acid A sour chemical substance that releases hydrogen ions with the ability to dissolve metals, neutralize alkaline materials and to combine with bases to form salts. Acid is used to lower (decrease) pH and total alkalinity of swimming pool and spa water. Examples are muriatic acid (hydrochloric) and dry acid (sodium bisulfate). All liquids with a pH lower than 7.0 are acidic or acid.

acid demand The amount of acid required to bring high pH and total alkalinity down to their proper levels. Determined by the Acid Demand Test.

acid demand test A reagent test usually used in conjunction with a pH test to determine the amount of acid needed to lower high pH and total alkalinity levels.

aggressive water Water that is corrosive because it is low in pH, alkalinity or calcium hardness.

air relief valve A brass or plastic manually operated valve located at the top of a filter tank for relieving the pressure inside the filter and for removing the air inside the filter (called bleeding the filter). Sometimes called a pressure relief valve.

air switch A pneumatic control device used to operate pool and spa equipment safely. A button located near the water and convenient for use is pushed which sends air pressure through a small flexible tube or hose to a switch.

algae Microscopic plant-like organisms that contain chlorophyll. Algae are nourished by carbon and use sunlight to carry out photosynthesis. They are introduced by rain or wind and grow in colonies producing nuisance masses. Algae are not disease-causing but they can harbor bacteria and they are slippery. There are 21,000 known species of algae. The most common pool types are black, blue-green, green and yellow or mustard. Pink or red colored algae-like organisms exist but are bacteria and not algae. Maintaining proper sanitizer levels, shocking and superchlorination will prevent their occurrence.

algaecide – A natural or synthetic chemical designed to kill destroy or control algae.

algaestat – Any chemical or substance that inhibits the growth of algae.

alkalialso called base – A class of compounds which will react with an acid to give a salt. A substance that creates a bitter taste and a slippery feel when dissolved in water. An alkali has a pH greater than 7.0 and is the opposite of an acid. Alkalis may include the soluble hydroxide, carbonate and bicarbonate salts of calcium, magnesium, potassium, and sodium. A reaction between an acid and a base is called a neutralization.

alkalinityalso more commonly called total alkalinity – A measure of the pH-buffering capacity of water. Also defined as the water’s resistance to change in pH. Composed of the hydroxides, carbonates and bicarbonates in the water expressed in ppm or mg/L. One of the basic water tests necessary to determine water balance. The ideal range is from 60 to 140 ppm.

anti-foam A chemical added to the water to make the suds or foam go away. These products do not remove the source of the sudsing. Most often, the water must be drained and refilled to remove the soaps, oils and other causes of foaming. Shocking, superchlorination, clarifiers and enzymes may help remove the causes.

anti-scalant A chemical that inhibits or delays precipitation and subsequent scale formation.

anti-vortex drain cover A cover or grate attached to the main drain that prevents a vortex (whirlpool) from forming as the water passes through.

ascorbic acid A chemical compound used to remove iron stains from fiberglass and vinyl pools. It is Vitamin C and can also be used to dechlorinate or debrominate water.

automatic pool cleaner Pool maintenance system that will agitate or vacuum debris from the pool interior.

available chlorine The amount of chlorine, both free and combined in the pool water that is available to sanitize or disinfect the water. Sometimes called residual chlorine.

backfill Dirt or other material used to fill in the area or gaps between the pool or spa walls and the surrounding excavation.

backflow The backing up of water through a pipe in the direction opposite to normal flow.

back pressure Resistance to flow, normally expressed in pounds per square inch (psi).

backwash The process of thoroughly cleaning the filter by reversing the flow of water through it with the dirt and rinse water going to waste.

Bactericide A chemical or element that kills, destroys or controls bacteria.

Bacteriostat A substance that prevents or retards bacteria growth.

baffle A deflector plate on the inlet of a typical filter that disperses the water over the top of the filter bed or media more evenly.

balanced water The correct ratio of mineral content, pH and alkalinity levels that prevents the water from being corrosive or scale forming.

ball valve A simple non-return valve consisting of a ball resting on a cylindrical seat within a liquid passageway.

base demand The amount of base required to bring low pH and total alkalinity up to their proper levels. Determined by the Base Demand Test.

base demand test A measurement indicating the amount of alkaline material required to raise the pH and alkalinity to their proper levels.

black algae A really tough strain of algae that grows in pools and spas. It can be difficult to get rid of. See algae.

bleed To remove air from a pipe or device, allowing water to fill the space.

bleeder valve A device, usually on a filter, that allows air to be vented from the plumbing system.

blister Refers to an air pocket in a plastic or fiberglass surface.

blower An electrical device that produces a continuous rush of air to create the optimal bubbling effect in a spa, hot tub or whirlpool. It is usually plumbed in with the hydrotherapy jets or to a separate bubbler ring.

blue fingernails Caused by too much copper in the pool water. Blue fingernails are not caused by chlorine. The copper may get into the water by the bad practice of placing trichlor tabs in the skimmer. This acidic product will cause low-pH water which will in turn dissolve metals in the equipment. The dissolved metal (usually copper) then stains hair, fingernails and eventually pool walls. It can also be caused by keeping the pH too low or misusing acid.

bond beam The top of a wall of a pool or spa that is built stronger than the wall so it will support the coping and surrounding deck.

booster pump A water pump added to a spa circulation system to add pressure to the jets. Also used for some automatic pool cleaners.

breakpoint chlorination The process of adding sufficient free available chlorine to completely oxidize all organic matter and ammonia or nitrogen compounds. All chlorine added after that point is free available chlorine. Requires a ratio of chlorine to ammonia of 7.6:1 but it is easier and leaves a chlorine residual to multiply the combined chlorine or ammonia content by 10 and add that as free chlorine to reach breakpoint.

bridging A condition that exists when DE (diatomaceous earth) and suspended particles (dirt) closes the intended gaps between the filter and the filter grids in a DE filter which reduces the water flow rate through the filter.

bromine tablets Common name for two chemical compounds containing bromine. They are used as disinfectants to destroy bacteria and algae. A separate oxidizer must be used to destroy swimmer waste. (Leslie’s Bromine Tablets)

brominator A mechanical or electrical device for dispensing bromine at a controlled rate. Most often a canister or floater filled with tablets of bromine.

buffer A substance or compound that stabilizes the pH value of a solution. It is also the water’s resistance to change in pH.

bypass An arrangement of pipes, gates and valves by which the flow of water may be passed around a piece of equipment or diverted to another piece of equipment; a controlled diversion.

calcification The formation of calcium carbonate on the walls or pipes, or in a heater due to the precipitation of calcium carbonate.

calcium carbonate Crystalline compounds formed in swimming pool and spa water when the calcium, pH and total alkalinity levels are too high. Once formed, the crystals adhere to the plumbing, equipment, pool walls and pool bottom. These crystals are better known as scale.

calcium chloride A soluble white salt used to raise the calcium or total hardness level in the pool or spa water.

calcium hypochlorite A compound of chlorine and calcium used as a disinfectant, sanitizer, bactericide, algaecide and oxidizer in swimming pool and spa water. It is available as a white granular material usually used for superchlorination or it is available as tablets used in a feeder for regular chlorination.

cartridge A replaceable porous element made of paper or polyester used as the filter medium in cartridge filters.

cartridge filter A pool or spa water filter that uses a replaceable porous element made of paper or polyester as its removal system.

centrifugal pump A pump consisting of an impeller fixed on a rotating shaft and enclosed in a casing or volute and having an inlet and a discharge connection. The rotating impeller creates pressure in the water by the velocity derived from the centrifugal force.

channeling Refers to the non-standard flow of water through a D.E. or sand filter wherein the liquid follows the pathway of least resistance rather than passing uniformly throughout the medium bed. Channeling does not permit uniform filtration of the water passing through the filter.

check valve A mechanical device in a pipe that permits the flow of water or air in one direction only.

chelate Pronounced KEY-late. Also called sequester. It is the process of preventing metals in the water from combining with other components in water to form colored precipitates that stain pool walls and bottom or produce colored water.

chemical feeder Any of several types of devices that dispense chemicals into the pool or spa water at a predetermined rate. Some dispense chlorine or bromine while other dispense pH-adjusting chemicals.

chlorinator A mechanical or electrical device for dispensing chlorine at a controlled rate. Most often a canister or floater filled with tablets of chlorine.

