The following list of definitions is excerpted from ANSI/APSP standards.

Acidic - Having a pH below 7.0.  Opposite of basic.

Acid Wash - A procedure using an acid solution to clean an interior surface of a pool with subsequent neutralization of the acid.

Action Pool - A wave pool that generates standing waves in an assortment of patterns.

Activated Carbon - A charcoal-like material used to remove colors, odors and/or excess oxidizer from water.

Activity Pool - A pool designed primarily for play activity that uses constructed features and devices including lily pad walks, flotation devices, small slides and similar attractions.

AF - Abbreviation for the alkalinity factor in calculating the saturation index of water.

Aggressive Water - Water that is corrosive because it is low in pH and/or calcium hardness and/or total alkalinity.

Air Blower - A device that produces a continuous flow of air.

Air Channel - A system for introducing a volume of air into hollow ducting built into a spa floor, bench or other location, activated by a separate air power unit (blower).

Air Control - A means for spas and hot tubs to regulate air flow in the air induction system, thereby increasing or decreasing hydrotherapy action.

Air Induction System - A plumbing system that provides the source of air for the air/water mixture to a hydrotherapy jet.

Air Pump Assist Backwash - The compression of a volume of air in the filter chamber (by means of an air compressor) that, when released, rapidly decompresses and forces water through the filter chamber elements in reverse.  This dislodges the filter aid and accumulated dirt, allowing it to be carried to waste.

Air Switch System - An isolated device that sends a pulse of air down a tube to remotely operate electrical equipment.

Alkaline - Having a pH above 7.0

Alkalinity - A measure of the pH buffering capacity of water, generally expressed in terms of the equivalent concentration of calcium carbonate in milligrams per liter (or ppm).

Antivortex Drain Cover (Antivortex Plate or Cover) - A plate or cover that is affixed to the main outlet of a swimming pool or spa to prevent a vortex from forming as water passes through to the main outlet.

Back Pressure - Resistance to flow, normally expressed in pounds per square inch (kilograms per square centimeter).

Backwash - The process of cleansing the filter medium and /or elements by the reverse flow of water through the filter.

Backwash Cycle - The time required to backwash the filter medium and /or elements, and to remove debris in the filter vessel.

Backwash Rate - The rate of flow water through the filter medium per unit of area (U.S. gallons per minute per square foot).

Ball Valve - A device that can partially or totally obstruct the flow of water, using a ball-shaped diverter.

Band - Device used in conjunction with a clamp to secure two halves of a filter together.

Basic - Having a pH above 7.0 Opposite of acidic.

Bather - Any person using a pool, spa or hot tub and adjoining deck area for the purpose of water sports, recreation, therapy or related activities.  Also user load.

Bleeder Valve - A device that allows air to be vented from a system.

Body Coat - A layer of diatomaceous earth or similar material on a filter element that acts as the filtering medium.

Body Feed - A controlled amount of diatomaceous earth that is continuously added to the filter element during the course of a filter run to help maintain filter porosity.

Booster Pump System - A pump that is completely independent of the filtration and heating system, used to provide support for hydrotherapy jets, cleaning systems and gas chlorinators or special waterfeatures.

Breakpoint Chlorination - The practice of adding a sufficient amount of chlorine to water to destroy the combined inorganic chlorine present.  Normally, the amount added is 10 times the combined chlorine concentration.

BTU (British Thermal Unit) - A unit of measurement used to define the capabilities of heaters.  One BTU is capable of raising the temperature of one pound of water by one degree Fahrenheit.

Burner - The component of a heater where the combustion of fossil fuel takes place.

Bypass Valve - A valve used to limit or adjust the amount of water flowing through a device in a bypass loop to divert flow to an alternate plumbing path. 

Calcification - Formation of calcium carbonate on walls of pools or pipes, or in a filter or heater, due to precipitation of calcium carbonate.

Calcium Hardness - A measure of the amount of calcium dissolved in water, expressed in ppm of equivalent carbonate.

Cartridge - A replaceable, porous element designed to retain suspended particles from water.

Cartridge Filter - A filter that uses a porous element to act as a filter medium.

