. When your pool water is over-saturated with calcium, it’s like making rock candy, calcium (as calcium carbonate) will want to grow all over your pool surface. The 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 will form under the right conditions. If your CSI is excessively negative, the water will dissolve calcium from any source it can find, eg, your plaster.
Unlike rock candy which only depends on how much sugar you dissolve in water and what the temperature is, CSI also has to factor in pH and TA as well as some minor variations based on salt and borates. A user should make every effort to stay between Trouble Free Pool’s Recommended Levels because the chances of a user etching the surface of their pool or forming scale is slim to none. From time to time however a pool owner cannot maintain TFP’s Recommended Levels mostly due to high Calcium levels or Total Alkalinity in their fill water. In these cases pool owners may benefit from learning and maintaining their CSI. Users who do decide to use CSI shouldn’t focus on achieving a “perfect CSI” as this number doesn’t exist. Users also shouldn’t stress if their CSI falls out of the “safe range” as it takes time for any problem to occur. Instead the user should use PoolMath to calculate their CSI and adjust their water chemistry to make sure their pool stays free of any issues.
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.