Pool Service Glossary: Key Terms and Definitions
Pool service is a technically dense field that spans water chemistry, mechanical systems, regulatory compliance, and physical infrastructure. This glossary compiles the core terminology used by technicians, inspectors, contractors, and facility operators across the residential and commercial pool service industry in the United States. Precise vocabulary is foundational to accurate diagnosis, compliant chemical handling, and clear communication between service professionals and the clients and agencies they work with. The definitions below reflect standard industry usage as codified by the Association of Pool & Spa Professionals (APSP), the Pool & Hot Tub Alliance (PHTA), and applicable federal safety frameworks.
Definition and scope
A pool service glossary establishes standardized definitions for terms encountered during routine maintenance, chemical treatment, equipment service, and regulatory compliance activities. The scope of applicable vocabulary spans at least 5 distinct technical domains: water chemistry, filtration systems, hydraulics, heating and supplemental treatment, and structural surface classification.
Terminology in this field carries operational weight. A technician who conflates "combined chlorine" with "free chlorine" will misread a test result and underdose or overdose a system. A service professional who misuses "turnover rate" may configure pump schedules incorrectly, creating conditions that violate the Model Aquatic Health Code (MAHC) published by the Centers for Disease Control and Prevention (CDC). The MAHC is the primary federal reference framework for public aquatic venue health and safety standards in the United States, though adoption is implemented at the state or local level.
For a broader orientation to the field, the conceptual overview of how pool service works provides context on where these terms apply across the full service lifecycle.
How it works
Pool service terminology is organized around functional subsystems. Each term below is classified by domain, and where two related terms are commonly confused, a contrast is provided.
Water Chemistry Terms
Free Chlorine (FC): The concentration of chlorine available to sanitize water, measured in parts per million (ppm). PHTA guidelines for residential pools typically target a free chlorine range of 1–3 ppm.
Combined Chlorine (CC): Chlorine that has bonded with nitrogen-containing compounds (ammonia, urea) and lost most sanitizing capacity. CC is also called chloramines. Combined chlorine above 0.2 ppm is the threshold at which the CDC MAHC recommends remediation action.
Total Chlorine (TC): The sum of free chlorine and combined chlorine. Formula: TC = FC + CC.
Cyanuric Acid (CYA) / Stabilizer: A chemical that slows UV degradation of chlorine. Excessive CYA (above 100 ppm) reduces chlorine efficacy, a condition called "chlorine lock." The CDC MAHC recommends a CYA maximum of 90 ppm for public pools.
pH: A logarithmic scale (0–14) measuring hydrogen ion concentration. The standard operational range for pool water is 7.2–7.8. Pool water outside this range causes equipment corrosion, surface damage, and reduced sanitizer effectiveness.
Total Alkalinity (TA): A measure of water's resistance to pH change (buffering capacity), expressed in ppm. The recommended range is 80–120 ppm for most pool types.
Calcium Hardness (CH): The concentration of dissolved calcium in pool water. Low CH (below 150 ppm) causes water to leach calcium from plaster surfaces; high CH (above 400 ppm) produces scale.
Langelier Saturation Index (LSI): A calculated index that predicts whether pool water will be corrosive, balanced, or scale-forming. LSI values between -0.3 and +0.3 are generally considered acceptable. Understanding water chemistry fundamentals is expanded in the pool water chemistry fundamentals resource.
Breakpoint Chlorination (Superchlorination): The process of adding sufficient chlorine to oxidize all chloramines, restoring free chlorine effectiveness. Breakpoint is reached when TC drops and FC rises sharply, typically at a chlorine dose 10 times the combined chlorine concentration.
Oxidation-Reduction Potential (ORP): An electrochemical measurement (in millivolts) of water's ability to oxidize contaminants. An ORP of 650–750 mV is the operating target for automated controller systems. ORP is used in automated chemical feed systems alongside pH probes.
