Basic pool Chemistry

Free chlorine shows the level of disinfecting chlorine available in the pool chemistry to keep your pool sanitary. FC should be tested and chlorine added daily, unless you have an automatic feeder or SWG, in which case you can test it every couple of days. FC is consumed by sunlight and by breaking down organic material in your pool. The level of FC you need to maintain depends on your CYA level and how much your pool is used. See The Pool Calculator's suggested FC levels section (towards the bottom) or the Chlorine / CYA Chart at TFP's Pool School for guidelines on the appropriate FC level to maintain based on your CYA level. It is important that you do not allow FC to get too low, or you run the risk of getting algae and/or having an unsafe pool.

Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. CC causes the "chlorine" smell many people associate with chlorine pools. If CC is above 0.5 you should shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. In an outdoor pool, CC will normally stay at or near zero as long as you maintain an appropriate FC level and the pool gets some direct sunlight.

Total chlorine is the sum of FC plus CC. Inexpensive chlorine tests, such as the common OTO test, which shows TC as different shades of yellow, measure TC because it is easier to test for than FC and CC. In normal operation TC can be used as if it was FC because CC is usually zero. However when you have algae or some other problems, CC levels can be significant and TC becomes useless.

pH indicates how acidic or basic the water is. pH should be tested daily at first. Once you gain experience with your pool, less frequent monitoring may be appropriate depending on your pool's typical rate of pH change. pH levels between 7.2 and 7.8 are suitable for swimming, while levels between 7.4 and 7.6 are ideal. pH below 7.2 tends to make eyes sting or burn. pH below 7.0 can cause damage to metal parts, particularly pool heaters with copper heat exchange coils. High pH can lead to calcium scaling. pH contributes to the CSI, which indicates the tendency for plaster damage or calcium scaling. Aeration will tend to cause the pH to rise. This can be mitigated by lowering TA.

Calcium hardness indicates the amount of calcium in the water. Over time, water with low calcium levels will tend to dissolve calcium out of plaster, pebble, tile, stone, concrete, and to some extent fiberglass surfaces. You can prevent this from happening by keeping the water saturated with calcium. In a vinyl lined pool there is no need for calcium, though high levels can still cause problems. A plaster pool should have CH levels between 250 and 350 if possible. Calcium helps fiberglass pools resist staining and cobalt spotting. If you have a spa you might want to keep CH at at least 100 to 150 to reduce foaming. CH contributes to the CSI which indicates the tendency for plaster damage or calcium scaling.

Total alkalinity indicates the water's ability to buffer pH changes. Buffering means you need to use a larger quantity of a chemical to change the pH. At low TA levels, the pH tends to swing around wildly. At high TA levels, the pH tends to drift up slowly, or even quickly in extreme cases. TA contributes to the CSI which indicates the tendency for plaster damage or calcium scaling.

Total hardness is the sum of calcium hardness and magnesium hardness. Most test strips report TH instead of CH. The ratio of calcium to magnesium varies. As an approximation you can multiply TH by two thirds to get a rough estimate of CH.

Cyanuric acid, often called stabilizer or sometimes conditioner, both protects FC from sunlight and lowers the effective strength of the FC. The higher your CYA level, the more FC you need to get the same effect. It is important to know your CYA level so you can figure out what FC level to aim for. If you don't have a SWG or problems from extremely high amounts of sunlight, CYA is typically kept between 30 and 50. If you have a SWG or very high levels of direct sunlight, CYA is typically kept between 60 and 80. If you are using an ORP controller, keep CYA below 50.

Borates can be used to control pH drift due to a SWG or high aeration levels. Borates also help to control algae. Various subjective water quality/feel improvements are also attributed to borates. If you are not intentionally using borates there is no need to test for them. To use borates to control pH rise and algae, add borates to between 30 to 50 ppm. With borate levels at 30 or above there is a chance that pets which drink the majority of their water from the pool might have some problems. Since pets should always be trained to not drink pool water, this shouldn't be a problem.

You need to adjust your FC target level based on your current CYA reading. Higher CYA levels bind up more of the FC, requiring higher FC levels to get the same disinfecting chlorine level. Many people find that a SWG will work with a slightly lower FC level than other forms of chlorine. The normal FC target is given as a range because different pools require different FC levels. If your water starts looking dull or just slightly cloudy try using a higher FC target.

Salt is required with a SWG. Salt can also be added to the water to enhance the subjective feel of the water. For a SWG, check the manual for the correct salt level for your unit. This level will typically around 3,000, but different models vary. For improved water feel, try levels around 2,000. These levels are less then one tenth of the salt level in ocean water, which has around 35,000 ppm of salt. People vary in their ability to taste low levels of salt. A few people can taste salt levels as low as 1,000, others not until 3,500 or more.

The calcite saturation index is a tool for estimating the likelihood of plaster corrosion or calcite scaling. The LSI, Langelier Saturation Index, is a very similar but slightly less accurate measure. The CSI uses pH, TA, CH, CYA, temperature, Borate, and Salt levels to estimate the likelihood of problems. A low saturation index means the water is likely to dissolve calcite out of plaster, pebble, tile, stone, and concrete surfaces (and perhaps fiberglass) which will eventually cause damage. A high saturation index means the water is likely to deposit calcite scale on the walls of the pool and in the plumbing.

The Pool Calculator can display suggested goal levels from either traditional sources, or from TroubleFreePool.com. Select the source you would like to use and also your primary source of chlorine and pool surface. Suggested goal levels will be displayed for FC, pH, TA, CH, and CYA. Suggested goals are simply a starting point. There are many situations that are not covered by these basic guidelines.

Common bleach sold in grocery stores is the same thing as liquid chlorine or liquid shock sold by pool stores in some states. You want bleach without any additives, typically labeled unscented or "original scent". Check the strength of the bleach when comparing prices. Bleach with a higher percentages of sodium hypochlorite is more effective and easier to carry.

A kind of sensor that reacts to disinfecting chlorine levels which is used in some systems that automatically feed chlorine into the pool based on the sensor's measurement of the current chlorine level in the pool.

Shocking your pool means bringing the FC level up to high levels and holding it there. This will kill anything that might be living in your water and to speed the breakdown of CC. The appropriate FC level to aim for depends on your CYA level.

When fighting algae you also want to brush the entire pool each day. This will expose any algae growing in biofilms on the walls or floor to the chlorine and allow the chlorine to do it's work. If you have black algae it is particularly important that you vigorously brush the algae as frequently as possible.