chlorine A term used to describe any type of chlorine compound used as a disinfectant in swimming pool and spa water or to kill, destroy or control bacteria and algae. In addition, chlorine oxidizes ammonia and nitrogen compounds (swimmer and bather waste).

chlorine demand The amount of chlorine necessary to oxidize all organic matter (bacteria, algae, chloramines, ammonia and nitrogen compounds) in the pool or spa water.

chlorine enhancer A chemical compound that when used in conjunction with chlorine makes the chlorine perform better as an algaecide.

chlorine generator An electrical device that generates chlorine from a salt solution in a tank or from salt added to the pool water.

chlorine neutralizer A chemical solution used to make chlorine harmless. Used in test kits to counteract the bleaching effect of the chlorine or bromine in order to increase the accuracy of pool water tests.

chlorine residual The amount of chlorine left in the pool or spa water after the chlorine demand has been satisfied.

clarity The degree of transparency of the water.

cloudy water The visible effect in water created by material in suspension. Also called turbidity. A condition of the water due to the presence of extremely fine particles in suspension that can not be trapped by the filter because they are too small. Adding a clarifier such as an organic polymer or alum will coagulate the particles and make the filter more efficient.

color comparator A device used for water tests such as chlorine or pH. People can determine concentrations of constituents by visually comparing a permanent standard to the color produced when a reagent is added to the test sample water.

coping The cap or top lip on the pool or spa wall that provides a finishing edge around the pool or spa. It can be formed, cast in place or precast, or pre-fabricated of extruded aluminum or rigid vinyl. It may also be part of the system that secures a vinyl liner to the top of the pool wall.

corrosion The etching, pitting or eating away of the pool or spa vessel or equipment. Caused by improper water balance, misuse of acid or acidic products or from “soft” water.

coupling A plumbing fitting that is used to connect to pieces of pipe.

D. E. filteralso called a diatomaceous earth filter – A filter designed to use diatomaceous earth (D.E.) as the filter medium. The D.E. is added through the skimmer with the pump on which takes the D.E. and deposits it on a grid. The D.E. then becomes the filter medium.

dead spot Usually refers to a spot or location in a pool or spa where water circulation is poor or non-existent.

dechlor or dechlorinatealso called chlorine neutralizer The process of adding a chemical to the water to remove the residual chlorine. Chemicals used for this are sodium thiosulfate, sodium bisulfite, sodium metabisulfite, sodium sulfite, hydrogen peroxide and Vitamin C.

diatomaceous earth also called D.E.– A white powder composed of fossilized skeletons of one-celled organisms called diatoms. The skeletons are porous and have microscopic spaces. The powder is added through the skimmer and deposits itself on a grid. The powder then becomes the filter medium.

diffuser A porous plate, tube or other device through which air is forced and divided into minute bubbles for diffusion in the water. A diffuser can also be an overdrain on a sand filter. A diffuser is also used on a closed-face impeller on a pump to concentrate water flow to the center of the impeller.

discolored water Usually refers to water that has taken on a color from dissolved minerals, metals or from algae. Iron in water will turn water red or brown. Copper in water will turn water green or turquoise. Algae can make water look green, brown, blue or black.

distilled water Water formed by the condensation of steam or water vapor.

diverter valve A plumbing fitting used to change the direction or re-direct the flow of water. Some diverter valves are used on pool/spa combination to allow the use of the spa and then switch the flow back to the pool. A brand name diverter valve is called an Ortega valve which is sometimes used to describe a diverter valve.

dynamic head The resistance caused by the friction of water flowing through an entire circulation system, including the plumbing and equipment and the head pressure measured from the suction and discharge side.

effluent The water that flows out of a pump, filter or heater, usually on its way back to the pool or spa.

elbowalso called an ell – A plumbing fitting shaped at a 90 ° or a 45 ° angle usually made of metal, PVC or some other plastic.

end point The stage in a water analysis test called a titration at which equivalence is attained and revealed by a change that can be observed or measured, such as color development or formation of a precipitate. The color change when performing a water hardness test is an example.

enzyme A very large class of protein substances that are produced by livings cells and are essential to life by acting as a catalyst in metabolism. In the pool and spa industry they are used to digest or eliminate oils, soap scum and other organic matter from the water and from filters.

erosion feeder An in-line or off-line chemical feed device (also called a chlorinator or brominator) for holding, dissolving and dispensing tablets of chlorine or bromine.

eye irritation Caused by too low or too high pH, too low or too high alkalinity, or a combined chlorine level greater than 0.2 ppm. It is not usually caused by too high of a chlorine level unless the level is greater than 10.0 ppm.

eye protection Glasses, goggles or face shields used to protect the eyes from dust, debris or vapors from hazardous or irritating chemicals.

fiberglass Finespun filaments of glass which are available in a rope or mat form. When used in a process with polyester resins, catalysts and hardeners, can be formed or molded into pools, spas and related shapes.

fill wateralso called makeup water – Water added to a pool or spa to replace water lost due to evaporation, splash out or drag out.

filter A device that removes undissolved or suspended particles from water by recirculating the water through a porous substance (a filter medium or element). The three types of filters used in pools and spas are sand, cartridge and D.E. (diatomaceous earth).

filter aid A chemical compound added to the water or added to the filter that allows the existing filter to become more efficient. Examples are alum, water clarifiers and D.E. (diatomaceous earth).

filter area The total surface area of the filter medium that is exposed to the flow of water from the pump expressed in square feet. Examples are: 36 sq. ft. (also 36 ft 2) D.E. filter, 100 sq. ft. (also 100 ft 2) cartridge filter.

filter cartridge A replaceable porous element made of paper or polyester used as the filter medium in cartridge filters.

filter cleaner A chemical used to aid in cleaning of the filter element.

filter cycle The operating time between cleaning or backwashing cycles of a filter. Also, the amount of time the filter has water flowing through it each day expressed in hours.

filter element A device within a filter tank designed to trap suspended solids as water from the pool or spa is flowing through it.

filter medium In essence, the material used in the filter to trap suspended dirt particles as the water is flowing through it. The polyester or paper used in making a cartridge filter element. The sand used in a sand filter. The D.E. (diatomaceous earth) used in a D.E. filter.

filter powder Common name for diatomaceous earth (D.E.) used as the filter medium in a diatomaceous earth filter.

filter rock Graded, rounded rock and/or gravel used to support the filter medium. Usually used with sand filters.

filter sand A type of filter media composed of hard, sharp silica, quartz or similar particles with proper grading for size and uniformity. The most common grade in pool and spa filters is #20.

filter septum That portion of the filter element consisting of cloth, wire screen or other porous material on which the filter medium or filter aid is deposited. The nylon grid on a D.E. filter is the septum.

filtration rate The rate at which the water is traveling through the filter expressed in U.S. gallons per minute (gpm) per square foot of filter area.

floater A chemical feeder whereby the chemical is placed in a container and the container is allowed to float around the pool dispensing the chemical. The chemical dissolves in the floater and is released into the water. Most often these are used for chlorine or bromine.

float valve A valve controlled by the liquid level of a fluid. Most toilets have float valves.

flow The rate of movement of water in gallons per minute (GPM).

flow meter A device for measuring the rate of water passing through a given pipe in gallons per minute (GPM).

flow rate The quantity of water flowing past a designated point within a specified time, such as the number of gallons flowing past a point in one minute – abbreviated as gpm.

foam A froth of bubbles on the surface of the water. Usually comes from soap, oil, deodorant, hair spray, sun tan oil, etc. that is shed into the water as swimmers enter.

free available chlorine (FAC) The amount of free chlorine in the pool or spa water that is available to sanitize or disinfect the water. Sometimes called residual or available chlorine.

freeboard The vertical distance between the top of a filter media and the overflow or collector. This space allows for bed expansion during backwashing.

free chlorine The amount of free chlorine in the pool or spa water that is available to sanitize or disinfect the water. Sometimes called residual or available chlorine.

gas chlorine A chemical element used as a sanitizer and disinfectant in swimming pool water. The greenish gas is compressed and becomes a liquid under pressure. It is 2 ½ times heavier than air and is highly toxic to humans if released.

gate valve A type of valve in which the closing element (gate) is a disc that moves across the stream, often in a groove or slot for support against pressure.

gazebo A free-standing, traditional garden structure usually made of wood and having a solid or open roof and open, latticed sides.

gelcoat A colored, polyester-resin material applied to the surface of a molded part. The gelcoat hardens to a smooth, durable form and becomes an integral part of the laminate. Fiberglass pools and spas have gelcoat finishes.

geothermal Having to do with the heat of the interior of the earth.

