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The Pool Encyclopedia


The following is a list of abbreviations, acronyms and other terms that you might encounter on the page or at the pool store.

Alkalinity Increaser (Also known as Baking Soda or Sodium Bicarbonate/Sodium Bicarb)

This is nothing more than common household baking soda. It is the same “chemical” that pool stores will sell you for more money to increase your total alkalinity.

Baquacil (PHMB (Polyhexamethylene Biguanide)

This is an alternative to chlorine. Those who have a true chlorine allergy may decide to use this, although these allergies are rare.

Bleach (Yup, just like what you put in the laundry!)

Go down to sodium hypochorite, below, as that is what bleach is. You can find it in concentrations from 3% to 6% at the grocery store (and we have heard of real cheap bleach with 1% concentration, so make sure you read the label!). Make sure that you buy the plain or unscented kind if you purchase your bleach here, and do not buy the scented ones! As a general rule of thumb, one gallon of regular bleach will raise 10,000 gallons of water approximately the percentage of the bleach in FC (Free Chlorine)-in other words, one gallon of 3% bleach will raise 10,000 gallons by approximately 3 ppm FC, and one gallon of 6% will raise the same by approximately 6 ppm FC.


Borates are an optional consideration that can be added to your pool water to stabilize the pH. It can also help prevent algae, is helpful in reducing chlorine consumption, and will improve the feel and appearance of your water. Should you decide to use borates, we recommend that you keep the level between 30 and 50 ppm.

BR (Bromine)

Bromine is an alternative to chlorine, and is most commonly used in hot tubs (spas). The biggest “challenge” with bromine is that it can not be effectively stabilized against sunlight, so is is typically used in indoor spas or ones that are kept covered. Bromine is more stable than chlorine at higher temperatures, and requires higher temperatures than most pools are typically kept at to be effective, so it is generally used in hot tubs only.

Calcium Hypoclorite (Cal Hypo)

Calcium Hypochlorite is a slow dissolving un-stabilized chlorine, found in powder or granular form. While it has a minimal impact on pH (good), it will raise calcium hardness with continued use (not so good!). Cal hypo is suitable for normal sanitation, and can be used for shocking. It is the most common form of chlorine sold in pool supply stores as “Shock”. It can cloud your water if your TA (Total Alkalinity) is high, and it is recommended that you premix it in a bucket of water before adding to your pool. Cal hypo is relatively inexpensive, and is sold in different strengths; most commonly 48% and 65% concentration. 2.5 oz of the 65% concentration will raise 1,000 gallons of water by 1 ppm FC.

CC (Combined Chlorine)

This is the stuff that smells like chlorine, gives chlorine a bad name, and burns your eyes that you get rid of by shocking your pool. Combined chlorine means that you have something in your pool that needs to be addressed (usually algae!). This smell is really chloramines, which are released during the “cleansing” process.

CH (Calcium Hardness, sometimes shown as CAL)

Calcium Hardness is required for all plaster pools, typically accepted as being in the 200-400 ppm range. CH is the measure of calcium in your water, and it is most important in protecting plaster, as too low of a concentration of CH will make the water “aggressive”, and it will go searching for calcium in the plaster (or grout) if allowed to be kept too low. It is not as important in vinyl pools (although equipment manufacturers suggest a level of CH to protect the equipment), and in fiberglass pools it helps prevent staining and cobalt spotting, if properly maintained. Having too high of a calcium level can lead to scaling and cloudy water, as well as impeding chlorine’s ability to properly sanitize the water. as mentioned above, the recommended range for CH is between 200-400 ppm for both plaster and fiberglass pools. It is really is not much of an issue for vinyl pools, unless it is allowed to get very high. Generally, anything above about 130 ppm is probably alright for vinyl pools, and 200 ppm, or greater, will work for fiberglass pools. Always check and see if there is a warranty on your pool or liner that requires you to maintain a specific calcium level. Make sure you then maintain that level and follow that recommendation to keep your warranty! Calcium hardness is easily tested with a titration (drop count) test, in which you see the color change from pink to blue, counting the drops to get the CH level.