Cavitation - The formation of partial vacuums when pump capacity exceeds the water replacement supply.

Centrifugal Force - A pump that circulates water using a shaft-mounted impeller powered b y an electric motor or gasoline engine.  The centrifugal force of the spinning impeller creates the flow through the pump.

CF - Abbreviation for the calcium hardness factor in calculating the saturation index of water.

CFM - Abbreviation from cubic feet per minute (of air).

Check Valve - A valve allowing flow in one direction and obstructing flow in the other.

Chemical Feeder - A device (floating or mechanical) used to add a chemical to pool/spa water.

Chlorinator - A device used to add or deliver a chlorine disinfectant at a controllable rate.  Chlorinators are designed for specific chlorine compounds and should only be used with the compounds for which they are designed.

Chlorine Demand - The amount of chlorine necessary to oxidize all organic matter present in pool water at any given moment, or over a period of time.

Circuit - The complete path of an electric current.

Circuit Breaker - A device designed to open and close an electrical circuit manually, and to open a circuit automatically at a predetermined overcurrent level, thus providing protection to the wiring and electrical components.

Circulation Equipment - The components of a circulation system.

Circulation System - A system of mechanical equipment and/or components designed to ensure even distribution of heat, chemicals and filtration of water throughout a pool.  Includes filters, heaters, pumps, chlorinators, piping, inlets, drains, skimmers and other devices.

Collector, Solar - An assembly of components used to collect solar energy for heating.

Concrete Shell - Various forms of concrete that, together with steel, form the structure of a pool or spa.

Coping - The cap on the wall that provides a finishing edge around a pool/spa.  Can be formed, cast in place, precast or prefabricated from metal or plastic materials, brick or stone.  May be used as part of the system that secures a vinyl liner to the top of the pool wall.

Corrosion - The etching, pitting or eating away of a material.

Cove - The radius that joins the floor and wall of a pool/spa.

Cover - Something that covers, protects or shelters a pool, spa or hot tub.

Safety Cover - As defined by ASTM in ES 13-89, Emergency Standard Performance Specification for Safety Covers and Labeling Requirements for All Covers for Swimming Pools, Spas and Hot Tubs.  It is a barrier (intended to be completely removed before entry of users) for swimming pools, spas, hot tubs or wading pools, attendant appliances and/or anchoring mechanisms that will -- when properly labeled, installed, used and maintained in accordance with the manufacturer's published instructions -- reduce the drowning risk for children under age 5.  Specifically, safety covers are meant to inhibit small children's access to the water and to provide for the removal of any substantially hazardous level of collected surface water.  (These covers may be power or manual.)

Solar Cover - A cover that, when placed on a pool or spa surface, increases the water temperature by solar activity and reduces evaporation.

Thermal Cover - An insulating cover used to prevent evaporation and heat loss from a pool or spa.

Winter Cover - A cover that is secured around the perimeter of a pool or spa that provides a barrier to debris when the pool or spa is closed for the season.

Crack (surface) - A repairable break in the surface, not major or self-curing.

Crack (structural) - A break or split that weakens the structural integrity of the pool.

Deck Dive - A dive performed from the deck area of a pool into 5 feet or more of water depth.

Decks - The areas immediately adjacent to or attached to a pool or spa that are specifically constructed or installed for sitting, standing or walking.  Generally made of concrete, wood or masonry.

Deep Area - Water areas exceeding 5 feet in depth.

Design Rate of Flow - The rate of flow used for design calculations in a system.

Diatomaceous Earth Filter - A filter that uses a coating of diatomaceous earth (DE) over a porous fabric as its filter medium.

Diatomite - The filtering medium of a diatomaceous earth filter composed of microscopic fossil skeletons of the diatom, a tiny freshwater plankton used to trap solids while allowing clear water to pass through the filter.  Also known as diatomaceous earth.

Diatomite Filter Element - A device used in a filter tank called a filter grid or element, coated with a fabric that traps diatomite on its surface.

Diffuser - A pump component that reduces velocity and increases static pressure of fluid passing through a system.

Digital Multimeter - A meter read by means of an LED or LCD display.

