Pool Water Chemistry
" What to Measure and look for in pool chemical levels "

Article by Jeff Bogdanof of Vineyard Pools in Clovis, CA (2013)

These are the most important chemical levels to monitor in swimming pools.
The yellow are levels to monitor and maintain weekly and frequently. The others are more monthly to seasonal measurements to monitor.

 

  1. FC - Free Chlorine - A sanitizer which keeps your pool water safe and free of germs. Chlorine must be constantly replenished. (# depends on CYA)
  2. pH - Acidity/Alkalinity - Needs to be kept in balance to prevent corrosion or scale build-up which will protect the pool equipment and plaster. (7.2 to 7.8)
  3. TA - Total Alkalinity - Appropriate levels help keep the pH in balance. High levels can cause pH to rise. (80 ppm to 120 ppm generally)
  4. CH - Calcium Hardness - High levels will add scale. Too low and it will suck it out of your plaster. High levels can cause calcium scaling. (150ppm 400ppm)
  5. CYA - Cyanuric Acid - Protects chlorine from UV sunlight and determines the required FC level. (outdoors 50 to 100, indoors 0 to 20 since there's no sunlight
  6. TDS - Total Dissolved Solids - Includes everything solid that is soluble and floating in the water. High TDS can = "tired water" (+2000 ppm = drain pool dillution)
  7. Phosphate - A pollutant that is an important nutrient for algae to grow. It's best to keep phosphates minimized close to zero if possible, less is better.
  8. Salt - Required with a Salt Chlorine Generator, otherwise an optional enhancement. Can have some minor corrosive elements to pool and deck.

 


Free Chlorine - FC 

Free chlorine shows the level of disinfecting chlorine available (active plus reserve) to keep your pool sanitary. Normally, FC should be tested and chlorine added daily. If you have an automatic feeder or Salt chlorine generator, 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 Cyanuric Acid Level (CYA) and how much you use the pool. See the Chlorine / CYA Chart 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. If FC ever gets down to zero, or you have algae, the pool is not safe to swim in. Maintaining an appropriate FC level is the most important part of keeping your water in balance.

Bleach, liquid chlorine, liquid shock, trichlor tablets/pucks/sticks, dichlor powder, cal-hypo powder/capsules, and lithium hypochlorite all raise the FC level. It is important to use bleach without any additives or special features, typically labeled unscented or "original scent". In addition to chlorine, trichlor and dichlor also add CYA, and lower the pH of the water. Cal-hypo adds chlorine and calcium. Lithium hypochlorite tends to be quite expensive. See How to Chlorinate Your Pool for information on choosing what kind of chlorine to use.

It is most efficient to raise the FC level in the evening, since none will be lost to sunlight until the next morning. FC normally goes down by itself. If you must lower the FC level quickly, you can use a chlorine neutralizer (sodium thiosulfate).

Combined Chlorine - CC  "Time to shock !"

Combined chlorine forms when there is often too much bather waste. Eww... Normal chlorine levels of 2.0-4.0 ppm will not necessarily burn off the organic and inorganic wastes left behind from your last pool party with the 3 year old child that didn't want to go inside to use the bathroom. CC causes the "chlorine" smell many people associate with chlorine pools. If CC is even a small amount such as 0.2ppm you can still be irritated and smell it. When CC is discovered it is recommended 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.

Potassium monopersulfate is a great way to shock your pool. Usually 1lb. per 10,000 gallons will do the trick (a common non-chlorine shock) will often show up on tests as CC. There is a special reagent you can get to neutralize the potassium monopersulfate so you can get a true CC reading.

Total Chlorine - TC 

Total chlorine is the sum of FC plus CC. Inexpensive chlorine tests, such as the common OTO test, which shows the TC level as different shades of yellow, measures 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. You can not use the OTO test, or other tests that only measure TC, when you have algae or certain other problems. Anything that might cause the CC level to be above zero, such as algae, makes the TC level different from the FC level. In these situations, TC is useless on its own.

Total Alkalinity - TA  "Father" to pH - controls and keeps pH maintained

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. TA contributes to the CSI which indicates the tendency for plaster damage or calcium scaling.

The ideal TA level depends on several factors. If you are using acidic chlorine sources, such as trichlor or dichlor, keep TA on the high side, perhaps between 100 and 120. If you have a SWG, or if you commonly run water features such as a spa, waterfall, or fountain, keep TA on the low side, between 60 and 80. Otherwise levels between 70 and 90 are good. Pools with plaster surfaces should factor their CSI into the preferred TA level decision. Pools with vinyl liners can tolerate high TA levels reasonably well.

You can raise TA with baking soda. It is often best to make large TA adjustments in a couple of steps, testing the water after each one, as adding baking soda will also affect the pH and you don't want the pH going out of range. Lowering the TA involves adding muriatic acid which in turn also lowers the pH of the water. Always adjust alkalinity first before adjusting pH levels. 


pH - Acidity/Alkalinity - "Son" to alkalninty 

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 7.5 and 7.8 are ideal, while levels between 7.2 and 7.8 are acceptable for swimming.

pH levels below 7.2 tend to make eyes sting or burn. pH below 6.8 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.

Many pools will drift up towards higher pH over time. This is particularly true for fresh plaster (particularly in the first month and continuing for perhaps a year) or when TA is high and the water is being aerated (because of a spa, waterfall, fountain, SWG, rain, kids splashing in the pool, etc).

