FIBERGLASS: fine spun 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

FILTER: mechanical device for straining suspended waste particles from the pool as water circulates through a porous substance called a filter medium; refers to the complete mechanism including all components; the three types of filters used in pools and spas are sand, cartridge and D.E. (diatomaceous earth)

FILTER AID: chemical compound added to the water or to the filter that allows the existing filter to become more efficient; examples are alum, water clarifiers and D.E. (diatomaceous earth)

FILTER AREA: total surface area of the filter medium that is exposed to the flow of water from the pump, expressed in square feet

FILTER CARTRIDGE: replaceable porous element made of paper or polyester used as the filter medium in cartridge filters

FILTER CYCLE: (1) operating time between cleaning or backwashing cycles of a filter (2) amount of time the filter has water flowing through it each day expressed in hours

FILTER ELEMENT: porous component in the filter tank that removes particles and other suspended wastes from water before its re-circulated back to the pool

FILTER LATERALS: see underdrain

FILTER MEDIUM: sand, diatomaceous earth, anthracite or other finely graded material used to filter particles out of water

FILTER MEDIUM: finely graded material, such as sand, diatomaceous earth, polyester fabric or anthracite coal that removes suspended particles from water passing through it

FILTER POWDER: common name for diatomaceous earth (D.E.), used as the filter medium in a diatomaceous earth filter

FILTER PUMP: device that pulls water from the pool and pushes it through the filter on its way back to the pool

FILTER ROCK: graded, rounded rock and/or gravel used to support the filter medium; usually used with rapid-rate sand filters

FILTER SAND: sharp silica or quartz particles graded for uniform size and used as a filter medium

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

FILTRATION RATE: rate of water pumped through a filter, in gallons per minute (gpm)

FIREMAN'S SWITCH: mechanical switch located inside the time clock, which opens a circuit and shuts off the heater 10 or 15 minutes prior to shutting off the water circulation pump, allowing the heater to cool down; this helps reduce lime buildup in the heat exchanger

FLOW METER: measuring device that determines the gallons per minute of water flow through a pool re-circulation system

FLOW RATE: volume of liquid flowing past a given point in a specific time period, expressed in gallons per minute or hour

FOOT BATH: shallow water area between bathhouse showers and pool deck through which pool patrons must walk; originally designed to contain a disinfectant solution for control of athlete’s foot; because it was proven ineffective, the footbath has either been eliminated or modified to contain a continuous flow of clean water

GAS VALVE: electronic valve in the pool heater that directs gas flow from the meter to the pilot and the burner tray

GELCOAT: colored polyester-resin liquid applied to a mold during the manufacturing of fiberglass pool or spa shells

GPD: gallons per day

GPH: gallons per hour

GPM: gallons per minute 

GRAB RAIL: (1) also called hand rail (2) 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

GREEN HAIR: condition caused by too much copper, not chlorine in the pool water; 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 in turn will dissolve metals in the equipment; the dissolved metal (usually copper) then stains hair, fingernails and, eventually, pool walls; keeping the pH too low or misusing acid can also cause it

GROUND-FAULT CIRCUIT-INTERRUPTER: interrupts (de-energizes) the electrical circuit whenever it detects the presence of excess electrical current going to ground (usually 1/40th of a second and 5/1000th of an ampere)

GUNITE: concrete and sand mixture sprayed onto a reinforced steel form to create a pool shell; dry mixture of cement and sand is mixed with water at the "gun or spraying device", hence the name

GUTTER: (1) overflow trough at the edge of the pool through which floating debris, oil and other "lighter-than-weight" items flow; pools with gutters usually do not have skimmers (2) also called a perimeter or recirculation system

HAND RAIL: see grab rail

HAND SKIMMER: screen attached to a frame which is then attached to a telepole used to remove large floating debris, such as leaves and bugs, from the water's surface

HEAT EXCHANGER: (1) 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 (2) set of 8 or 10 ribbed copper tubes that absorb the heat produced below it and transfer it to the water cycling through its tubes

HEAT PUMP: using a cooling coil, it removes heat from the air while the condenser coil transfers it to the water being cycled through the pump

HEATER: fossil-fueled, electric or solar device used to heat the water of a pool, spa or hot tub

HOT TUB: a circular, wooden vessel with the sides and bottom formed separately and joined together by hoops, bands or rods; filled with heated and circulated water

HYDROJET: 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: force involving built up ground water, which creates upward pressure beneath the pool shell

HYDROSTATIC RELIEF VALVE: fitting(s) installed in the floor of the pool designed to manually or automatically release hydrostatic pressure beneath the pool by allowing ground water into the pool

HYDROTHERAPY SPA: unit primarily for therapeutic use to ease stress, muscle strains and other physical problems

IMPELLER: rotating member of a pump; its action creates the flow of water

INFLUENT: (1) water flowing into a filter, pipe, pool, chemical feeder or other space; water going into the pump is called in influent, while water leaving the pump is called the effluent (2) the water coming into and up to the impeller from the suction lines; these pipes are under vacuum pressure

IN-GROUND POOLS: are partially or completely installed in the ground

INLET: fitting in the pool or spa on the water return line from the equipment that water returns to the pool