GFCI (Ground Fault Circuit Interrupter) A mechanism that cuts the current to an electrical device the moment a change in voltage occurs.

Giardiasis Infestation with or disease caused by Giardia lamblia.

GPD Abbreviation for gallons per day.

GPH Abbreviation for gallons per hour.

GPM Abbreviation for gallons per minute.

grab rail also handrail – A tubular steel or plastic device that can be gripped by swimmers or bathers for the purpose of steadying themselves. Usually located near the steps in the pool.

grain A unit of weight equal to 0.0648 grams, 0.000143 pounds, or 1/7,000 th of a pound. Water and total hardness is often expressed as grains per gallon.

grain per gallon Common basis of reporting water analysis in the U.S. One grain per gallon equals 17.1 ppm (parts per million) or 17.1 mg/L (milligrams per liter).

green hair Caused by too much copper in the pool water. Blue fingernails are not caused by chlorine. The copper may get into the water by the bad practice of placing trichlor tabs in the skimmer. This acidic product will cause low-pH water which will in turn dissolve metals in the equipment. The dissolved metal (usually copper) then stains hair, fingernails and eventually pool walls. It can also be caused by keeping the pH too low or misusing acid.

grid Frame covered with fabric used as a filter media. Also called the septa or element.

gunite A mixture of cement and sand sprayed onto contoured and supported surfaces to build a pool. Gunite is mixed and pumped to the site dry and water is added at point of application. Plaster is usually applied over the gunite.

gutter An overflow trough at the edge of the pool through which floating debris, oil and other “lighter-than-water” things flow. Pools with gutters usually do not have skimmers.

hair & lint strainer Usually a basket before the pump that is design to catch and prevent hair, lint and other small material from entering the pump where it may damage the impeller.

halogens The chemical elements either individually or collectively that constitute Group VIIA of the periodic table: namely, fluorine, chlorine, bromine iodine and astatine. Of these, only chlorine and bromine are used as disinfectant and sanitizers in pool and spas.

hand rail A tubular steel or plastic device that can be gripped by swimmers or bathers for the purpose of steadying themselves. Usually located near the steps in the pool.

hand skimmer A screen attached to a frame that is then attached to a telepole used to remove large floating debris such as leaves and bugs from the water’s surface.

hard water Drinking water term that indicates water that contains one grain per gallon (17.1 ppm or mg/L) or higher of total hardness. The ideal range of hardness for pools and spas is 200 to 400 ppm.

hardness The amount of calcium and magnesium dissolved in the water. “Water” or “total” hardness refers to the total of the magnesium and calcium dissolved in the water. Calcium hardness refers to just the calcium. Measured by a test kit and expressed as ppm. Proper range is 200 – 400 ppm.

hardness increaser A chemical used to increase calcium or water hardness. It is usually calcium chloride.

head A measure of the pressure at a given point in a water system, expressed as pounds per square inch (psi), or the height of a column of water that would produce the pressure.

header A manifold in the heater that directs the flow of water into and out of the heat exchanger.

heater A fossil-fueled, electric or solar device used to heat the water of a pool, spa or hot tub.

heat exchanger A device located inside the heater providing for the transfer of heat from the heat source to the water. This is usually a series of metallic tubes with fins located just above the flames.

heat pump A refrigeration compressor, usually electrically driven, that is operated in reverse. To obtain heat, the evaporator side (cooling coil) is exposed to water, air or ground. The coil transfers the heat from this source to the condenser coil, where it discharges the heat to the water in the pool or spa.

heavy metal A metallic element with a specific gravity greater than 5, such as copper, iron, manganese, silver and zinc.

herbicide A chemical compound used to kill or control plant growth or algae.

high-rate sand filter A filter using sand for the filtration media designed for flows in excess of 5 GPM but less than 20 GPM per square foot.

horsepower Work done per unit of time. One horsepower equals 33,000 foot pounds of work per minute or approximately 746 watts. Motors for pumps are rated in horsepower.

hot tub A spa constructed of wood with sides and bottom formed separately and joined together by hoops, bands or rods.

hydrochloric acid (HCl) – also called muriatic acid – A very strong acid used in pools to control pH and total alkalinity. It can also be used for various cleaning needs. Used in “acid washing” a pool. Use extreme care in handling.

hydrogen The lightest chemical element. A component of water and a frequent product of many chemical reactions. pH is a measure of hydrogen in its ionic form in water.

hydrogen ion The positively charged nucleus of a hydrogen atom. The relative degree of acid or base of a solution (called pH) is a measure of hydrogen ions.

hydrogen peroxide An unstable, colorless heavy liquid used as a bleach in industry and as an antiseptic in households. It is used as an oxidizing agent in pools and spas. May also be used to dechlorinate pool or spa water.

hydrojet A fitting in the pool or spa on the water return line from the equipment that blends or mixes air and water creating a high velocity, turbulent stream of air-enriched water.

hydrostatic pressure The force created by the water that tends to push pools up out of the ground.

hydrostatic valve A check valve located in the main drain of a pool to relieve hydrostatic pressure created by rising ground water. The valve allows ground water into the pool, but does not let water out.

hydroxide The ion formed by an oxygen and hydrogen atom. The term used to describe the anionic radical () that is responsible for the alkalinity of a solution. Hydroxides include sodium hydroxide (NaOH).

hydroxyl The univalent group or radical consisting of one atom of oxygen and one atom of hydrogen.

hypobromite ion The ionized form of hypobromous acid .

hypobromous acid The most powerful disinfecting form of bromine in water. Sometimes called the killing form of bromine.

hypochlorite Name given to a family of chlorine-containing compounds including calcium hypochlorite, sodium hypochlorite and lithium hypochlorite that are used as disinfectants and sanitizers in pool and spa water.

hypochlorite ion An ionic form of chlorine in water. Hypochlorite can kill but is 30 to 60 times less effective than hypochlorous acid depending on the organism or chemical being oxidized. The ratio of HOCl to OCl in water is determined by the pH. The lower the pH, the more HOCl.

hypochlorous acid The most powerful disinfecting form of chlorine in water. Sometimes called the killing form of chlorine. The ratio of HOCl to OCl in water is determined by the pH. The lower the pH, the more HOCl.

ideal level or range The universally accepted ranges for the chemical parameters for swimming pool or spa water. For instance, the ideal pH range is 7.4 to 7.6; the ideal chlorine level is 1.5 to 3.0 ppm; the ideal total alkalinity is 80 to 140 ppm depending on the sanitizer being used; the ideal hardness level is 200 to 400 ppm and so on.

Impeller The rotating member of a pump. The part of the pump that moves the water.

indicator (1) A substance that gives visible change, usually of color, at a desired point in a chemical reaction. (2) A device that indicates the result of a measurement. (3) A reagent in a test kit.

influent The water entering the pump, the filter or other equipment or space. Water going into the pump is called the influent, while water leaving the pump is called the effluent.

inlet A fitting in the pool or spa on the water return line from the equipment that water returns to the pool. Usually the last thing on the return line.

ion An atom or group of atoms (a radical) that has lost or gained one or more electrons, and such possess an electrical charge. Positively charged ions are cations and negatively charged ions are anions.

ionizer A water-sanitation device that uses electricity to generate metal ions which are dispersed in the water. It works by passing a low-voltage, DC current through a set of metallic (usually copper and silver) electrodes placed in line with the circulation equipment. The copper is an algaecide while the silver is a bactericide. Does not remove swimmer waste.

iron Iron in water causes the water to be brown or green colored. Can be controlled by addition of a sequestering agent or a chelating agent. Water can be tested with an iron test kit. Also see ferrous and ferric iron.

