Pool glossary

Water chemistry

The numbers you test, and what each one does.

Aeration

Aeration is breaking up the pool's surface so dissolved carbon dioxide escapes, which raises pH. Aeration is the only way to raise pH without raising total alkalinity. Fountains, bubblers, deck jets, spa jets, spillovers and return jets aimed up all aerate the water.

Baking soda (sodium bicarbonate)

Baking soda is sodium bicarbonate, the identical chemical sold at the pool store as "Alkalinity Increaser" or "Alkalinity Up". In a pool it raises total alkalinity and barely touches pH, nudging it toward about 8.3.

Black algae

Black algae is not algae but cyanobacteria, a bacterium that glues itself into the pores and micro-cracks of plaster, gunite and concrete under a waxy cap chlorine can't penetrate. It shows as dark, raised spots that regrow in the same places after brushing, and no bottle kills it.

Borates

Borates are a pH buffer added to pool water as boric acid or borax and held at 30–50 ppm. Borates slow the upward pH drift every pool has and cut scale inside a salt cell. Borates do not sanitize, and they do not let a pool run less chlorine.

Calcium hardness (CH)

Calcium hardness (CH) is the amount of dissolved calcium in pool water. Plaster, gunite and pebble surfaces are calcium, so the water needs enough dissolved calcium that it isn't pulling more out of the finish, while too much calcium falls out as scale on tile, heaters and salt cells.

Calcium scale

Calcium scale is calcium carbonate coming out of oversaturated water and hardening into a white-to-grey crust that won't wipe off, on tile, at the waterline, on salt cell plates or inside a heater. It is what a positive saturation index (LSI or CSI) looks like once it lands on something.

Chlorine lock

Chlorine lock is a pool-store term for water where the chlorine reads fine but does nothing. Chlorine lock does not exist: Trouble Free Pool calls it a myth, and what a store calls chlorine lock is almost always over-stabilized water, meaning the cyanuric acid (CYA) is too high.

Combined chlorine (CC)

Combined chlorine (CC), also called chloramines, is chlorine that has already reacted with sweat, sunscreen, oils and other contaminants and been used up. The target for combined chlorine is 0, a reading of 0.5 or below is acceptable, and above 0.5 it is time to SLAM the pool.

Cyanuric acid (CYA)

Cyanuric acid (CYA), sold as stabilizer, is sunscreen for chlorine: without some in the water, UV burns off chlorine in hours. Cyanuric acid also binds most of the free chlorine into a reserve, so the higher the CYA, the more free chlorine a pool needs.

FAS-DPD test

A FAS-DPD test is a drop-titration chlorine test: you count drops until the sample turns from pink to colorless. A FAS-DPD kit measures free and combined chlorine precisely, accurate to 0.2 or 0.5 ppm, and it reads the high chlorine levels that test strips and the DPD color block cannot.

FC/CYA chart

The FC/CYA chart is the table that gives the free chlorine (FC) a pool needs at each cyanuric acid (CYA) level: a minimum of about 7.5% of CYA, a target band above that minimum, and a SLAM level of about 40% of CYA.

Free chlorine (FC)

Free chlorine (FC) is the chlorine in pool water that is still available to sanitize, and it is the number a chlorine test reports as FC. Free chlorine has no single correct level: the minimum is about 7.5% of the pool's cyanuric acid (CYA) reading.

Hypochlorous acid (HOCl)

Hypochlorous acid (HOCl) is the active form of chlorine in pool water, the form that actually kills algae and bacteria. A free chlorine test counts all the available chlorine, but only the HOCl fraction does the work, and cyanuric acid (CYA) controls how much of it is active at once.

Metal stain

A metal stain is dissolved iron, copper or manganese that chlorine has oxidized and plated onto the pool surface: rust-brown or orange for iron, teal or blue-green, sometimes black, for copper. Under a crushed vitamin C tablet an iron stain lightens and a copper stain goes darker or black.

Muriatic acid

Muriatic acid is hydrochloric acid, the liquid pool owners add to lower pH. Every dose lowers total alkalinity at the same time, and no dose or pour technique drops one without the other.

Mustard algae

Mustard algae is a yellow, dusty algae that grows on walls and floor, especially the shaded side, tolerates normal chlorine levels, and clears during a SLAM only to return when chlorine drops back to normal. It is resistant to chlorine, not immune.

