What are all of these numbers and what do they mean?!?

Understanding Pool Chemistry


You’ve been told since day one with your pool how important water chemistry is. Did you ever wonder what these numbers are and what they really mean? Proper water balance is necessary to provide safe, comfortable, and healthy water as well as ensuring the longevity of your equipment and pool surface. As a new or even as an experienced pool owner having your water professionally tested will help you to enjoy your pool and keep it clean, clear, and blue!

There are 3 different chlorine readings you may be given when you have your water tested. Free, total and combined. The free chlorine is the chlorine in your water that is working to kill germs and bacteria. This is your good chlorine and should be maintained at a level of 1-3ppm. The total chlorine is all the chlorine in the water; both used and free. This number should be equal to your free chlorine. The combined chlorine is basically used up chlorine or chloramines. Combined chlorine is formed when there is heavy organic waste (sweat, body oils, etc.) and not enough free chlorine to get rid of it. You want the combined chlorine reading to be 0, less than .02 is acceptable.

Next thing on your water analysis sheet is probably pH. Your pH is the number that determines if your pool is acidic, base, or neutral. The acceptable range for pH should be between 7.2-7.6. When your pH is low the water is acidic and can cause red eyes in swimmers and corrosion that damages your equipment and pool surface. When the pH is too high, the chlorine is not as effective at doing its job and you will be more likely to develop some pesky algae. High pH can also cause scale to form on your equipment and pool surface. Keeping your pH in range is easier to do when your total alkalinity is balanced. Total alkalinity acts as a buffer for the pH and should be maintained at 80-120ppm. When you have low alkalinity your pH will fluctuate, the water becomes corrosive and staining is more likely to occur. High alkalinity will lead you to cloudy water, clogged filters and heaters, and may make pH adjustments more difficult.

Calcium hardness, much like the calcium vitamins you might take serves a vital role in keeping pool surfaces and equipment healthy. The calcium hardness level in a pool should be maintained between 200-400ppm. Low calcium hardness can cause the water to etch at your plaster surface and draw the natural calcium from the surface. Too much calcium however, can cause scale to form on your pool surface and equipment. There is no way to reduce the calcium hardness of the water, so be sure not to add too much when making adjustments!

Cyanuric acid is used as a sunblock for your chlorine to prevent it from burning off too quickly. The cyanuric acid level should be maintained between 30-50ppm in a residential pool. Cyanuric acid can also not be reduced in the water, so take caution that you do not add too much. This is only used in outdoor pools.

Now that you have a better understanding of all those numbers they give you at your local pool store, it probably makes more sense when you hear them tell you to add 20lbs of this or 10lbs of that. Keeping your water balanced will keep your swimmers from getting sick and keeps your pool looking and running like new.