Discharge Head - Resistance to water flow encountered on the discharge side of a pump, measured in feet of head.  Each foot of water on the suction side of the pump is equal to one foot of head on the discharge side.

Disinfecton - The killing of pathogenic organisms with chemicals or other proven means, such as ozone, copper or silver.

Dive - A free-fall entry as a planned acrobatic maneuver into the designated diving area of a pool.

Diving Area -  The area of a swimming pool that is designed to be used for diving.

Diving Board - A flexible board secured at one end to be used for diving.

Diving Platform - A stationary platform designed for diving.

Diving Stand - Any supporting device for a springboard or diving board.

Draft Hood - Part of an indoor heater used to vent to the outside to prevent a down draft.

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

Elbow - A plumbing fitting shaped at a 90 - or 45 - degree angle. 

Effective Filter Area - The total surface area through which designed flow rate will be maintained during filtration.

Cartridge Type - The total effective filter area is the cartridge fabric area that is exposed to the direct flow of water, figured in square footage of fabric.  This excludes cartridge ends, seals, supports and other areas where flow is impaired.

Diatomaceous Earth Type - The actual area of the filter element is the total effective area of the porous fabric septum, less any area of a septum support member greater than 1/4 inch wide contacting the septum during filtration.

Permanent Medium Type - The effective filter area is the filter surface that is perpendicular to the flow direction.

Sand Filtration Type - The effective filter area is the top surface area of the sand within the filter calculated in square inches or square feet.

Effluent - The outflow of water from a filter, pump or pool.

Electrolytic Chlorinator - A device that converts dissolved chloride salt (sodium chloride) into chlorine and its reaction products.

Enclosure- A fenced-in, secured area.

Energy, Electric - The amount of electricity consumed when power is used over a period of time, measured in kilowatt-hours.

Equipment Area - An area used to house recirculation and disinfecting equipment and related appliances.

Etching - Corrosion on the surface; the pitting or eating away of a material such as the surface of plaster (marcite).

Evaporation - The conversion of liquid molecules into vapor.

Exercise Bar - A tubular device installed in the wall as a handhold.

Feet Of Head - In a hydraulic system, resistance based on the equivalent height of a column of water that causes the same resistance (100 feet of head equals 43 pounds per square inch). The total dynamic head is the sum of all resistances in complete operating system.

Fence - A boundary between a pool/spa area and the outside, intended to deter unauthorized entry from the outside.  Not intended as a structural barrier to be sat, walked or climbed on.

Filter - A vessel that removes undissolved particles from water by recirculating the water through a porous substance (a filter medium or elements).

Cartridge Filter - Uses a porous element that acts as a filter medium.

Diatomaceous Earth Filter - Uses a thin coating of diatomaceous earth (DE) over a porous fabric as its filter medium.

Permanent Medium Filter - Uses a filter medium (sand).

Filter Agitation - Mechanical or manual movement to dislodge the filter aid and dirt from the filter element.

Filter Aid - A powderlike substance such as diatomaceous earth or volcanic ash used to coat a septum-type filter.

Filter Cartridge - A filtering element, usually of fibrous material.

Filter Cycle - The operating time between cleaning or backwash cycles.

Filter Element - A device within a filter tank designed to entrap solids and conduct water to a manifold, collection header, pipe or similar conduit and return it to the pool, spa or hot tub.  A filter element usually consists of a septum and septum support, or a cartridge.

Filter Medium - A finely graded material (such as sand, diatomaceous earth, polyester fabric, anthracite, etc.) that removes solid particles from water.

Filter Sand - A hard, silicalike substance free of carbonates or other foreign material and used as the medium in sand filters.

Filtration - The process of capturing suspended particles and clarifying water.

Filtration Flow - The design rate or flow, in volume over time (gallons per minute, gallons per hour), through the filter system when installed per the manufacturer's instructions with a new, clean filter medium.

Filtration Rate - The rate of water flowing through a filter during a given period of time, expressed in U.S. gallons per minute per square foot of effective filter are.  Also known as filtration flow rate.

Firebox - A chamber in the pool/spa heater where combustion takes place.

Fireman's Switch - A mechanism adapted to a time clock that will turn the heater off long enough for it to cool down before the time clock turns the pumps off.