Overnight chlorine loss test (OCLT)

The overnight chlorine loss test (OCLT) measures free chlorine after sundown and again before sunrise. If free chlorine drops more than 1.0 ppm overnight, with no sunlight to blame, something organic such as algae is eating the chlorine.

pH

pH is the measure of how acidic pool water is. Aim for pH 7.6–7.8 and treat 7.2–8.0 as acceptable, which is Trouble Free Pool's recommended level and is the same for vinyl, plaster and fiberglass, and for liquid-chlorine and salt pools.

Phosphates

Phosphates are algae food that gets into every outdoor pool through fill water, leaves, pollen and lawn runoff. Phosphates do not cause algae when free chlorine is held at the right level for the pool's cyanuric acid (CYA), so a normally run pool does not need phosphate remover.

Polyquat 60

Polyquat 60 is a quat-based algaecide sold at 60% strength, the most effective of the quat algaecides and the gentlest in a chlorinated pool. Polyquat 60 is the one algaecide Trouble Free Pool suggests keeping, and its real use is winterizing a closed pool, not a weekly dose.

Saturation index (LSI / CSI)

The saturation index, known as the Langelier Saturation Index (LSI) or its modern refinement the CSI, folds pH, temperature, calcium hardness and carbonate alkalinity into a single value that says whether pool water is corrosive, balanced or scaling. Balanced is −0.3 to +0.3.

Sequestrant

A sequestrant, also called a chelating agent, binds dissolved metal such as iron so the metal stays in the pool water instead of plating out as a stain. A sequestrant does not remove metal from the pool, and it does not lift a stain that is already on the wall.

Shock (shocking a pool)

Shock, in pool care, is a level of free chlorine and not a product: shocking means raising free chlorine to a high level, about 40% of the pool's cyanuric acid (CYA), and holding it until a specific problem clears. The bag labeled shock at the store is just chlorine, often with extras.

SLAM (Shock Level And Maintain)

SLAM stands for Shock Level And Maintain, the name Trouble Free Pool gives to the method that clears algae: raise free chlorine to the shock level for the pool's cyanuric acid (CYA), about 40% of CYA, and hold it there by retesting and re-dosing until the pool passes three exit tests.

Soda ash (sodium carbonate)

Soda ash is sodium carbonate, sold at the pool store as "pH Up" or "pH Increaser". It raises pH strongly and raises total alkalinity along with it, so it is the right tool only when both numbers are low.

Total alkalinity (TA)

Total alkalinity (TA) is the buffer in pool water that keeps pH from swinging. Hold total alkalinity at 60–80 ppm, with 50–90 acceptable, which is Trouble Free Pool's recommended level; the 80–120 on pool-store charts and salt-cell manuals is the public-pool operator figure.

Total chlorine

Total chlorine is free chlorine plus combined chlorine added together. Most test strips report total chlorine instead of free chlorine, so a pool full of chloramines can look chlorinated on a strip while the true free chlorine is near zero.

Chlorine & salt

Where the chlorine comes from, and what comes with it.

Bromine

Bromine is a halogen sanitizer chemically close to chlorine, with a softer feel and milder odor. There is no stabilizer for bromine the way CYA protects chlorine, and direct sunlight destroys roughly half of it in 60 to 90 minutes, so it belongs in indoor pools and hot tubs.

Cal-hypo (calcium hypochlorite)

Cal-hypo is calcium hypochlorite, a granular chlorine sold at about 50–73% strength and often bagged as pool shock. It dissolves fast and raises free chlorine quickly, but every dose also adds calcium hardness, and it raises pH.

Dichlor (granular stabilized chlorine)

Dichlor is a granular chlorine, about 56–62% strength, that adds cyanuric acid (CYA) with every dose. It raises free chlorine and stabilizer together, which makes it a startup tool rather than an everyday chlorine or a routine shock.

Liquid chlorine (sodium hypochlorite)

Liquid chlorine is sodium hypochlorite in water, sold for pools at about 10–12.5% strength. Household bleach is the same chemical at about 6–8.25%. Liquid chlorine adds only chlorine and a trace of salt, so nothing accumulates in the pool water.

Mineral system (copper ionizer)

A mineral system or ionizer (Nature2, Pool Frog, electronic copper-silver ionizers) releases copper, and sometimes silver, into pool water. Copper is a real algaecide, but copper is not a sanitizer, so a mineral pool still needs chlorine. It just runs a lower level.