Jacuzzi A brand name of a spa or whirlpool. The term has been used so frequently to describe a generic spa that Jacuzzi has come to mean spa – just like Kleenex, Xerox or Scotch Tape.

jet also called hydrojet or hydrotherapy jet – A fitting in the pool or spa on the water return line from the equipment that blends or mixes air and water creating a high velocity, turbulent stream of air-enriched water.

k ilowatt (kW) A unit of electrical energy measurement equal to 1000 watts. One kilowatt equals 1,000 joules per second, 1.341 horsepower, or 238.9 calories per second. One kilowatt also equals 3413 BTU/hour.

kilowatt hour A standard unit of energy measurement (electrical) equal to 1,000 watt hours or 3,600,000 joules. Also expressed as the amount of electrical energy needed to burn a 100 watt light bulb for one hour.

ladder A structure for climbing up or down; consists of two parallel sides joined by a series of crosspieces that serve as footrests. It is used for getting in and out of the pool. A double-access ladder straddles the pool wall of an above-ground pool. An in-pool ladder is located in the pool only.

Langelier Index A means of expressing the degree of saturation of a water as related to calcium carbonate solubility. A calculated number used to predict whether or not a water will precipitate, be in equilibrium with, or dissolve calcium carbonate.

laterals The horizontal filter grids at the bottom of a sand filter installed in the underdrain.

leaf bagger A device that attaches to a telepole and a garden hose. Pressure from the garden hose creates a suction by which leaves and large debris are sucked into a large mesh bag.

lime A calcined chemical material with the formula CaO. Lime is used in lime softening and in lime-soda ash drinking water treatment. It is not used in pool or spa water treatment.

lineralso vinyl liner – The vinyl membrane that acts as the container to hold or contain the water in a swimming pool.

lint strainer A device attached to the front of a pump to which the influent water line (suction line) is connected, used to trap lint, hair and other debris that could damage the pump impeller or volute.

liquid acidalso called muriatic acid – Hydrochloric acid that is 31.45%. (Concentrated HCl is 37%). It is used for controlling pH and total alkalinity in pool and spa water, and for various cleaning needs. It is also used for acid washing.

liquid chlorine Sodium hypochlorite solution, chemical formula usually provides 10-12% available chlorine; has a pH of 13 and requires that small amounts of acid be added to the pool to neutralize the high pH. Good for regular chlorination and superchlorination. It is the same chemical used for household or laundry bleach except that laundry bleach is about 5.6% available chlorine. One gallon of liquid chlorine 12.5% is equal to about one pound of gas chlorine.

light niche The area in a pool or spa that houses the underwater light.

lithium hypochlorite A dry, granular chlorinating compound with an available chlorine content of 35%. It is rapid dissolving and should be used to superchlorinate vinyl-liner pools, painted pools or fiberglass pools as well as spas and hot tubs.

lye See sodium hydroxide.

magnesium hardness A measure of the amount of magnesium dissolved in the water. It is part of total or water hardness. It also causes scale if levels are too high. In tap water, magnesium hardness is about 25% of the total hardness and calcium is about 75%.

main drain Usually refers to a plumbing fitting installed on the suction side of the pump in pools, spas and hot tubs. Sometimes called the drain and is located in the deepest part of the pool, spa or hot tub. It is not a drain such as a drain on a kitchen sink. Main drains do not allow the water to drain to waste but rather connect to the pump for circulation and filtration.

make-up water Sometimes called “tap” or “refill” water. It is the water used to replace water lost to evaporation, splash out, leaks or swimmer drag out in the pool.

manganese (Mn) An element on the Periodic Table. Symbol Mn. Sometimes found in ground or surface water usually in combination with iron. Manganese is noticeable because it causes black staining when concentration is greater than 0.05 ppm.

manifold The branch pipe arrangement that connects several input pipes into one chamber or one chamber into several output pipes. A filter manifold connects several input pipes from the filter septa back into one common pipe.

Marcite Originally a brand name for a white plaster finish coat from 1/8 th to ½ inch thick applied over the gunite or shotcrete.

mechanical seal A mechanical device or assembly that forms a seal between a pump casing and a rotating shaft to prevent leakage along the shaft.

media A selected group of materials used in filters and filter devices to form barriers to the passage of certain solids or molecules that are suspended or dissolved in water. Media is the plural form of medium and refers to more than one type of barrier material. In the pool and spa industry, filters use either sand, polyester or diatomaceous earth (D.E.) as the media.

metal An element that forms positive ions when its compounds are in solution and for which the oxides form hydroxides rather than acids with water. Most metals are crystalline solids with metallic luster, are conductors of electricity, and have rather high chemical activity. Some metals are quite toxic.

metal components Plumbing, fixtures or equipment that are made of or have metal in them. For instance, a “plastic” pump may have a brass impeller. The brass impeller is a metal component.

mg/L The abbreviation for milligrams per liter. Generally considered as equal to ppm (parts per million), but not exactly. Multiply mg/L times 1.001142303 to get ppm. Multiply ppm times 0.9988590 to get mg/L.

microbe Minute plant or life.

micrometer Same as micron .

micron A unit of length equal to one millionth of a meter — it is .0000394 inch. Microns are used to describe the pore size of filter media. Sand filters have openings of 25-30 microns; cartridge filters have openings of 8-10 microns; and D.E. (diatomaceous earth) filters have openings of 1-3 microns. Humans, without magnification, can see objects 35 microns or larger. A
granule of table salt is between 90 and 110 microns. Abbreviated as m m.

microorganism Microscopic organisms, either plant or animal, invisible to or barely visible to the naked eye. Examples are bacteria, algae, fungi and virus.

milligram per liter Abbreviated as “mg/L” and generally considered as equal to ppm (parts per million) – but not exactly. Multiply mg/L times 1.001142303 to get ppm. Multiply ppm times 0.9988590 to get mg/L.

millivolt A unit of electrical measurement equal to 1/1000 th of a volt. Abbreviated as mV.

mineral Any substance that is neither animal nor vegetable. It is any class of substances occurring in nature, usually comprising inorganic substances, such as quartz or feldspar, of definite chemical composition and definite crystal structure. It sometimes includes rocks formed by these substances. Ground water dissolves these rock substances and the dissolved
minerals are present in tap water. Depending on the kinds of rocks the water comes in contact with, the minerals dissolved in the water may be just a few or they may be many. Water hardness is mostly comprised of these minerals.

molecule A chemical unit composed of one or more atoms. The smallest unit of matter which can exist by itself and retain all of its chemical properties. A molecule of water contains two hydrogen atoms and one oxygen atom

monopersulfateSee potassium peroxymonosulfate.

MPS Abbreviation for monopersulfate.

MSDS Abbreviation for Material Safety Data Sheet. Companies that make and distribute hazardous substances have to provide an MSDS on each substance and mixtures of hazardous substances. That is because you will have one place to look for everything you need to know about a substance’s or chemical’s hazards. It is part of the Worker Right To Know Law.

multiport valvealso called a rotary-type backwash valve – This valve replaces as many as six regular gate valves. Water from the pump can be diverted for various functions by merely turning the valve. The water may be sent to waste, used for backwashing, bypassing the filter for maximum circulation, normal filtration, filter to waste (rinse) or the valve may be
closed to pass no water.

muriatic acid 31.45% hydrochloric acid, chemical formula HCl. An acid used to reduce the pH and alkalinity levels in pool water. It is also used in acid washing, a process that removes stains and scale from pool plaster.

mustard algae A particularly hardy form of algae that is yellow colored. See algae.

N-bromo compound Any number of compounds formed in pool and spa water by the combining of bromine with nitrogen or ammonia compounds, complexing of bromine with nitrogen or ammonia, or partial oxidation of ammonia and nitrogen-containing compounds by bromine.

