Getting familiar with the correct terms will make caring for your pool and spa quick and easy. Click on any letter to get to the glossary section you want.



Acid- A Chemical compound which releases hydrogen ions into water, decreasing pH. Products like muriatic acid or Sodium Bisulfate (Lo n Slo) 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.

Aeration- 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. There are 21,000 known species of algae.

Algicide- A chemical added to 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)- A Nitrogen containing compound that combines with free chlorine to form chloramines.

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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 and Bromine and Biguanide are used to kill bacteria in pool & spa water.

Bactericides- Chemical compounds that kill bacteria.

Balanced Water- Pool or spa water that has a proper pH and the appropriate mineral content to prevent corrosion and scaling.

Base- See Alkali.

Bather- In a Spa, a bather is anyone using a spa for a duration of 20 minutes at 102° F.

Biguanide (PHMB)- Polyhexmethylene biguanide hydrochloride. A long chain polymeric molecule with both bactericidal and algicidal characteristics. Must be used in pool maintenance with a supporting oxidizer (Hydrogen peroxide).

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. This point is achieved when Free Available Chlorine is 10 times higher than Combined Chlorine.

Bromine (Br2)- A halogen element; alternative sanitizer for pools. Most effective in spas and indoor pools.

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Calcium Carbonate (CaCO3)- Scale that forms from calcium compounds when pool water is too alkaline, calcium hardness is too high or total alkalinity is too high. These hard deposits accumulate on pool surfaces and equipment.

Calcium Chloride (CaCl2)- A soluble salt added to pool water to raise the calcium hardness level.

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(OCL2)- A chlorine compound using calcium as the carrying salt for application.

Cartridge- A porous, replaceable element in some filters. Particulates are removed when they penetrate into the medium. Surface type cartridges have a medium less than ¾ 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.

Chloramines- Substances formed when chlorine combines with swimmer wastes (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. 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 therefore 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 of hypochlorite for water treatment. The chlorine is typically generated from Sodium Chloride (NaCl- salt) 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 the 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.

Contaminated- An impure condition indicating the presence of undesirable matter in pool water.

Corrosion- Etching, pitting and other destructive erosion of the spa surfaces and equipment due to low pH or other chemical imbalance.

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Diatomaceous earth (DE)- A powdery filtering agent composed of the skeletal remains of a form of plankton (diatoms). Use in Diatomaceous Earth filters.

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Effluent- The water that flos out of a filter, pump or other device.

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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 the most convenient.

Filter Element- A device (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. The material of a filter cartridge.

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.

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Gunite (shotcrete)- A concrete and sand mixture sprayed into a reinforced steel form to create a pool shell. Plaster, paint, of some other form of cosmetic finish is applied on top of the gunite structural shell.

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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 (unstabilized) family of chlorine compounds used in various forms to provide chlorine for water treatment. Includes Calcium hypochlorite, lithium hypochlorite, and sodium hypochlorite.

Hypochlorous Acid (HOCl)- The active sanitizing compound formed when any type of chlorine is put in water.

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Influent- The water entering a pump, filter, heater, or pipe.

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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 (3# of salt per gallon), and inconvenient to apply and handle. Should not be confused with Cloroxâ that has even lower levels of available chlorine.

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Make-Up Water- Fresh water used to fill the spa.

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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.

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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 7.6. 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 goes from 6.8 to 8.2.

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 the water a cloudy look.

Pump- A motor powered device that creates pressure and water flow by spinning an impeller to provide circulation through the filter and heater. Some spas combine a pump for filtration/heating with an additional “therapy pump.”

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Reagent- Chemical testing compounds that are used to test for chlorine, bromine, pH, total alkalinity, calcium hardness, etc.

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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.

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 “dip” 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 the Total Alkalinity. The water treatment grade of Sodium Bicarbonate is used in pool water. The baking grade (baking soda) is use for cooking. The two grades do not share the same physical characteristics, and should not be interchanged.

Soft Water- Water that contains less than 100 ppm of calcium and magnesium.

Stablized 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 the chlorine’s degradation due to exposure to the UV rays of the sun.

Superchlorination- See Shock Treatment and Breakpoint.