Salt cell

A salt cell is the part of a salt chlorine generator that makes the chlorine: titanium plates coated with a precious-metal catalyst. It is a consumable rated in hours of chlorine generation, not years, typically 3,000–10,000 hours.

Salt chlorine generator (SWG)

A salt chlorine generator (SWG) runs a low-voltage current through salt dissolved in the pool water and produces chlorine, the same sanitizer (hypochlorous acid) that comes out of a jug of liquid chlorine. A salt water pool is a chlorine pool that makes its own chlorine.

Supplemental sanitizer (ozone, UV, AOP)

A supplemental sanitizer is an ozone, UV or AOP unit on the equipment pad that treats water only as it passes through. It leaves no residual in the pool, so a chlorine or bromine residual is still required. It reduces chlorine demand; it does not replace chlorine.

Trichlor (chlorine tablets)

Trichlor is the slow-dissolving chlorine tablet, about 90% strength, used in a floater or feeder. Every tablet also adds cyanuric acid (CYA) and is acidic, so over weeks CYA climbs while pH and total alkalinity fall.

Equipment & filtration

Pumps, filters, heaters and the words on their labels.

Backwash

Backwashing cleans a sand or DE pool filter by reversing the flow to flush the trapped dirt out to waste. Backwash when the pressure gauge reads about 8 to 10 PSI above the filter's clean baseline, never on a calendar. A cartridge filter does not backwash at all.

Cartridge filter

A cartridge filter is a pool filter built around a pleated element, like a large paper coffee filter folded into a cylinder. Water passes through the pleats and dirt stays behind. There is no multiport valve and no backwashing: you pull the element and rinse it with a hose.

Clean baseline pressure

Clean baseline pressure is the reading on a pool filter's pressure gauge right after the filter is freshly serviced, with the air bled out and the pump at its normal running speed. Every cleaning rule is relative to it: clean the filter when pressure runs about 8 to 10 PSI above that baseline.

DE filter (diatomaceous earth)

A DE filter is a pool filter whose fabric-covered grids are coated with diatomaceous earth, a fine powder of fossilised algae skeletons poured in through the skimmer. That powder coating makes DE the finest of the three filter types. Every backwash flushes the powder out, so every backwash ends with a fresh charge.

Flow rate (GPM)

Flow rate is how much water a pump actually delivers against your system's resistance, measured in gallons per minute (GPM). Pool equipment publishes its minimum flow as a GPM, never an RPM, because the pump speed that produces a given flow depends on your own plumbing.

Flow switch

A flow switch is the sensor on a salt cell or pool heater that stops the device from running until enough water is moving through it. A salt cell stops generating chlorine below its flow switch, and a heater refuses to fire or locks out below its minimum flow.

Glass filter media

Glass filter media is crushed, sized recycled glass that drops into the same tank as filter sand and does the same job. Compared with #20 silica sand, glass filters finer, lasts longer at about 5–7+ years, and weighs about 20% less for the same job.

Micron rating

A micron rating is the particle size a pool filter is said to trap, in microns (µm). The ladder quoted everywhere, sand 20–40, cartridge 10–20 and DE 2–5, is a dealer convention, not a manufacturer specification. Treat a micron rating as a rough ranking, not a measurement.

Multiport valve

A multiport valve is the multi-position valve on a sand or DE pool filter that sends water to FILTER, BACKWASH, RINSE, WASTE or recirculate. A cartridge filter has no multiport valve. Always turn the pump off before changing the valve position.

Pool automation

Pool automation is the controller that schedules and runs the equipment pad from one system: pumps, heater, lights, valves and water features. Pentair, Jandy and Hayward each run their own equipment over a proprietary RS-485 bus, so the automation platform you pick quietly picks the rest of the pad.

Pool heat pump

A pool heat pump moves heat from the outside air into the pool water using a little electricity: a fan pulls air over a coil, and a compressor concentrates that heat and hands it to the water. A heat pump is the cheapest heater to run in mild weather.

Sand filter

A sand filter pushes pool water down through a bed of media, #20 silica sand or recycled glass, and dirt lodges in the gaps between the grains. When the bed fills and the pressure climbs, you backwash: reverse the flow and flush the trapped dirt out to waste.