N-chloro compound Any number of compounds formed in pool and spa water by the combining of chlorine with nitrogen or ammonia compounds, complexing of chlorine with nitrogen or ammonia, or partial oxidation of ammonia and nitrogen-containing compounds by chlorine.

needle valve A valve with a circular outlet through which the flow is controlled by means of a tapered needle that extends through the outlet, reducing the area of the outlet as it advances and enlarging the area as it retreats.

neutralizer A chemical solution used to make chlorine or bromine harmless. Used in test kits to counteract the bleaching effect of the chlorine or bromine in order to increase the accuracy of pool water tests. Sold as chlorine and bromine neutralizer, it is used to destroy excessive amounts of chlorine or bromine so the high levels will not affect swimmers. The most common ones are sodium thiosulfate, sodium sulfite Leslie’s Chlor Neutralizer), sodium metabisulfite, hydrogen peroxide, and vitamin C.

niche The housing built into the wall of a pool or spa to accommodate a light fixture.

nitrates An oxidized ion of nitrogen. The presence of nitrates indicates the final stages of the end of the nitrogen cycle. Nitrates can be checked with a nitrite or nitrate test kit. The level should not exceed 10 ppm. Nitrate causes algae to bloom and disables free available chlorine. Nitrites are often reduced to nitrates in pool water by bacteria and algae. Humans, rain bringing in smog and NOx, bird droppings and fertilizer run off are the most common sources. They are not oxidizable and are not destroyed by Superchlorination or non-chlorine oxidizers. Draining and refilling is the only practical way to lower them. Keeping free chlorine in the water at all times destroys the precursors to nitrate formation.

nitrites Nitrites can become nitrates in pool and spa water. They disable chlorine and can cause algae to bloom. The level can be checked with a nitrate/nitrite test kit or strip. Nitrites can be oxidized by Superchlorination.

nitrogen A gaseous element (molecular formula N 2) that constitutes 78 percent of our atmosphere. Causes algae to bloom and disables chlorine. It is brought into the water each time it rains. Maintaining proper chlorine levels will prevent nitrogen from becoming a problem. Superchlorination will remove nitrogen and its related compounds.

non-chlorine shock A term given to a class of chemical compounds that are used to oxidize or shock the water (destroy ammonia, nitrogen and swimmer waste). They contain no chlorine or bromine and do not kill living organisms. Swimmers may re-enter the water in only 15 minutes after adding a non-chlorine shock. Examples are monopersulfates, hydrogen peroxide and percarbonates.

NSPI Abbreviation for the National Spa & Pool Institute.

NTU Abbreviation for Nephelometric Turbidity Units. A unit of measurement to determine a water’s clarity.

oocyst The oocyst is usually the infectious and environmental stage, and it contains sporozoites. See also cryptosporidium.

organic Refers to volatile, combustible and sometimes biodegradable chemical compounds containing carbon atoms bonded together with other elements. The principal groups of organic substances found in water are proteins, carbohydrates, fats and oils. See also organic waste.

organic chloramines Chloramines formed from the combination of chlorine and organic ammonia or nitrogen (swimmer waste, perspiration, urine, sweat, saliva, etc.) these are not considered disinfectants like inorganic chloramines are.

organic wastealso called swimmer or bather waste – All of the soap, deodorant, sun tan lotion, lipstick, makeup, cologne, body oils, sweat, spit, urine, etc. brought into the water. They also form chloramines which are foul-smelling and they are body irritants. Requires large amounts of chlorine or non-chlorine shock to destroy.

organochlorine compound Any organic compound containing chlorine as a constituent. Some organochlorine compounds are formed when chlorine or free chlorine reacts with organic substances. Others are synthetic organic compounds or chlorine-substituted by-products.

orifice An opening (hole) in a plate, wall, or partition.

orifice plate An orifice plate or flange placed in a pipe that consists of a slot or a calibrated circular hole smaller than the pipe diameter. The difference in pressure between a point in the pipe upstream and a point at the orifice may be used to determine the flow in the pipe.

O-ring A round, circular rubber-material gasket shaped like the letter O that is used to seal around round objects, such as the ends of two pipes.

ORP Abbreviation for Oxidation Reduction Potential. It is a measurement of a body of water’s ability to oxidize contaminants. Measured with an electrode and an electronic meter. It is an indication of the sanitizing level or degree of safety from disease in the water. Measured in millivolts with the accepted minimum level being 650 mV (millivolt).

orthotolidine A chemical reagent used to test the total chlorine level in pool and spa water. It does not measure free available chlorine. See DPD.

OSHA The Occupational Safety and Health Administration, A federal agency in the Department of Labor that administers safety and health regulations and their enforcement.

osmosis The natural tendency for water molecules to pass through a semipermeable membrane, from the side low in dissolved impurities to the side high in dissolved impurities.

OTO Abbreviation for orthotolidine. A chemical reagent used to test the total chlorine level in pool and spa water. It does not measure free available chlorine. See DPD.

outlet The downstream opening or discharge end of a pipe. A discharge opening for water from the water distributing system to a fixture or a water operated device.

over-acid An incorrect term used to describe water that is acidic or water that has a pH lower than 7.4.

overdrainalso known as a diffuser or distributor – An internal sand filter device that evenly distributes influent pool water over the sand filter bed.

overflow system A system for the removal of pool surface water through the use of overflows, surface skimmers, and surface water collection systems.

oxidation To rid the water of ammonia, nitrogen compounds and swimmer waste (organic compounds). These organic compounds disable chlorine, are body irritants and have a foul smell. Removal is accomplished by superchlorination or by shock treating with a non-chlorine oxidizer.

oxidizer A non-chlorine shocking compound that removes or destroys built-up contaminants and chloramines in pool water without raising chlorine levels as required when “superchlorinating.”

ozone A gaseous molecule comprised of three atoms of oxygen. It is generated on site from air or oxygen and used for oxidation of water contaminants.

ozonator Electrical devices which produce ozone from air or oxygen used for oxidation of water contaminants. Corona Discharge or CD ozone is produced by a discharge of electricity at ranges from 1 to 7 percent ozone in air. Ultraviolet or UV ozone is produced by a UV light or lamp at ranges from .001 to .01 percent.

part per million Abbreviated as ppm. It is a “weight-to-weight” expression. It means one part in one million parts, such as, 1 pound of chlorine in one million pounds of water or 119,904 gallons. Many of the common pool water tests as well as acceptable ranges are stated as ppm. For instance, Free Available Chlorine should be kept between 1.0 and 3.0 ppm; Total Alkalinity should be between 80 and 120 ppm; and Water Hardness should be between 200 and 400 ppm. A ppm and a milligram per liter (mg/L) are considered equal.

pathogenic organism An organism that causes disease.

perimeter The distance around the outer edge of a shape.

peristaltic pump A positive-displacement pump using a rotary head that squeezes a flexible tube to deliver the pumped liquid.

permeability A measure of the relative ease with which water flows through a porous material.

peroxide Any compound containing the two oxygen atoms united together into a bivalent– O-O group. Readily releases abnormally active atomic oxygen, and therefore any peroxide is a strong oxidizer. Examples are hydrogen peroxide and potassium peroxymonosulfate

pesticide Any chemical or compound that kills insects, weeds, rodents, or any other thing considered a pest. In pools that is bacteria, virus, algae and the like. All pesticides must be registered by the U.S. EPA.

petcock A small, manually-operated faucet or valve for draining off liquids or releasing air pressure. The air-relief valve on a filter is an example.

pH A term used to indicate the level of acidity or alkalinity of pool water. Too low pH causes etched plaster, metal corrosion and eye irritation. Too high pH causes scale formation, poor chlorine efficiency and eye irritation. Proper range for pH in swimming pools is 7.4 to 7.6. It is always written with a lower case “p” and a capital “H.”

pH decreaser A chemical that decreases or lowers the pH of the water. Muriatic acid, dry acid and sodium bisulfate are examples.

pH increaser A chemical that increases the pH of the water. Soda ash, sodium sesquicarbonate and sodium bicarbonate are examples of pH increasers.

pH meter A sensitive volt meter used to measure pH via an electrode placed in the water.

phenol red A chemical reagent dye used to test for pH. It can measure pH from 6.8 to 8.4.

phosphate An essential growth constituent of organisms and can be a nutrient that limits the primary productivity of a body of water. Excess phosphates or phosphorus in water can cause excessive algal growth stimulation to sometimes nuisance quantities. Phosphate in water is measured as phosphate ion.

plaster A mixture of white cement and white marble dust used as an interior finish which can be tinted, colored or left white, applied to the gunite or shotcrete of a pool or spa.

pollutant Any contaminant high enough to endanger the aquatic environment or the public health.

polymer A substance made of giant molecules formed by the union of simpler molecules. Many water clarifiers are made from organic polymers. An example would be polymerized ethylene is called polyethylene.

poly vinyl chloride (PVC) An artificial polymer made from vinyl chloride monomer and frequently used in pipes, sheets and vessels for transport, containment, water treatment facilities, pools and spas.

potable water Any water, such as an approved domestic water supply, which is bacteriologically safe, non-pathogenic and otherwise suitable for human consumption.

