Pool Terminology
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Acid      A chemical compound which releases hydrogen ions into water, decreasing pH.Products like muriatic acid or Sodium Bisulfate are used to lower pH and total Alkalinity in pool water.
Acid Demand   The amount of acid needed to lower pH to the proper level for pool water.

The process of mixing air and water. In a spa this can happen two ways:

1. Using an Air Blower to force air into an air channel or through the spa jets.

2. With Venturi Air Controls which allow air to be pulled into and mixed with the water in the spa jet.

Algae         Microscopic plants deposited in pool or spa water by wind, rain, and dust. They thrive in sunlight and warm water, clogging filters, increasing the need for sanitizers and oxidizers, and causing slippery surfaces. These are 21,000 known species of algae.
Algaecide    A chemical added to the water to prevent or control algae growth.
Algistat   Any substance that retards algae growth.
Alkali   A basic solution that neutralizes acids by releasing carbonates and/or hydroxides.
Alkali Demand   The amount of alkali (base) needed in the water to raise pH and/or Total Alkalinity to the proper level.
Alkaline   When the pH of a solution measures above 7.0 on the pH scale. Alkaline (often referred to as "basic") is the opposite of acid.
Alkalinity   The amount of bicarbonate, carbonate, and hydroxide compounds present in the water. Total Alkalinity is a measure of the water's ability to resist pH change due to the presence of these compounds in the water.
Ammonia (NH3)   Nitrogen containing compound that combines with free chlorine to form chloramines.
Backwash        Reversing the flow of water through the filter to clean the elements and filter medium. Typical part of maintenance for sand filters and some DE filters.
Backwash Cycle   The time needed to backwash (clean) the filter and its components.
Bacteria   Single-celled microscopic organisms. Pathogenic bacteria can cause infections, disease and bather irritation. Chlorine, Bromine and Biguanide are used to kill bacteria in pool and spa water.
Bactericides   Chemical compounds that kill bacteria.
Balanced Water   Pool or spa water that has proper pH and the appropriate mineral content to prevent corrosion and scaling.
Bather Load   The number of people in a pool or spa at a particular time or during a specific period of time.
Borate   An elemental mineral used for conditioning water to provide clearer, more comfortable water.
Breakpoint   During chlorination, this is the point at which all combined chlorine is oxidized (removed) and only Free Available Chlorine remains in the water to kill bacteria. The point is achieved when Free Available Chlorine is 10 times higher that Combined Chlorine.
Bromine   A halogen element; alternative sanitizer for pools. Most effective in spas and indoor pools.
Buffer   Chemicals that serve to slow fluctuations in pH. (See Alkalinity)
Calcium Carbonate (CaCO3)         Scale that forms from calcium compounds when pool water is too alkaline and/or calcium hardness or total alkalinity is too high. These hard deposits accumulate on pool surfaces and equipment.
Calcium Hardness      The amount of dissolved calcium in water. Low levels of calcium hardness will promote deterioration in the pool surfaces and equipment. High levels will promote scale formation.
Calcium Hypochlorite Ca(OCL)2   A chlorine compound using calcium as the carrying salt for application. This chemical is considered to be relatively stable and has greater available chlorine than sodium hypochlorite (liquid bleach).
Cartridge   A porous, replaceable element in some filters. Particulates are removed when they into the medium. Surface type cartridges have a medium less than 3/4 inch thick. Particulates are retained on the surface of the cartridge for removal. Loose debris can be hosed off, oils must be chemically removed by soaking the cartridge in a solution of water and filter cleaner.
Chelant   A chemical compound that ties-up iron, copper, or calcium to prevent staining and scaling. Also called a sequestering agent.
Chloramines   Substances formed when chlorine combines with swimmer waster (nitrogen or ammonia), causing chlorine odor and irritation to skin and eyes. This compound has little sanitizing value compared to active chlorine.
Chlorine (Cl)   One of five members of the Halogen family of chemical elements. As an element it is a gas. Other forms are compounds. It is the most widely used bacteria-killing agent for recreational water treatment. Two forms of chlorine are:

1. Organic chlorine - less vulnerable to the UV rays of the sun and therefore longer lasting

2. Inorganic chlorine - susceptible to degradation by the UV rays of the sun and therefor less convenient for pool use. Also see Hypochlorite.