Self-priming pump

A self-priming pump can pull water up to itself, so it can sit above the pool's water line. Pumps designed for in-ground pools are self-priming. Most above-ground pumps are not: they sit on the ground below the water line and gravity fills them before they run.

Smart Relay

A Smart Relay is what a pool automation controller falls back to when it runs another brand's equipment. A Smart Relay does exactly one thing: cut power on and off. There is no variable-speed scheduling, no colour syncing on lights, and no in-app diagnostics or error reporting.

Total horsepower (THP)

Total horsepower (THP) is the horsepower number on most pool pump spec sheets, and the figure the federal pool-pump motor rule uses to sort motors into size classes. Roughly 1.15 total horsepower matches 0.711 hydraulic horsepower, the size where a new in-ground pump is variable-speed in practice.

Turnover

Turnover is the time it takes a pool's pump and filter to move a volume of water equal to the whole pool. Residential codes use turnover to size equipment: a pump and filter able to turn the pool over in 12 hours or less. Turnover is a sizing rule, not a daily quota.

Variable-speed pump (VS pump)

A variable-speed pump is a pool pump that can be programmed to run at different speeds, so it filters slowly for long hours instead of at one fixed speed. Flow falls in step with speed but power falls much faster, so a slow pump moves more water per kilowatt-hour.

Pool build & materials

Shells, finishes and the features poured once.

Bubbler

A bubbler is a pool water feature that sends an upward burst of water in shallow water. Bubblers are almost always installed in 6 to 12 inches of water, which is why a tanning ledge is their natural home and the two are usually one decision.

Bucket test

The bucket test separates evaporation from a leak. Fill a 5-gallon bucket with pool water to about an inch from the top, set it on a pool step immersed a few inches, mark the level inside and outside, and compare the drops. Equal means evaporation; a bigger drop outside means a leak.

Bump-out

A pool bump-out is an alcove pushed outward from the pool wall, so the seating sits in an added pocket rather than being carved out of the swimming area. Builders also call it a bump-out bench or an extended bench.

Fiberglass pool

A fiberglass pool is a one-piece shell molded in a factory, delivered whole, craned into a prepared hole and backfilled. Its smooth, non-porous gelcoat surface resists algae and needs roughly 25–50% fewer chemicals than concrete, but the shape is limited to factory molds.

Fire bowl

A fire bowl is a gas burner in a bowl, set on the bond beam, a pedestal or a raised wall beside a pool. It is plumbed gas and permanent, and it needs electricity only if it has an igniter.

Gunite

Gunite is the structure of a concrete pool: concrete sprayed over a steel rebar frame to form the shell. Plaster is the finish troweled on top of that shell, so a plaster pool is a gunite pool and the two are not competing choices.

No-see-um screen

No-see-um screen is 20x20 pool-cage mesh: the same fiberglass material as standard 18x14 screen, woven tighter to stop the no-see-ums, gnats and sand flies that walk right through 18x14. The numbers are the mesh count, strands per inch each way.

Pebble finish

A pebble finish is a pool interior with small natural pebbles exposed at the surface, the most durable finish you can put on a gunite pool at 15–25 years. The cement binder is largely washed off during installation, so the water contacts the pebbles instead of cement.

Plaster finish

Plaster, also called white marcite, is the traditional interior finish of a concrete pool: a cement-and-marble-dust coat troweled onto the gunite shell. It is the smoothest finish when new, the cheapest, and the shortest-lived, at 7–12 years before it needs replastering.

Polished finish

A polished finish is the fourth rung of the pool-finish ladder, above plaster, quartz and exposed pebble. The finish is allowed to cure and is then ground smooth with diamond abrasive discs, and its real argument is that it can be re-polished years later rather than replastered.

Pool cage

A pool cage is a screen enclosure over a pool: aluminum frames with screen mesh stretched across them. It keeps most leaves, windblown debris and biting insects out, and the Florida Solar Energy Center measured a standard screen blocking 30–40% of the sun's energy on a clear day.

Pre-gunite inspection

The pre-gunite inspection is the first of four inspections in a standard in-ground pool build (pre-gunite, pre-deck, pre-plaster, final). With the rebar tied and every pipe in the open, it verifies pipe sizes, the number and location of main drains, skimmers and returns, and the step and bench geometry against the approved plan.