CYA (Cyanuric Acid, also know as Stabilizer or Conditioner)

CYA helps protect the chlorine from ‘burning off’ in direct sunlight. CYA is sometimes called “sunscreen for chlorine” for this reason. If there is too little in your pool, most of your chlorine can be lost in about 30 minutes of direct sunlight. Having too much CYA in your pool interferes with chlorine’s ability to sanitize and kill algae effectively. The recommended range is 30-50 ppm for a pool sanitized with liquid or granular chlorine, and between 50-100 ppm if you have a salt water chlorine generator, depending on the manufacturer of your unit. We like to see salt water pools between 60-80 ppm for the most part, as excess CYA is somewhat difficult to remove. Try holding a lower CYA residual for a while and see if your system will maintain your FC at the lower level. Remember, you can always add CYA easily, but removing is much harder!

DE (Diatomaceous Earth)

DE is made up of fossilized diatoms, which are porous, sponge like microscopic organisms. It is used in DE filters and provides particularly fine filtering of your pool water. Pool grade DE is a heat treated product and is a mild carcinogen, should the powder be inhaled (although it would take inhaling quite a bit, and most pool owners will never encounter it in a high enough concentration to worry). DE can also be found without heat treating for use in gardening, although gardening DE is considered safer to handle, it doesn’t work as all well in pool filters.

Di-chlor (Sodium dichloro-s-triazinetirone, Sodium dichloroisocyanurate)

This is a stabilized, granular, fast dissolving form of chlorine (chlorinated isocyanurate), and because of its fast dissolving nature, it is sometimes sold as ‘shock’ at pool stores. Di-Chlor is not a good idea to use generally, as it will raise CYA levels fairly quickly. Di-chlor is also mildly acidic, and it will lower pH and TA. Di-chlor will add .9 parts CYA for every 1 part of chlorine it adds.

DPD (N, N-diethyl-paraphenylenediamine)

DPD effectively tests both FC and TC (Total Chlorine). You will need to subtract the FC from the TC to get your CC (Combined Chlorine) total. This test turns different shades of red when chlorine is present, and uses a color comparator that you will match up the shade of red to get the totals you are looking for. Some have problems with this test because they are unable to differentiate the shades of red, and find it somewhat challenging. Please note that this test can bleach out when chlorine levels are above 10 ppm, and will turn colorless. This occurrence can lead you to believe that there is no chlorine in your pool when in fact it is actually very high. One trick you can use if this is the case is to use OTO (Orthotolidine) to verify if chlorine is present, since it will not bleach out at high chlorine levels, but will show a very bright yellow (or sometimes almost brown!) coloration.

FAS-DPD (Ferrous ammonium sulfate-N, N-diethyl-paraphenylenediamine)

The only true way to really tell how much chlorine is in your pool, this is a titration, or drop count, test for both FC and CC. This test measures chlorine levels up to at least 25 ppm (and possibly to 50 ppm) with an accuracy of .2 ppm. If you are serious about maintaining a safe swimming pool, this is the test you need. It will not suffer from the bleach out problem like the DPD test does, and is very easy to read, since the color change is from red to colorless when you are at the end point.

FC (Free Chlorine)

This is the stuff that sanitizes your water. Without chlorine, you do not have a safe swimming pool, as chlorine is effective at killing 99.9% of everything in your pool and providing a safe environment for you and your guests. FC, in proper balance, is completely safe.

HP (Horse Power)

HP is a rough indication of the power of a pool pump. Because HP can be calculated in several different ways, HP ratings cannot always be directly compared to each other when looking at pumps by different manufacturers. A more useful number would be the SFHP, which is the rated HP times the rated SF (Service Factor). SFHP numbers can be compared to each other when comparing different pumps made by different manufacturers.


Ionizers add metals to the water as an alternative to chlorine to help control algae. Most commonly, copper, silver, and zinc are used for this purpose. Typically, it is the cost, and risk of metal staining, that make these systems a poor choice.