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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 Aklalinity- 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.

Turnover Rate- The period of time (in hours) required to circulate- through the pump & filter, a volume of water equal to the spa or pool capacity.

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Underwater Light- A Fixture designed to illuminate from beneath the water surface.

  • Dry Niche Light- A light unit placed behind a watertight window in a pool, spa or hot tub wall.
  • Wet Niche Light- A watertight and water-cooled light unit placed in a submerged niche in a pool, spa or hot tub wall and accessible only from the pool.

Upper Distribution System- A system designed to distribute the water entering a permanent medium. Also properly collects water during filter backwashing unless other means are provided.User- Any person using the a pool, spa or hot tub and adjoining deck area for the purpose of water sports, recreation, therapy or related activities.

User Load- The number of persons in the pool/spa area at any given moment, or during a stated period time.

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Vacuum- The reduction of atmospheric pressure within a pipe, tank, pump or other vessel. Measured in inches of Mercury, or 33.9 feet of head.

Vacuum Filter- A filter, usually of diatomite type, through which water is pulled by a pump mounted on the effluent side of the filter.

Valve- Any device in a pipe that will partially or totally obstruct the flow of water (as in a ball, gate or glove) or permit flow in only one direction (as in a check or foot valve)

  • Bleeder Valve- Allows air to be vented from a system
  • Multiport Valve- Allows for multidirectional control of the flow of water through a system.
  • Push-pull Valve- Allows for dual directional control of the flow of water through a system.

Velocity- The speed at which a liquid flows between two specific points, expressed in feet per second.Velocity Head- The equivalent distance a liquid must fall to attain a given velocity. The amount of energy, expressed in feet of head, required to accelerate the liquid to a given velocity.

Venting- A system that introduces air for combustion into the combustion chamber and disperses spent fossil fuel or flue products.

Venturi Jet- See hydrotherapy jets

Venturi Tube- A tube mounted in a waterline, meant to cause restriction of flow. The constriction causes a change in velocity of water through the tube, resulting in a pressure differential that is proportional to the flow rate.

Vertical Wall- Refers to the wall up to a positive 11-degree angle toward the pool’s interior from plumb.

Vinyl Liner- A plastic membrane constructed of vinyl or vinyl compounds that acts as a container of water.

  • Expandable Liner- Constructed of material that has the capacity to stretch into a shape other than the original construction dimensions.
  • Hopper Liner- Used to obtain greater depth by geometrical pattern construction on the liner bottom or floor to fit a predetermined size and shape.

Volts- The measure of electrical potential or electromotive force in units.Volume- Capacity; the amount of water expressed in gallons or liters that a pool/spa will hold.

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Wading Pool- A pool that has a maximum of 18” depth (shallow depth) used for wading. There are no requirements for residential wading pools (refer to ANSI/NSPI-1 Standard for Public Swimming Pools for public wading pools.)

Walls- The interior pool or spa wall surfaces consisting of surfaces from the plumb to a 45-degree slope.

Wastewater Disposal System- All water disposal systems approved by (state/local) authority, such as a storm sewer, sanitary sewer, open pit, leach field or irrigation system.


  • Overflow System- The waterline is at the top of the overflow rim.
  • Skimmer System- The waterline is at the midpoint of the operating range of the skimmers when there are no users in the pool/spa.

Water Pressure Switch- A switch that, from water flow, detects abnormal low pressure on a piece of equipment and can shut it off.Watt- The measure of electrical power computed by multiplying the voltage times the current (volts X amperes). Equal to 1/746th of one horsepower unit.

Watts Valve- A mechanical valve that opens with a high pressure or high temperature, used as a safety device on fossil-fuel heaters.

Weir- A device included with a through-the-wall or over-the-wall skimmer that controls the amount of surface water (flow) drawn into the skimmer and filtration system. See also skimmer weir.

Wet Niche- See underwater light.

Winterized Liner- A plastic liner that is manufactured with sufficient plasticizers to withstand exposure to its lowest rated temperature of –20 degrees F.

Winterizing- The process of preparing a pool or spa for freezing weather. Includes chemical treatment of the standing water, plus physical and chemical protection against freezing of the pool or spa and its equipment.

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