Float Valve - A valve controlled by the level of a fluid.

Flooded System - An underwater pump that does not require priming.

Floor - The interior bottom surface of a pool or spa

Floor Slope - The slope of the pool floor, usually expressed in feet or inches of vertical rise per feet or inches of horizontal distance.

Flow - The rate of the movement of water typically in gallons per minute.

Flow Meter - A device that measures the rate of flow of liquid through piping.

Flow Rate - The volume of liquid flowing past a given point in a specified time period.  Usually expressed as U.S. gallons per minute (gpm) or gallons per hour (gph).

Flow Switch - A safety device that prevents a heater from firing if water flow through the system is not adequate.

Flume - A troughlike or tubular structure, generally recognized as a water slike, that directs the path of travel and rate of descent of the rider.

Flume Slide - Slides fo various configurations that are characterized by having deep riding channels, vertical and lateral curves, and high water flows.

Free Available Chlorine - The portion of the total chlorine remaining in chlorinated water that is not combined with ammonia or nitrogen compounds and will react chemically with undesirable or pathogenic organisms.  The most active disinfectant in pool/spa water.

Freeboard - In a sand filter, the clear vertical distance between the top of the filter medium and the lowest outlet of the upper distribution system. 

Fusible Link - A thermal safety cut-off device in the control circuitry that melts at blowback of flame.

Gate Valve - A device in a pipe that can partially or totally obstruct the flow of water using an internal "gate" that moves in and out as the valve is operated.

Grab Bar/ Rail - A rail used to enter or leave a pool/spa.

Grounding - Connecting or providing a conducting path to earth or a conducting body that serves in place of earth.

Gunite - A pneumatically applied (sprayed) concrete that is a dry mixture of cement, aggregate and/ or sand.  Water is applied to the mix at the hose nozzle.

Gutter - Overflow trough in the perimeter wall of a pool, a component of the circulation system.

Halogen - Any of the active chemical elements fluorine, chlorine, bromine and iodine, used as disinfectants or oxidizers.

Handhold/Handrail - A device that is intended to be gripped by a user for the purpose of resting and /or steadying him/herself.  Typically located inside or outside the pool/spa, or as part of a set of steps or deck-installed equipment.

Head - A measure of the amount of pressure or resistance in a hydraulic system, expressed in feet.

Head Loss - The amount that flow reduces as head increases.

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

Heat Exchanger - A device with coils, tubes or plates that transfers heat from air or a fluid to another fluid without intermixing them.

Heat Loss - The natural drop in water temperature as heat is transferred to the surrounding air.

Hertz - The measure of the frequency of alternating current, expressed in cycles per second.

High-Limit Switch - A termperature control switch that can deactivate a control circuit at a preset temperature.  Normally preset at the factory and nonadjustable; must be manually reset.

Horsepower - A unit for expressing the power of motors or engines, equal to a rate of  33,000 foot-pounds per minute.

Hot Tub - In the past, this term applied to a spa constructed of wood.  Now can also be used to refer to spa units formed of plastic, concrete, metal or other materials.

Impeller - The rotating vanes of a centrifugal pump; its action creates the flow of the water.

Inlet Fitting - The fitting aperture through which water under positive pressure is returned to a pool or spa.

Intermittent Ignition Device - An electrical spark device used to ignite a gas heater.

Ionization - The process whereby a compund, in solution, separates into charged ions.

Kilowatt - A unit of power equal to 1,000.

Kilowatt Hour - A unit of work or energy equal to the energy expended by one kilowatt in an hour.

Kinetic Energy - The energy from the motion imparted to a liquid by the action of impeller vanes, causing the liquid to flow out of the pump casing and away from the impeller.

Ladder -

• A-Frame - An entry ladder that straddles an aboveground/onground pool wall and is either removable or has a built-in entry-limiting feature.
• Double Access - An A-Frame ladder that straddles the pool wall of an abouveground pool and provides pool ingress and egress, and is intended to be removed when not in use.
• Limited Access - A removable A-Frame ladder that straddles an aboveground/onground pool wall with a built-in entry-limiting provision for making the pool inaccessible when not in use.
• Staircase - A gound-to-deck staircase ladder that allows access to an aboveground pool deck and has a built-in, entry-limiting feature.  Used with In-pool staircase ladder to provide a means of ingress and egress from the pool to the deck.