Chlorine Demand   The amount of chlorine needed to establish a stable, residual for effective sanitation.
Chlorine Generator   On-site equipment that generates its own supply of chlorine, hypochlorous acid or hypochlorite for water treatment. The chlorine is typically generated from Sodium Chloride (NaCl) by exposing it to a low voltage (DC) electrical current.
Chlorine Neutralizer   Sodium Thiosulfate or other similar compound used to neutralize excessive chlorine in a water sample in order to permit more accurate testing of the water balance factors. Sodium Sulfite is typically used in pools and spas to neutralize high levels of Chlorine or Bromine.
Chlorine Residual   The amount of chlorine which is readily available to sanitize pool water.
Coagulant   A polymeric chemical compound added to water to gather suspended particles together for filtration.
Combined Chlorine   Chlorine which is bonded to other compounds; a chloramine. See Chloramines.
Contaminated   An impure condition indicating the presence of undesirable matter in pool water.
Corrosion   Etching, pitting and other destructive erosion of the pool surfaces and/or equipment due to low pH or other chemical imbalance.
Cyanuric Acid (Triazinetrione)   A chemical compound added to pool water to reduce the degradation of chlorine by the UV rays of the sun. Chlorinated Isocyanurates are the group of chlorine compounds that combine Chlorine and Cyanuric Acid into a form for pool and spa sanitizing.


Diatomaceous Earth (DE)     A powdery filtering agent composed of the skeletal remains of a form of plankton (diatoms). Used in Diatomaceous Earth filters.
DPD #1   A test reagent used to measure the amount of Free Available Chlorine in the water.
Dry Acid   A Bisulfate compound used to lower the pH and Total Alkalinity. Safer to handle than Muriatic Acid.
Effluent   The water that flows out of a pump, filter, heater, or pipe.

Filter    A device that removes undissolved particles from water through a porous filter medium (sand, cartridge, DE).
Filter Cycle   The operating time between filter cleaning or backwash cycles. Long filter cycles are most convenient.
Filter Element   The cartridge within a filter housing designed to remove suspended debris from the water.
Filter Medium   Sand, Diatomaceous Earth, or other finely graded material used to filter particles out of the water.
Filter Sand   Sharp silica or quartz particles graded for uniform size and used as a filter medium. #20 Silica Sand is the industry standard grade of filter sand.
Flow Meter   A measuring device that determines the gallons per minute of water flow through a pool recirculation system.
Flow Rate   The volume of liquid (water) flowing past a given point in a specific time period, expressed in gallons per minute.
Free Available Chlorine   Hypochlorous acid, the chlorine in pool water that is not combined with ammonia or nitrogenous compounds, and therefore is available to kill bacteria entering the water. See also Available Chlorine.
Gelcoat     The colored surface layer of a fiberglass pool or spa shell. This resin is applied to the mold during the manufacturing process, and is either of polyester or vinylester composition.
Gunite   A concrete and sand mixture sprayed into a reinforced steel form to create a pool shell. Plaster, paint, or some other form of cosmetic finish is applied on top of the gunite structural shell. 
Halogen       A family of chemical elements containing Chlorine, Bromide, Fluorine, Iodine, and Astatine. With the exception of Astatine, the Halogen family is widely used for a variety of sanitizing situations.
Hydrogen Ion (H+)   The positively charged nucleus of a hydrogen atom. Increasing levels of the hydrogen ion in the water will cause pH to be lowered.
Hydrotherapy Jets   A spa fitting that blends air and water creating a high-velocity, turbulent stream of air enriched water.
Hydrotherapy Spa   A non-wooden vessel containing hot moving water for therapeutic use to ease stress, muscle strains and other physical problems. Popular construction types include thermoplastic shells and gunite/plaster interiors. 
Hypochlorite   An inorganic (unstabalized) family of chlorine compounds used in various forms to provide chlorine for water treatment. Includes Calcium Hypochlorite, Lithium Hypochlorite, and Sodium Hypochlorite.
Hypochlorous Acid      The active sanitizing compound formed when any type of chlorine is put in water.


Impeller       The heart of the centrifugal spa or pool pump. Rotating vanes create the suction flow of the water into the pump.
Influent   The water entering a pump, filter, heater or pipe. 
Leaching   The process of extracting a mineral from plaster interiors or tannic acid from wooden hot tubs.
Liquid Chlorine (Bleach)   Sodium Hypochlorite (NaOCl) solutions added to water as a disinfectant. Characteristics include very low levels of available chlorine (12-15%), high contribution to Total Dissolved Solids, and inconvenient to apply and handle.