You can raise pH with borax or soda ash/washing soda. Soda ash/washing soda will increase TA more than borax will. You can lower pH with muriatic acid or dry acid. How much you will need for a given pH change depends on several other numbers, most importantly your TA and borate levels. Higher TA and/or borate levels cause you to need larger amounts of chemicals to change the pH.


Calcium Hardness - CH 

Calcium hardness is primarily the topic. But Total Hardness covers all mineral salts in the water, such as calcium (75% of total hardness) can also be 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.

You increase CH with calcium chloride, you lower calcium by replacing water or using a reverse osmosis water treatment.

Total Hardness -TH

 
Total hardness is the sum of calcium hardness and magnesium hardness. 75% of TH is calcium, so calcium and TH can be used interchangeably. 


Cyanuric Acid - CYA 

Cyanuric acid, often called stabilizer or conditioner, both protects Free Chlorine from sunlight and lowers the effective strength of the FC (by holding some of the FC in reserve). The higher your CYA level, the more FC you need to use 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 Salt Chlorine Generator or you have problems from extremely high amounts of sunlight, CYA is typically kept between and 40 and 70. If you have a chlorine generator or very high levels of direct sunlight, CYA is typically kept between 70 and 100.

You increase CYA by adding cyanuric acid, often sold as stabilizer or conditioner. CYA is available as a solid and as a liquid. The liquid costs a lot more, and generally isn't worth the extra expense. Solid stabilizer can take up to a week to fully register on the test, so don't retest your CYA level for a week after adding some. Solid stabilizer is best added by placing it in a sock in the skimmer basket. The pump should be run for 24 hours after adding solid stabilizer and you should avoid backwashing/cleaning the filter for a week.

In nearly all cases the best way to lower CYA is to do a partial or majority drain and refill the water. It cannot be chemically removed or taken out of the water unless in some circumstances you have a high degree of bacteria in your pool that literally eats the CYA and will proceed to poop nitrates. In that case, your pool is in a bad condition and draining is still a likely solution if not other complex problem.


Salt

Salt is required with a Salt Chlorine Generator. Keep in mind when water evaporates, it only evaporates PURE water, meaning salt and all other solids such as calcium, minerals, cyanuric acid, etc. can only be physically drained from the pool .Salt can also be added to the water to enhance the subjective feel of the water. For a SCG, check the manual for the correct salt level for your unit, Pentair Intelli Chlor is 3500 ppm. For improved water feel without a SCG, 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.

Having salt in the water just slightly increases the risk of corrosion, particularly to surfaces that water splashes onto where the water can evaporate, leaving high concentrations of salt behind. This is normally only a problem for stone work above the water line made from softer kinds of stone. There is a lack of solid information about the salt corrosion risks for many materials, leading to debate about the overall level of risk. Most people with salt in their water do not experience any problems.

Salt can be added using solar salt, sold for use in water softeners (sodium chloride). You want the kind that is 99.4% pure or better and which doesn't have any rust inhibitor or other additives. Crystals are fine. Pellets will work but dissolve slightly more slowly. Pool store salt generally costs more and is more finely ground, but even pellets dissolve quickly enough so that isn't really any advantage.

Total Dissolved Solids - TDS

TDS is the measure of the total of all of the soluble solids dissolved in the water. Measuring your TDS levels is usually done by assessing the electrical conductivity of the pool water.  The primary contributing factor to higher TDS levels is evaporation. The evaporation level in Phoenix is high due to our extreme heat.  The average pool evaporates the equivalent of the pool's capacity 1.5 times a year. As the water evaporates, only the water itself is removed, the minerals, chemical residue and other particles are left behind in the remaining pool water.  You then add more water to your pool due to the lower levels, but the remnants of the minerals, etc. remain, allowing higher levels even with the  water being added.

Distilled or pure water has a TDS value of 0 ppm (parts per million). Drinking water can have a maximum TDS value of 500 ppm according to EPS Water Standards. For regular fresh water swimming pools, the maximum recommended TDS level is 2000 ppm.  Any TDS level that gets any higher than that can lead to some or all of the following problems: cloudy water, staining of the pool surfaces, scaling, hard water and salty taste, along with additional chemical usage.

The TDS value should be used as an indicator of the volume of soluble particles in the pool relative to that of the fill water used. If the pool water has become saturated with contaminants and the TDS level confirms this, the pool should ideally be drained and refilled with fresh water. There is no method to reduce the TDS effectively without replacing some or all of the swimming pool water. Lower TDS levels allows the sanitation chemicals to do their job quickly and effectively without much waste.  The higher the levels - the more possibilities of algae and difficulty balancing your chemistry and keeping it balanced.

 

Phosphates

Phosphates are a pollutant that is also a nutrient for algae to grow. Sometimes removed from the pool as a way of keeping algae in check. A sanitizer, (Chlorine) is still always necessary. There are large doses, or maintenance doses of many products that can keep levels close to zero which can assist with the overall water quality. Most phosphates come from soaps, detergents, shampoos, soda, ferilizers, organic debris like leaves, bark, and even some other pool chemicals. Some argue that phosphate levels by themselves are not very important in the scheme of the other important factors to measure and maintain. I would not suggest phosphates as the #1 problem for most pools, it's likely one of the last problems to check off.