Lithium Hypochlorite

Lithium Hypochlorite is a fast dissolving un-stabilized chlorine, found in powder form. It has a minimal impact on pH or other water parameters, and is suitable for shocking or normal sanitation. While it is very convenient as a powder, its convenience is offset by it’s extremely high cost.


“Minerals” is most often used in place of “metals” added to the water to help control algae. As mentioned above, the most commonly used metals are copper, silver, and zinc. Once again, the costs, and risk of metal staining, usually make this a poor choice.

MPS (KMPS, Potassium Monopersulfate, Potassium Peroxymonosulfate)

MPS is a non chlorine oxidizer or ‘shock’ product. While it is acidic and will lower pH and TA, it also adds sulfates to the water. MPS will not break down CC, but it does help prevent it from forming when used on a regular basis. If using MPS, please remember that it will interfere with TC and CC tests unless special reagents are used in testing, since it will show up in the test as CC! MPS is a good choice to use in indoor pools that are not exposed to sunlight, but it really has little or no advantage in an outdoor pool. Also remember that it is only an oxidizer and not a sanitizer, and should not be used as such.

MSDS (Material Safety Data Sheet)

MSDS is an informational sheet that all manufacturers of pool chemicals must provide, by law, if there are any hazardous chemicals in any of their products. The manufacturers are only required to list the hazardous chemicals in a product, and they are allowed to say that it is a “proprietary mixture”, give only a general description, and not list the actual ingredients, should they choose.

OTO (Orthotolidine)

While OTO will only test TC, it is a ‘bulletproof’ test that will not bleach out, and will always tell you if there is chlorine in your pool or not. Typically, it will be different shades of yellow when chlorine is present, and will turn orange to reddish or even brown when very high chlorine levels are present. OTO is a good test for daily checks to see if there is chlorine in your pool, and it is the test that is usually found in the common two (2) way chlorine/pH testers.


Ozone is occasionally used as a supplement to chlorine, and helps oxidize organic material in the pool. The biggest challenge with Ozone is that is does not provide any residual sanitizer in the bulk pool water, so it is not suitable as a stand alone sanitizer. Ozone may allow you to lower the ppm residual of another sanitizer (such as chlorine) however, which appeals to many pool owners.

pH (Potential of Hydrogen)

pH is a measure of how acidic or basic (alkaline) a solution is. While a pH of 7 is considered neutral, anything above 7 is considered basic, and anything below is considered acidic. Swimming pools, and spas, are usually kept at a pH between 7.2 to 7.8. pH, and is tested with the indicator Phenol Red. A high sanitizer level (above about 5 ppm) can interfere with this test and lead you to believe that your pH is very high, when it actually is not. Better test kits (such as those made by Taylor Technologies) will allow for having a higher sanitizer level by adding a chlorine neutralizer to the indicator. This will allow you to calibrate their color comparator blocks to compensate for the added chlorine neutralizer, thereby allowing you to get accurate pH readings when your sanitizer levels are at 10-15 ppm range.

Polyquat (Polyoxyethylene(dimethyliminio)ethylene(dimethyli minio)ethylene dichloride)

This is the only algaecide we suggest that you use. Polyquat is most often sold in a 60% concentration, as “Algaecide 60”, or something similar. Most other types of algaecides have various associated problems, such as adding metals, foaming, bad smells, etc, depending on what they contain. An algaecide is most affective at preventing algae from getting started, but it does not do very well against an active algae bloom (high dosages of chlorine will do a better job against this).

Pool Salt

Also called Sodium Chloride, is used for Salt Water Chlorine Generators. Pool salt usually has a smaller crystal size than water softener/solar salt, and costs about twice as much. You can use ordinary water softener/solar salt or pellets that have a purity of at least 99.5% and do not have any additives for cleaning water softeners or removing iron. These are fine to use in your pool and are what most manufacturers of SWCG’s recommend. The advantage to pool salt is that it dissolves slightly faster than the other versions of sodium chloride. It is important that you do not use food grade salt in a swimming pool. Food grade salt contains yellow prussiate of soda, also known as sodium ferrocyanide, which is an anti-caking ingredient, and can cause iron staining in your pool.