Quartz finish

A quartz finish is pool plaster with crushed quartz blended in, which makes it tougher, more stain-resistant and better at holding color than plain plaster. It sits between plaster and pebble on the finish ladder for price and lifespan, at about 12–15 years.

Sheer descent

A sheer descent is a pool water feature: a wide slot that drops a flat sheet of water, the waterfall-curtain look. It is sold by width, but flow is what makes it work, at roughly 12 gallons per minute per linear foot for a clean sheet.

Sun shelf

A sun shelf, also called a tanning ledge or Baja shelf, is a shallow platform inside or beside the main pool, typically holding somewhere between 6 and 15 inches of water, meant for lounging rather than swimming. The three names describe the same feature.

Vinyl-liner pool

A vinyl-liner pool is an in-ground pool built from steel or polymer wall panels set on a footing, with a custom vinyl liner, typically 20–30 mil, fitted inside to hold the water. It is the least expensive in-ground type, but the liner is a consumable replaced every 7–10 years.

Waterline tile

Waterline tile is the band of tile at the waterline that almost every pool has, whether the surface below it is plaster, pebble, fiberglass or full tile. It is the most-seen strip of the pool and takes the worst of the scale, oils and sun.

Above-ground pools

The terms that only apply when the pool stands on the ground.

Easy Set pool

An Easy Set pool is an above-ground pool whose wall is a soft liner lifted by an inflatable ring as the pool fills. Intex says one is ready for water in about ten minutes on level ground, and its manual sets the recommended fill line at the bottom of the inflated ring.

Metal-frame pool

A metal-frame pool is an above-ground pool that hangs a soft liner on a steel tube frame. Against an inflatable-ring Easy Set it gives more depth and a real rectangular swim area, at the cost of a longer, fiddlier build.

Pool barrier

A pool barrier is the fence, wall or railing that keeps children out of a pool. Under the ICC's model pool code (ISPSC section 305) it stands at least 48 inches, measured on the side facing away from the pool, passes no 4-inch sphere, and has a gate that opens outward, self-closes and self-latches.

Rigid-wall pool

A rigid-wall pool is a permanent above-ground pool with a steel or resin wall, and it is a real installation: base, wall, coving and liner. Rounds run 12 to 33 feet in 3-foot increments, and most are now offered with 52- or 54-inch walls.

Semi-inground pool

A semi-inground pool is a rigid-wall above-ground pool recessed into the ground rather than standing on it, and only a model the manufacturer rates for burial should be buried. It buys the look of an in-ground pool, not the features: the floor is still flat and a vinyl liner still hangs inside a wall.

Covers, closing & opening

The calendar jobs and the gear they use.

Air pillow

An air pillow is an inflatable cushion tied off at the centre of an above-ground pool under the winter cover. Its documented job is holding the cover up out of the gap between the top rail and the lowered water line, so the cover doesn't sag in and fill.

Hard-freeze close

A hard-freeze close is the full winter shutdown for a pool whose water reliably drops below 60°F for weeks and whose air goes below freezing for sustained stretches: balance the water, lower it, clear or remove the plumbing, and cover the pool. Warm-climate pools do not close; they run in winter mode.

Mesh pool cover

A mesh cover is a pool cover that lets rain and snowmelt drain through it, so there is no standing water to pump and no weight load building on the cover. The price is a dirtier spring opening, because fine debris and some sunlight get through.

Safety cover

A safety cover is an in-ground pool cover, mesh or solid, that anchors into the deck and is certified to ASTM F1346, the standard for holding a person's or a pet's weight without letting them into the water. A winter debris cover that simply lies on the water is not one.

Solid pool cover

A solid cover is a pool cover that blocks sunlight, so the pool opens to cleaner water in spring, while rain and snowmelt collect on top of it instead of draining through. On a basic solid cover with no drain, a cover pump is mandatory.

Winter cover

A winter cover is a debris cover for a closed pool: it keeps leaves and light out and holds water on top, and it does not hold a person or a pet. No above-ground winter cover, mesh or solid, is a safety cover certified to ASTM F1346.

Winterizing

Winterizing is getting a pool through winter, and it is two entirely different jobs depending on where you live. In freeze country it means a hard-freeze close: drain the plumbing so it doesn't split and seal the pool under a cover. In Florida you don't close; you switch to winter mode.

A–Z index