Liquid Propane Gas - The liquid form of propane gas, a heavy hydrocarbon that occurs naturally in petroleum.

Low-Watt Density - In a heating element, the amount of watts generated per inch.  The lower the amount of watts per inch, the lower the density.

Main Drain - An outlet located at the bottom of a pool or spa that conducts water to the recirculating pump.

Manifold - A pipe with several openings for making multiple connections.

Manometer - An instrument that measures vacuum or pressure differential across an orifice by means of a  column of liquid, usually mercury.  In pools, usually calibrated to show rate or flow of water in gallons per minute (gpm).

Marcite-  A kind of pool plaster

Mechanical Seal- A device used to prevent the passage of water in or out of a centrifugal pump at the motor shaft.

Micron - One-millionth of a meter.  Used to describe the size of particles that filters are capable of trapping.

Millivolt - One-thousandth of a volt.

Millivolt Ignition, Pilot - Heat from a continuous pilot is used to generate electrical energy that opens and closes the main gas valve and  operates a system's safety controls.

Moment - Rotation

Motor - A machine that converts electrical energy into mechanical energy.  When electrical current is supplied to a series of wires (windings), a magnetic field is created that drives the rotor and shaft of a pump impeller.

Multiple Filter-Control Valve - A multiport valve having a number of control positions for various filter operations, combining the function of two or more single valves in one unit.

Multiport Valve - A device that allows for the multidirectional control of the passage or flow of water through a system.

Natural Gas - Admixture of gaseous hydrocarbons, chiefly methane, occurring naturally underground, often in association with petroleum products.

Orifice - An opening, usually carefully calibrated in size, through which water flows.

OTO (Orthotolidine) - A colorless reagent that reacts with chlorine or bromine to produce a series of yellow to orange colors, indicating the amount of chlorine or bromine in water.  Effectively measures total chlorine.

Outdoor Stack - A type of equipment used on outdoor heaters in areas of extreme, constant winds.

Outlet - An aperture or fitting through which the water under suction is drawn from a pool, spa or hot tub.

Overflow System - A system for the removal of pool/spa surface water through the use of overflows, surface skimmers, and surface water collection systems of various design and manufacture.

Oxidizers - Products used to destroy organic waste in water.

Ozone - A gaseous molecule composed of three oxygen atoms, generated on site and used for the oxidation of water contaminants.

Ozone, Low-Output Generating Equipment (Ozonator) - A device that will product ozone in air at a concentration of less than 500 ppm. Usually refers to ultraviolet (UV) generators.

Ozone Contact Concentration - A measure of the amount of ozone dissolved in pool/spa water.

Ozone Generator - A device used to produce ozone.

pH - A value for the relative acidity or basicity of a substance such as water, as indicated by the hydrogen ion concentration.  Expressed on a scale of 0 to 14, 0 being most acidic, 7 being neutral and 14 being most basic.

pH Meter - An instrument that measures pH by electrical conductivity across a membrane of an electrode.

Phenol Red - A dye used in measuring pH.

Pilot Generator - The component in a millivolt system that transforms heat from the pilot into electrical energy.  Also referred to as a thermal coupling or thermocouple.

Pilot Light - A small, constant flame used to ignite gas at the first burner.

Pitting - A form of etching.

Plaster - A type of interior finish, made from a mixture of white cement and aggregate that will accept a tint or color; applied to a concrete pool or spa.  Also called marcite or marblite.

PPM - Abbreviation for parts per million, the unit of measurement used in chemical testing that indicates the parts by weight in relation to one million parts by weight of water.  Essentially identical to the term milligrams per liter (mg/L).

Pressure - In a liquid, a form of energy proportional to the depth of a column of liquid.  Expressed as pounds per square inch, feet of liquid or feet of head.

Pressure Check - A test for the rate of water flow; also a test for leaks in a system.