potassium peroxymonosulfate Active ingredient and chemical name of a non-chlorine shock treatment or non-chlorine oxidizer. Does not kill bacteria or algae but it will oxidize or destroy ammonia, nitrogen and swimmer waste. It has a low pH and it does not increase chlorine or bromine levels the way that superchlorination does, so water may be entered in 15 minutes after addition. Also, it will reactivate bromine to its killing form, hypobromous acid.

ppm Abbreviation for parts per million. It is a weight to weight expression. It means one part in one million parts, such as, 1 pound of chlorine in one million pounds of water or 119,904 gallons. Many of the common pool water tests as well as acceptable ranges are stated as ppm. For instance, Free Available Chlorine should be kept between 1.0 and 3.0 ppm; Total Alkalinity should be between 80 and 120 ppm; and Water Hardness should be between 200 and 400 ppm.

precipitate A substance separating, in solid particles, from a liquid as a result of a chemical or physical change. It also means to form a precipitate.

precoat Refers to the depositing of diatomaceous earth (D.E.) onto the filter grids or elements. A very fine granular filter medium, such as D.E., applied (usually by slurry) to a retaining membrane or fabric surface prior to a service run.

pre-dilute To dilute a chemical before adding it to the pool or spa water.

pressure The force pushing on a unit area. Water pressure is normally measured in pounds per square inch (psi) or feet of head.

pressure gauge A gauge with an analog dial indicating the pounds per square inch (PSI) of pressure that has built up within a closed container such as a filter.

prime To fill a pump casing or siphon with water to remove air. Most pumps must be primed before start-up or they will not pump water.

protoplasm Cell material inside the cell membrane.

protozoa Small, one-celled organisms, including amoebae, ciliates and flagellates.

pseudomonas aeruginosa A gram-negative, aerobic, motile bacterial species. This organism is found in a variety of environments, generally aquatic, and is pathogenic (disease-causing) for humans. It has been associated with infections of wounds, burns, middle-ear infections, pneumonia, meningitis, gastrointestinal infections and systemic infections. It also causes rash.

PSI An abbreviation for pounds per square inch.

pump A mechanical device, usually powered by an electric motor, which causes hydraulic flow and pressure for the purpose of filtration, heating and circulation of pool and spa water. Typically, a centrifugal pump is used for pools, spas and hot tubs.

pump capacity The volume of liquid a pump is capable of moving during a specified period of time. This is usually gallons per minute (gpm).

pump curvealso called a pump performance curve – A graph that represents a pump’s water flow capacity at any given resistance.

pump strainer basket A device placed on the suction side of the pump, which contains a removable strainer basket designed to trap debris in the waterflow without causing much flow restriction. Sometimes called a “hair and lint trap.”

PVC Abbreviation for Poly Vinyl Chloride. An artificial polymer made from vinyl chloride monomer and frequently used in pipes, sheets and vessels for transport, containment, water treatment facilities, pools and spas.

PVC pipe A type of pressure pipe for water distribution manufactured from poly vinyl chloride compound. Many pools and spas are plumbed with this type of pipe.

quat See quaternary ammonium compounds.

quaternary ammonium compoundsalso called quats – A type of organic compound in which the molecular structure includes a central nitrogen atom joined by four organic groups as well as to an acid radical. Cationic quaternary ammonium compounds adsorb on the cell walls of microbes and algae and react chemically with the negative charges carried by the cell walls. These chemical compounds of ammonia are used as algaecides and algaestats.

radius The distance from the center of a circle to its edge; one half of the diameter.

rate of flow The quantity of water flowing past a designated point within a specified time, such as the number of gallons flowing past a point in one minute – abbreviated as gpm.

reagents The chemical agents, dyes, indicators or titrants used in testing various aspects of water quality.

rebar An abbreviation for reinforcing bar. Steel rods laid in concrete to add strength.

red water Rust-colored water. Such color is usually due to the presence of precipitated ferric salts or to dead microorganisms that depend on iron and manganese.

redox See Oxidation Reduction Potential.

remote switch Any switching device that is used to activate equipment or apparatus from a distance. They can be wireless, wired or air operated.

residual bromine The amount of measurable bromine remaining after treating the water with bromine. The amount of bromine left in the pool or spa water after the bromine demand has been satisfied. Includes hypobromous acid (HOBr), hypobromite ion, free and total bromine and all combined bromine.

residual chlorine The amount of measurable chlorine remaining after treating the water with chlorine. The amount of chlorine left in the pool or spa water after the chlorine demand has been satisfied. Includes hypochlorous acid (HOCl), hypochlorite ion free and total chlorine and all combined chlorine.

return line Effluent piping.

rust A reddish corrosion product occasionally found in water. Rust is formed as a result of electrochemical interaction between iron and atmospheric oxygen in the presence of moisture. It is called ferric oxide.

salt generator An electrical device that generates chlorine from a salt (sodium chloride, NaCl) solution in a tank or from salt added to the pool water. Sometimes called a chlorine generator or electrolytic cell.

salt water Water containing relatively high concentration of salt, usually over 1,000 ppm or mg/L but less than the sea. The ocean has about 35,000 ppm of TDS.

sand Usually refers to the filter medium used by a sand filter. The grade most often specified by filter manufacturers is grade #20 with a particle size of 45 to 55 mm (millimeters).

sand filter A filter using sand or sand and gravel as the filter medium. The oldest (patented in 1790) method of improving water quality, generally using two layers of sand supported by an underbed layer of gravel.

sanitize To render sanitary; to kill all living things including bacteria and algae. Similar to sterilize.

saturationalso called the saturation point – The condition of the water when it has taken into solution the maximum possible quantity of a given substance at a given temperature, alkalinity and pH. Water hardness is calcium and magnesium dissolved in the water. When the water can hold no more of these elements, they deposit or precipitate out of the water causing scale. Other elements that can be saturated in pool and spa water are copper and iron. Usually, as the pH and alkalinity rise, the elements become less soluble causing them to come out of solution.

Saturation Index (SI) The relation of calcium carbonate to the pH, alkalinity and hardness of a given water to determine its scale forming tendency. Employs the use of factors and values for five constituents of the water (pH, alkalinity, hardness, TDS and temperature) substituted into a mathematical equation.

scale The precipitate that forms on surfaces in contact with water when the calcium hardness, pH or total alkalinity levels are too high. Results from chemically unbalanced pool and spa water. Scale may appear as grey, white, or dark streaks on the plaster, fiberglass or vinyl. It may also appear as a hard crust around the tile.

scale inhibitor A chemical added to the water that prevents scale from forming.

seal Packing gland material or a mechanical device that fits around a pump shaft and prevents either air intake to the pump or water leakage.

scum The extraneous or foreign matter which rises to the surface of the water and forms a layer or film there. It can also be a residue deposited on the tile or walls of the pool or spa. Sources of scum are soap, oil, deodorant, hair spray, sun tan lotion and various others.

sediment Solid material settled out from the water.

sediment trap A device for trapping sediment that is required on all gas heaters.

self-priming A classification given to centrifugal pumps that indicates the pump is capable of operating above the pool water level after initially filling with water. Most self-priming pumps have a hair and lint strainer basket attached to the front of the pump.

sensor An electrical or electronic device that measures water quality. Examples are pH electrodes, pH meters, ORP (Oxidation Reduction Potential) probes and meters, TDS meters and probes.

separation tank A container used in conjunction with a DE filter to trap DE and dirt when backwashing.

septum That portion of the filter element consisting of cloth, wire screen or other porous material on which the filter medium or filter aid is deposited. The nylon grid on a D.E. filter is the septum.

sequestering agentalso called chelating agent – A chemical that will combine with dissolved metals in the water to prevent the metals from coming out of solution (precipitating or causing stains). May also be a chemical that removes dissolved metals from water. Leslie’s Stain & Scale

shock treat The practice of adding significant amounts of an oxidizing chemical (usually non-chlorine oxidizers such as sodium persulfate or potassium peroxymonosulfate) to the water to destroy ammonia and nitrogen compounds or swimmer waste. Shock treat used to mean superchlorinating to 10-20 ppm of free chlorine.

shotcrete A mixture of sand and cement sprayed onto contoured and supported surfaces to build a pool or spa. Plaster is applied over the shotcrete. Shotcrete is premixed and pumped wet to the construction site.