Oxidation     A chemical process for removing undesirable compounds from the water
Ozone          A gaseous molecule composed of three atoms of oxygen (O3). It is created in ozone generators for oxidation. Its instability and short life in the water require that it be used only to supplement chlorine or bromine.
pH        A measurement that indicates the acidic or basic nature of a solution. Measured on a scale from 0 to 14 the ideal pH should be 7.4 to 4.6. A pH of 7.0 is neutral. A pH below 7.0 is acidic. A pH above 7.0 is basic.
Phenol Red   A colored reagent for measuring the pH of water in a range from 6.8 to 8.2. It changes from yellow to purple in color as the pH raises. 
Plaster   The interior finish of a gunite (concrete) spa or pool. Composed of white marble dust and portland cement.
Ppm   Parts per million, a unit of measurement used in measuring chemical application. It indicates the amount, by weight, of a chemical in relation to one million parts by weight of water.
Psi   Pounds per square inch, the unit by which filter pressure is measured on a pressure gauge, Psi increases as the filter gets dirtier.
Precipitate   Solid particles forced out of solution by a chemical reaction. They may settle on the bottom of the spa or pool or remain suspended in the water giving it a cloudy look.
Pump   A motor powered device that creates pressure and water flow by spinning an impeller to provide circulation through the pump room.


Reagent      Chemical testing compounds that are used to test for chlorine, bromine, pH, total alkalinity, calcium hardness, etc. 
Re-Bar   Reinforcement bar, used to add strength to concrete. After excavation of an in-ground poo,, a steel cage is formed out of re-bar and the gunite shell is shot over and surrounding it.
Residual   Usually refers to free available chlorine levels remainng in the pool after initial treatment or activity with contaminants.
Saturation Index (SI)      A numeric value indicating whether water is scale forming or corrosive. It factors in pH, Total Alkalinity, Calcium Hardness, and water temperature. Ideal range is between -.3 and +.3.
Sanitizer   A chemical agent used to remove unwanted contaminants.
Scale   Mineral deposits that form on spa surfaces and equipment due to excessive calcium in the water. Scale  is more likely to form in heated water, especially on the heater element or heat exchanger if proper water balance is not maintained.
Shock Treatment   The addition of an oxidizing compound to the pool or spa water to chemically breakup (oxidize) contaminants such as suntan oils, cosmetics, perspiration, metal ions and windblown dirt which interfere with normal sanitizer performance and/or cause cloudiness or colored water.
Skimmer   A device in the pool or spa wall that continuously removes the surface water and floating debris to be taken away by the filter. A hand skimmer net can be used manually to remove large floating debris from the water.
Sodium Carbonate (Na2CO3)   Soda Ash, added to water to increase pH.
Sodium Bicarbonate (NaHCO3)   Added to water to increase Total Alkalinity. The water treatment of Sodium Bicarbonate is used in pool water. The baking grade (baking soda) is used for cooking. The two grades do not share physical characteristics and should not be interchanged.
Soft Water   Water that contains less than 100 ppm of calcium and magnesium.
Stabilized Chlorine   An Organic (stabilized) compound of chlorine and cyanuric acid. The two most common compounds are trichloro-s-triazinetrione and sodium dichloro-s-trazinetrione. Their popularity is due to the protection that cyanuric acid provides to prevent chlorine's degradation due to exposure to the UV rays of the sun.
Stabilizer   A chemical compound added to pool water to reduce the degradation of chlorine by the UV rays of the sun. Chlorinated Isocyanurates are the group of chlorine compounds that combine Chlorine and Cyanuric Acid into a form for pool and spa sanitizing.


Suction Side   The plumbing prior to and carrying water to the pump. This side is under vacuum pressure.
Titration        A method of testing water. The end point of the titration process is determined by a pH change, caused by the titration solution being added to the test sample. The changing pH triggers the reagent to change colors.
Total Alkalinity   A measure of the water's ability to prevent pH change. It measures the amount of Carbonates, Bicarbonates, Hydroxides, and Borates, in the water.
Total Dissolved Solids (TDS)   A measure of the amount of dissolved matter in the water. High TDS (1500 ppm and higher) can interfere with the sanitizer's ability to combat bacteria growth.
Turbidity   Cloudy condition of water caused by finely divided microscopic material in suspension interfering with the passage of light.
Turnover Rate   The period of time (in hours) required to circulate water through the pump and filter, a volume of watr equal to the spa or pool capacity.