Pressure Differential - The difference in pressure between two parts of a hydraulic system, such as the influent and effluent of a filter.

Pressure Gauge -  A gauge that measures the amount of pressure built up within a closed container, such as a filter.

Pressure Switch - A device that will not allow a heater to fire unless there is adequate water pressure in the system.

Priming - Refers to re-establishing the water flow by the recirculating pump.

PSI - 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 ans spa water.  Typically, a centrifugal pump design is used for pools and spas.

Pump Capacity - The volume of liquid that a pump is capable of moving during a specified period of time against a given total head.

Pump Curve - A graph of performance characteristics of a given pump under varying power, flow and resistance factors.  Used in checking and sizing a pump.

Pump Strainer - A device, placed on the suction side of a pump, that contains a removable strainer basket designed to trap debris in the water flow with a minimum of flow restriction.  Sometimes called a hair/lint pot or trap.

PVC (polyvinyl chloride) - A thermoplastic resin commonly used for pool piping and plumbing components.

Rate of Flow - The quantity of water flowing past a designated point within a specified time, such as the number of gallons flowing in one minute (gpm).

Rated Pressure - Pressure that is equal to or less than the pressure designated on the data plate of the equipment.

Rehabilitation - The activity of restoring all or part of a pool or spa structure and its components to the original design condition, including the rebuilding and/or replacing of worn and broken parts or components.

Remote Switch - Any switching device used to activate an apparatus from a distance.

Return Inlet - The aperture of fitting through which the water under positive pressure returns into a pool or spa.

Return Piping - Effluent piping.

Reverse Circulation - A circulation system in which water is taken from the surface and returned through inlets at the bottom of the structure.

Salinity - The sodium chloride or slat content of water.

Sand Filter - A filter that uses sand, or sand and gravel, as the filter medium.

Saturation Index - A rating that indicates whether water will have a tendency to deposit calcium carbonate from a solution, or whether it will be potentially corrosive.  Four factors are used in the computation: pH, total alkalinity, calcium hardness and temperature.  When correctly balanced, water will be neither scale-forming nor corrosive.

Scale - The precipitate that forms on surfaces in contact with water whose calcium hardness, pH or total alkalinity level is too high.

Sealant - A liquid, usually clear, that is painted on a surface to prevent the entry of moisture.

Sediment Trap - A device for trapping sediment that is required on all gas-heater piping.

Self-Priming - A rating given to centrifugal pumps to indicate that the pump is capable of operating above pool water level, after the initial filling with water.

Service Factor - The degree to which an electric motor can be operated above its rated horsepower without danger of overload failure.

Shotcrete - A mixture of cement and sand that is applied to contoured and supported surfaces to build a pool or spa.  Shotcrete is premixed and pumped wet to the construction site.

Shut-Off Head - The amount of head at which the pump can no longer circulate water.

Skimmer - 

• Thru-Wall - A device installed in the wall of an inground pool or spa or aboveground/onground pool that allows for continuous removal of floating debris and surface water to the filter.

• Over-The-Wall - A device installed over the wall of an aboveground/onground pool that allows for continuous removal of debris and surface water to the filter.

Skimmer Cover - A device or lid to close the deck opening to the skimmer housing.

Skimmer Equalizer Pipe - The connection from the skimmer housing to the pool, spa or hot tub below the weir, sized to satisfy pump demand and prevent air lock or pump loss of prime.

Skimmer Housing - The structure that attaches to or contains the skimmer weir, strainer basket, float valve and other devices used in the skimming operation.

Skimmer Weir - The part of a skimmer that adjusts automatically to small changes in the water level to ensure a continuous flow of water to the skimmer.

Slope - An inclined surface.

Slurry Feed - Diatomaceous earth continuously added to filter elements during the course of a filter run to help maintain filter porosity.

Soft Water - Water that has very low calcium and magnesium content - less than 7 grains per gallon (less than 100 ppm.)

Spa (Hydrotherapy Spa) - A hydrotherapy unit of irregular or geometric shell design.

• Permanent Residential - A spa in which the water-heating and circulating equipment is not an integral part of the product.  Intended as a permanent plumbing fixture and not intended to be moved.