silica As used in water chemistry, a collective term encompassing dissolved, undissolved, and colloidal silica. In undissolved form, it exists as minute particulate and as encapsulated silica. In dissolved form, it can appear as a silicate, silicon dioxide and as silicic acid

silicone Any of a large group of siloxane polymers used as caulking, adhesives, lubricants, protective coatings and many other uses.

silt Soil particles having diameters between 0.004 and 0.062 mm (millimeters). Sometimes they may be too small to be trapped by the circulation system. In those cases, a clarifier or an alum product may be needed.

silver algaecide Algaecides that have colloidal silver or silver ions as their active ingredient.

skimmer A device installed through the wall of a pool or spa that is connected to the suction line of the pump that draws water and floating debris in from the surface of the water. The skimmer basket catches large debris while the filter traps smaller particles.

skimmer basket A removable, slotted basket or strainer placed in the skimmer on the suction side of the pump, which is designed to trap floating debris in the water flow from the surface without causing much flow restriction.

skimmer weir Part of a skimmer that adjusts automatically to small changes in water level to assure a continuous flow of water to the skimmer. The small floating “door” on the side of the skimmer that faces the water over which water flows on its way to the skimmer.

slime Any of numerous substances of viscous organic nature that are usually formed from microbial growth and that attach themselves to other objects, forming a coating.

slurry Water or a liquid containing a high concentration of suspended solids. Diatomaceous earth (D.E.) is usually added to the filter as a slurry by mixing a small amount of D.E. in a bucket of water and then pouring the slurry into the skimmer with the filter on.

soap residue Residual soap left on our bodies from bathing or residual soap left in bathing suits or towels from laundering. This can accumulate in the water and cause foaming or a scum line to develop.

soda ashSee sodium carbonate.

sodium bicarbonate (baking soda or bicarb] Used to raise total alkalinity in pool and spa water with only a slight affect on the pH. (Leslie’s Alkalinity Up)

sodium bisulfate (dry acid] A chemical used to lower the pH and total alkalinity. 2 1/2 pounds of dry acid are equal to 1 quart of muriatic acid. (Leslie’s Dry Acid)

sodium borate See sodium tetraborate.

sodium bromide A salt of bromine, chemical formula NaBr. It is used to establish a bromide “bank” in pool and spa water prior to beginning the use of bromine tablets.

sodium carbonate (common name, soda ash) A chemical used to raise the pH and total alkalinity in pool and spa water. (Leslie’s Soda Ash)

sodium dichlor Common name for sodium dichloro-s-triazinetrione. A fast-dissolving, granular, stabilized organic chlorine compound providing either 56% or 63% available chlorine. Used for regular as well as superchlorination. Contains an ingredient (cyanuric acid or stabilizer) that prevents the chlorine from being destroyed by the ultraviolet (UV) rays of the sun.
(Leslie’s Chlor Brite®)

sodium hypochlorite Liquid chlorine, hypochlorite solution, chemical formula NaOCl; usually provides 10-12% available chlorine (or about 1 lb. of pure chlorine per gallon); has a pH of 13 and requires that small amounts of acid be added to the pool to neutralize the high pH. Good for regular chlorination and superchlorination. Not recommended for spas. Does not contain conditioner or stabilizer to protect it from sunlight but it is protected if stabilizer or conditioner is already in the water.

sodium sesquicarbonate A chemical mixture of equal parts of soda ash and sodium bicarbonate used to increase pH and total alkalinity in pool and spa water. It has a pH of 10.1.

sodium sulfite A chemical used to neutralize, de-chlorinate or debrominate pool and spa water.(Leslie’s Chlor Neutralizer)

sodium tetraborate pentahydrate White crystals or powder used to scavenge from water and as a buffer which supplies alkalinity to water so pH can be more easily maintained.

sodium thiosulfate A chemical used to neutralize, de-chlorinate or debrominate pool and spa water.

soft water Water that has a very low calcium and magnesium content (water hardness) – usually means less than 100 ppm or 6 grains. Also, water that has gone through a water softener. Pools and spa should never be filled with soft water from a softener. Water with less than 100 ppm of hardness should be increased to a minimum of 150 to 200 ppm using calcium chloride.

solar cover A cover that when placed on the water’s surface of a pool, spa or hot tub, increases the water temperature by absorption and transmission of solar radiation, reduces evaporation and prevents wind-borne debris from entering the water.

solar heating system Usually panels or coils of plastic or metal through which water passes to increase the temperature from the sun’s radiant heat.

solenoid valve An electrical device operated by a magnetic coil to make the valve either open for flow or closed to shut off water flow.

source wateralso called “tap” water – It is the water used to fill or refill the pool or spa.

spring boardalso called a “diving board” – A recreational mechanism for entering a swimming pool consisting of a semirigid board that derives its spring from a fulcrum mounted below the board and attached to the deck.

stabilized chlorine A family of chlorine pool sanitizers that contain conditioner (cyanuric acid or isocyanuric acid) to protect the chlorine from the degrading UV rays in sunlight. Most common types are sodium dichlor and trichlor (“dichlor” and “trichlor”). Granular form is dichlor which is fast dissolving and can be used for regular chlorination or superchlorination by broadcasting into the pool or spa. Tablet or stick form is trichlor which is usually used in a chlorine feeder either the floating type or the in-line erosion type used for regular chlorination only. (Leslie’s Chlor Brite®, Leslie’s 1″ Chlor Tabs and Leslie’s 3″ Jumbo Tabs)

stabilizeralso known as conditionerSee cyanuric acid.

stain A discoloration or a colored deposit on the walls or bottom of a swimming pool or spa. Most often stains are metallic oxides, hydroxides, carbonates, sulfates, silicates and phosphates of such metals as iron, copper and manganese. They may appear as green, gray, brown or black. They may even discolor the water. Sometimes a sequestering agent or chelating agent will remove them. If not, usually an acid wash is necessary to remove them from the walls and bottom. The metals get in the water because the pH was too low or someone has added a low-pH chemical directly into the circulation system. The low-pH chemical then dissolves a small amount of metal from the equipment. Then, under certain conditions, the metals begin to come out of solution and deposit or stain the walls and bottom. Stains are sometimes confused with scale which is a deposit of calcium or magnesium on the walls, bottom or in the equipment.

stain inhibitorSee sequestering agent.

static head Head loss (pressure drop) caused by the weight of a standing water column to be moved. This occurs on the suction and discharge side of the pump.

static suction lift The vertical distance between the imaginary center line of the pump impeller and the surface level of the water. This distance can be expressed as Positive Suction Head (pump below water level) or Negative Suction Head (pump above water level).

sterilize To destroy or kill by any of several means all living microorganisms in water.

strainer basket An easily removable perforated or otherwise porous container used to catch coarse material such as leaves, twigs, insects and other debris before it gets into the pump.

suds A froth of bubbles on the surface of the water. Usually comes from soap, oil, deodorant, hair spray, sun tan oil, etc. that is shed into the water as swimmers enter.

sump A tank or pit that receives drainage and stores it temporarily and from which the discharge is pumped or ejected.

superchlorination The practice of adding an extra large dose (5-10 ppm) of chlorine to the water to destroy ammonia, nitrogen and swimmer waste which can build up in the water. This level of chlorine is required to destroy all of the combined chlorine in the water, which is called break-point chlorination. Technically it requires 7.6 moles of chlorine for each mole of ammonia to reach breakpoint chlorination. If the ratio of 7.6:1 is not reached, ammonia destruction does not happen. Common practice in the pool and spa industry is to multiply the amount of combined chlorine times 10 and add that amount of chlorine to the water. This way you are sure the ratio has been met and there is some chlorine left over to provide a residual.

surface area The two-dimensional area of an object. The surface area of a pool is just the length times the width – no depth. The surface of the water. Also refers to the filter surface area.

surface tension The tendency of any liquid to pull in its open surface, so that surface is as small as possible. A function of cohesion. The attraction between molecules of a liquid that causes the surface to act as a thin elastic film under tension. Polar liquids including water have high surface tensions. Surface tension makes it possible to float a razor blade on the surface of a glass of water, even though the blade is much heavier than the water.

surfactant A soluble chemical compound that reduces the surface tension between two liquids. It is used in many detergents and soapy cleaning compounds.

surge tank A tank that a ccumulates water in response to a pressure surge and supplies water in instances of moderate negative gauge pressure, thereby preventing damage to pumping and piping equipment.

suspended solids See TDS.

swimout A recessed area outside the general perimeter of a pool. Swimouts are designed to allow entry and exit from the pool.