PPM (Parts per Million)

PPM is a measure of the concentration of the chemicals in your pool. Most measured items in your pool require a certain ppm level to keep your pool safe.

Salt (Sodium Chloride)

Do not be afraid of salt, as most all swimming pools contain some salt, as it is found in most of the chemicals used in swimming pools. Adding salt, at levels between 2000-3000 ppm, can be used to improve the subjective feel of the water without concern. Most SWCG’s require salt levels between 2800-3200 ppm, though some require higher levels to convert the salt to chlorine properly. Salt, in too high of concentration (somewhere above 5000 ppm), can contribute to corrosion of some materials, predominantly some of the softer kinds of natural stone and lesser grades of stainless steel. Ocean water, by comparison, typically has salt levels around 35,000 ppm; more than ten times the levels commonly used with a SWCG!


Shocking is the process (not a one time application!) of raising your FC high enough to break down the CC that has formed. Sometimes called break-point chlorination, this is the level required to completely kill any bacteria or algae in the water. Shock levels must be elevated to the proper level until the kill is complete, and is considered successful once you lose no more than 1 ppm FC overnight (always check FC levels at night after the sun has set and before the sun rises the next day, as sunlight consumes FC and will not allow you to accurately determine when you are done shocking). Shock is something you do to your pool, and is not a special product that you use. Any un-stabilized chlorine will work for use in shocking.

Sodium Hypochorite

Sodium Hypochorite is a liquid, un-stabilized chlorine that is suitable for shocking or normal day to day chlorination. Sodium Hypochlorite has minimal impact on pH and other water parameters, a sit consists of water, salt and chlorine. Sodium Hypochorite comes in different strengths, and the 12.5% strength is most commonly used in swimming pools, and is easy to dose. Each ounce of Sodium Hypochorite will raise 1000 gallons 1 ppm FC.

TA (Total Alkalinity)

TA is also occasionally called carbonate hardness or kH, and itt has nothing to do with water hardness. TA is the measure of the carbonates and bicarbonates in your water, and it helps keep your pH from swinging up and down. TA is tested with a titration (drop counting) test that has a distinct color change from green to red, or from blue to light yellow, depending on which test kit you are using.

TC (Total Chlorine)

TC is the measure of the total amount of both FC and CC in your pool, (FC + CC equals TC), and can be measured if your test kit uses FAS-DPD to test chlorine. TC minus FC equals CC, if your test kit uses DPD to test chlorine.

TDS (Total Dissolved Solids)

TDS measures all of the dissolved solids in a swimming pool, and is not a real useful or effective gauge for swimming pools. In the past, TDS was used as a proxy for CYA levels, before we were able to easily and accurately test CYA. TDS measures total dissolved solids, but does not break out what those solids actually are. What you really need to know is which particular chemicals are present, and in what concentration. TDS is a composite of CH, magnesium, salt, etc.

TH (Total Hardness)

TH is the combined levels of both calcium and magnesium in your swimming pool water and will always be higher than your calcium hardness alone. It’s not a test that will be real useful for a swimming pool, but many of the less expensive test kits, and just about every test strip, tests for total hardness and not calcium hardness. If you are using a drop-based test and it only uses 2 reagents (instead of three), it is most likely testing total hardness only. This test also changes from pink to blue.

Tri-chlor (trichloro-s-triazinetrione, trichloroisocyanuric acid)

Tri-chlor is a stabilized form of chlorine ( also known as chlorinated isocyanurate), and is usually sold in tablet or stick form. It is used in automatic chlorinators or in floating dispensers, since it is very slow dissolving. Tri-chlor has a very low pH (around 2.5) and will lower pH and TA, and is considered a good choice for new plaster pool start-up chlorination. Tri-chlor will cause CYA levels to rise with continued use, making it a poor choice for those with cartridge filters, or those in climates that do not winterize pools and have an extended swim season. Tri-chlor adds .6 part CYA for every 1 part of chlorine it adds to your pool.

UV (Ultraviolet)

UV is sometimes used as a supplement to chlorine to help oxidize organic material in the pool. UV does not provide any residual sanitizer in the bulk pool water, and cannot be used without a sanitizer.