• Public - Any spa, other than a permanent residential spa or residential portable spa, an owner, lessee, operator, licensee or concessionaire, regardless of whether a fee is charged for use.

Residential Portable Spa - 

• Non-Self-Contained Spa - A spa in which the water-heating and water-circulating equipment is not an integral part of the product.  Non-self-contained spas may employ separate components such as an individual filter, pump, heater and controls, or they may employ assembled combinations of various components.

• Self-Contained Spa - A spa in which all control, water-heating and water-circulating equipment is an integral part of the product.  Self-contained spas may be permanently wired or cord-connected.

Spread - The clear distance between the side rails of a ladder.

Stabilizer (also Cyanuric Acid Isocyanuric Acid, Conditioner or Triazentrione) - A chemical that helps reduce the excell loss of chlorine in water due to the ultraviolet rays of the sun.

Stairs -  A series of steps, each consisting of a riser and a tread.

Static Head - Head loss caused by the weight of a standing water column to be moved; encountered on both the suction and discharge sides of a pump.

Static Suction Head - The vertical dimension between the pump center line and the level of the liquid being pumped when it is below the impeller plane, expressed in feet of head.

Static Suction Lift - The vertical distance in meters (or feet) from the center line of the pump impeller to the pool water level.

Steps, Recessed Steps, Ladders and Recessed Treads - Means of pool and spa ingress and egress, intended to be used separately or in conjunction with one another.

• Step(s) - A riser/tread or series of risers/treads extending down from the deck and terminating at the pool or spa floor.  Includes recessed steps with risers located outside of user areas.

• Ladder - A series of vertically separated treads or rungs connected by vertical rail memebers or independently fastened to an adjacent vertical pool wall.

• Deck Ladder - Provides deck access from outside the pool or spa.

• Recessed Treads - A series of vertically spaced cavities in the pool or spa wall, creating tread areas for stepholes. 

Strainer Basket - An easily removable, perforated or otherwise porous container used to catch coarse material in the pump lint strainer.

Suction Head - Resistance to water flow encountered on the suction side of a pump, measured in feet of head.  Reflects the vertical lift required by the pump's intake.

Suction Outlet - An aperture or fitting, other than a skimmer, on the side wall of a pool pressure (vacuum) is drawn from the pool or spa to the pump or circulation system.

Suction Piping - Influent piping.

Surface Skimming System - Refers to perimeter-type overflows, surface skimmers and design and manufacture.

Swimming Area - Any area of a pool more than 3 feet in water depth that is devoted to swimming.

Tee - A T-shaped plumbing fitting used to connect three pipes.

Temperature Rise - The difference between the desired water temperature and the current water temperature.

Test Kit - A device used to monitor specific chemical residual or demands in pool or spa water.

TF - Abbreviation for the temperature factor in calculating the saturation index of water.

Therm - A unit of thermal measurement equal to 100,000 BTU.

Thermostat - A temperature-control device that cycles the heater on and off to maintain the desired temperature.

Time Clock - A mechanical device that automatically controls the time periods that a pump, filter, blower or other electrical device is in operation.

Time Delay - A time-controlled switch used to provide a rest period or to stop repeated false starts.

Turbidity - A cloudy condition of water due to the presence of extremely fine particles in suspension that interfere with the passage of light.

Turnover Rate - The period of time (usually expressed in hours) required to circulate a volume of water equal to the pool/spa capacity.

Two-Speed Pump - A centrifugal pump with a motor that operates at two different rpm speeds.

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.

Vacuum - The reduction of atmospheric pressure within a pipe, tank, pump or other vessel.  Measured in inches of mercury.  One inch of mercury equals 1.13 feet of head.  The practical maximum vacuum is 30 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 valve) or permit flow in only only 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 specified 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 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.

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

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.

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

Waterline - 

• 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 the water flow, detects abnormally low pressures on a piece of equipment and can shut it off.

Watt - The measure of electrical power computed by multiplying voltage times current (volts x amperes).  Equal to 1/746th of one horsepower unit.

Watts Valve - A mechanical valve that opens with 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.

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.

Wrinkle - A small ridge or crease in an otherwise smooth surface.