TAC Abbreviation for Total Available Chlorine. It is the total amount of chlorine in the water and includes both free available chlorine and combined available chlorine.

TDSalso called Total Dissolved Solids – A measure of the total amount of dissolved material in the water. It is comprised of the spent or carrier chemicals added every time chemicals are added as well as the hardness, alkalinity, chlorides, sodium, magnesium, calcium, etc. Maximum amount in pools is 2500 ppm. Maximum in spa, 1500 over starting TDS. The only way to effectively lower TDS is to drain part or all of the water and replace it.

telepole A long-handled aluminum pole designed to extend in length onto which various pool-cleaning tools such as brushes or vacuums may be attached.

test kit An apparatus or device used to monitor specific chemical residuals, levels, constituents or demands in pool or spa water. Kits usually contain reagents, vials, titrants, color comparators and other materials needed to perform tests. The most common pool and spa water tests are: pH, total alkalinity, free available chlorine, water hardness, cyanuric acid, iron and copper.

test strips Small plastic strips with pads attached that have been impregnated with reagents that can be used to test pool water for residuals, levels, constituents or demands. The strips are usually dipped in the water and the resulting colors of the pads compared to a standard set of colors to determine concentration.

thermostat A temperature – control device that cycles a heater on and off to maintain a desired temperature.

time clock A mechanical or electrical device that automatically controls the periods that a pump, filter, heater, blower, automatic pool cleaner or other electrical devices are on.

titration The method used to test for total alkalinity and water hardness in swimming pool and spa water. An indicator reagent is added to a sample and then another reagent (a titrant) is added until the sample changes color. The drops or amount of titrant used are equal to the concentration in ppm.

total alkalinity The total amount of alkaline materials present in the water. Also called the buffering capacity of the water. It is the water’s resistance to change in pH. Low total alkalinity causes metal corrosion, plaster etching and eye irritation. High total alkalinity causes scale formation, poor chlorine efficiency and eye irritation. The test measures for hydroxides, carbonates and bicarbonates.

total chlorinealso Total Available Chlorine – The total amount of chlorine in the water. It includes both free available and combined chlorine.

total dissolved solidsSee TDS.

total dynamic head The sum of all resistances in an operating circulation system.

toxic Means that a substance has an adverse physiological effect on human beings or other living organisms if the substance is contacted, ingested or inhaled.

trichlor Common name for trichloro-s-triazinetrione or the new name trichloroisocyanurate. A slow-dissolving, tabletted, stabilized organic chlorine compound providing 90% available chlorine. Used for regular chlorination but must be dispensed using a floating feeder or an in-line feeder (chlorinator). Trichlor contains an ingredient (cyanuric acid or stabilizer) that prevents the chlorine from being destroyed by the ultraviolet (UV) rays of the sun. Trichlor has a pH of 2.8 and regular trichlor tabs should not be placed in the skimmer as the low pH will corrode the metal components in the equipment.

turbidity Cloudy condition of the water due to the presence of extremely fine particles in suspension that can not be trapped by the filter because they are too small. Adding a clarifier such as an organic polymer or alum will coagulate the particles and make the filter more efficient. Turbidity is measured with a Nephelometer and expressed in units of opaqueness called NTU (Nephelometer Turbidity Units).

turnoveralso called turnover rate – The period of time (usually in hours) required to circulate a volume of water equal to the volume of water contained in the pool or spa. Pool capacity in gallons divided by pump flow rate in gallons per minute (gpm) divided by 60 minutes in one hour will give hours for one turnover.

underdrainalso known as filter laterals or lower collection system – Slotted finger-like tubes that are attached to a sand filter manifold. The slots are on the bottom side to prevent the sand from passing through. Water comes into the filter tank, through the sand and into the underdrain and then back to the pool.

underwater light A light fixture designed to illuminate a pool or spa from beneath the water’s surface.

ultraviolet (UV) Those light rays beyond the visible light spectrum at its violet end. Radiation having a wavelength shorter than wavelengths of visible light and longer than X-rays.

ultraviolet (UV) disinfection A device that uses intense UV light to irradiate and disinfect water. It is chemical free and does not change the water. However, only water that passes through the chamber is irradiated. Also, UV light destroys chlorine and bromine, so UV can not be used with them.

vacuum This term can be used to define any number of devices that use suction to collect dirt from the bottom and sides of a pool or spa. Most common is a vacuum head with wheels that attaches to a telepole and is connected to the suction line usually via the opening in the skimmer. It must be moved about by a person and debris is collected in the filter. Other vacuum type cleaners are listed under automatic pool cleaners.

vacuum breaker A mechanical device that automatically vents a water line to the atmosphere when subjected to a partial or full vacuum, thus preventing entrapment on main drains or skimmers.

vacuum filtration One of the oldest types of filtration in use. A filtration process where a partial vacuum is applied to increase the rate of filtration by causing the liquid (water) to be sucked through the filter medium.

valve A mechanical device installed in a water line to close off or regulate the flow of water.

velocity The speed at which a liquid flows between two points. Usually measured in feet per second.

venturi A fitting or device that consists of a tube constricted in the middle and flared on both ends. A fluid’s velocity will increase and a fluid’s pressure will decrease while passing through the constriction. Placing a tube or pipe at the constriction point creates a vacuum. Fluid or air can then be drawn in through the tube. A hydro-therapy jet draws air in and mixes it with the water using this principal.

vinyl liner The vinyl membrane that acts as the container to hold or contain the water.

virus The smallest (10 to 300 micron diameter) lifeform capable of producing infection and diseases in man.

volume The amount of water a vessel will hold. It is a three-dimensional space occupied by an object. Formula for volume is: Length × Width × Average Depth = cubic feet. Then, cubic feet × 7.48 gallons per cubic foot = gallons in vessel.

volute A spiral casing for a centrifugal pump designed so the pump rotation will be converted to pressure in a smooth transition as the water leaves the impeller and is discharged from the pump.

vortex A revolving mass of water that forms a whirlpool. A funnel-shaped opening is created downward from the water surface.

walls The interior pool wall surfaces. May be plaster, vinyl, fiberglass, and various other materials.

water balance Water that has a pH of 7.4 to 7.6, a total alkalinity of 80 to 140 ppm, Calcium Hardness of 200 to 400 ppm, and a TDS of less than 2500 ppm. The water is said to be “in balance” when the water is within these parameters.

waterborne disease A disease caused by a bacteria or organism able to live in or be carried by water. A disease transmitted through the ingestion of contaminated water. Water acts as a passive carrier of the infectious agent.

water clarifieralso called coagulant or flocculant – A chemical compound used to gather (coagulate or agglomerate), or to precipitate suspended particles so they may be removed by vacuuming or filtration. There are two types: inorganic salts of aluminum (alum) and other metals or water-soluble organic polyelectrolytes. Leslie’s Ultra Brite)

water hammer The shock wave or series of waves produced by the abrupt acceleration or deceleration of water flow through a pipe, caused by inertia.

waterline The height to which the water is filled in the pool or spa. This is usually in the middle of skimmer openings, the center of the tile pattern or to the overflow if so constructed.

weight per gallon Water weighs 8.34 pounds per gallon at 77° F.

weiralso called skimmer weir – Part of a skimmer that adjusts automatically to small changes in water level to assure a continuous flow of water to the skimmer. The small floating “door” on the side of the skimmer that faces the water over which water flows on its way to the skimmer.

wetting agent A surfactant, similar to a detergent.

winter cover A cover that is secured around the perimeter of a pool, spa or hot tub that provides a barrier to bathers and debris when the pool, spa or hot tub is closed for the season.

winterizing The procedure for preparing the pool or spa for freezing weather. Includes chemical treatment and physical treatment to protect equipment and vessel – draining, anti-freeze, drain plugs, etc.

Y-connection A pipe fitting with three branches positioned in one plane in the pattern of the letter Y.

yellow algae A particularly hardy kind of algae that is yellow or mustard colored.

z eolite Generic name for a group of sodium hydrated alumina silicates either naturally occurring mined products or synthetic products with ion exchange properties. Clinoptilolite, one of the zeolites, is used in place of silica sand in high-rate sand filters. When activated with a sodium chloride solution, clinoptilolite removes ammonia from water via ion exchange.