Filtration Terms
Turnover Rate: The time required to filter the entire pool volume once. Calculated as: Pool Volume (gallons) ÷ Flow Rate (gallons per hour). A 6–8 hour turnover is standard for residential pools; commercial pools often require a 4-hour or shorter turnover under state health codes.
Filter Media: The material within a filter that traps particulate matter. The three primary media types are:
- Sand — Typically #20 silica sand, rated to trap particles 20–40 microns in size.
- Diatomaceous Earth (DE) — Ground fossilized diatom shells; filters particles as small as 2–5 microns.
- Cartridge — Pleated polyester fabric, no backwash required; rated 10–15 microns.
For a full breakdown of filter type service requirements, see pool filter service types and protocols.
Backwashing: The process of reversing water flow through a sand or DE filter to flush accumulated debris to waste. Backwashing is initiated when filter pressure rises 8–10 psi above the clean starting pressure.
Filter Pressure (Head Pressure): The resistance measured at the filter pressure gauge in pounds per square inch (psi). Elevated pressure indicates a dirty filter; low pressure can indicate a flow restriction before the filter or pump cavitation.
Hydraulics and Equipment Terms
Flow Rate: The volume of water moved by a pump per unit of time, measured in gallons per minute (GPM) or gallons per hour (GPH).
Head Loss: Total resistance to flow in a hydraulic system, expressed in feet of head. Head loss is the sum of friction losses from pipe diameter, length, fittings, and equipment resistance.
Priming: The process by which a pump fills its volute with water to generate suction. Loss of prime is a common failure mode caused by air leaks in suction-side plumbing or low water level.
Cavitation: A damaging condition where vapor bubbles form in the pump impeller due to insufficient inlet pressure. Cavitation produces a grinding noise and causes rapid impeller wear. Variable-speed pump selection and impeller sizing affect cavitation risk, which is covered in detail at variable-speed pump service considerations.
Impeller: The rotating component inside the pump volute that creates centrifugal force to move water.
Volute: The casing surrounding the impeller; its spiral shape converts kinetic energy to pressure.
Skimmer Weir: A floating flap at the skimmer opening that maintains surface suction while allowing debris to pass into the skimmer basket.
Main Drain (Suction Outlet): The fitting(s) at the pool floor that draw water to the pump. Following the Virginia Graeme Baker Pool and Spa Safety Act (VGB Act), all public pool suction outlets must be equipped with anti-entrapment drain covers compliant with ANSI/APSP-16 or ASME A112.19.8.
Anti-Entrapment Cover: A drain cover engineered to prevent body and hair entrapment, as mandated by the VGB Act enforced by the U.S. Consumer Product Safety Commission (CPSC).
Sanitizer and Treatment System Terms
Salt Chlorine Generator (SCG) / Salt Water Chlorinator (SWC): A device that electrolyzes dissolved sodium chloride (NaCl) in pool water to produce hypochlorous acid. Typical operating salt concentration is 2,700–3,400 ppm. SCG systems do not eliminate the need for pH management or supplemental oxidation.
UV System: An ultraviolet light system installed inline to neutralize microorganisms by disrupting DNA replication. UV does not maintain a residual chlorine level and must be paired with a primary halogen sanitizer. Protocols for servicing these systems are detailed at UV and ozone system service protocols.
Ozone Generator: A device producing ozone gas (O₃) as an oxidizer and supplemental sanitizer. Ozone destroys chloramines and organic contaminants but dissipates rapidly and cannot serve as a standalone sanitizer under the MAHC.
Phosphate Remover: A chemical treatment that precipitates orthophosphates out of solution, reducing the nutrient load that feeds algae. Phosphate levels above 500 ppb are associated with elevated algae risk.
Structural and Surface Terms
Marcite / White Plaster: The standard interior pool finish composed of white cement and marble dust. Typical service life is 7–15 years before replastering is required.
Pebble Finish (Pebble Tec): An aggregate finish incorporating small pebbles into the plaster matrix; more durable than standard marcite, with a typical life of 15–25 years.
Fiberglass Shell: A gel-coat fiberglass interior; non-porous, lower chemical demand, but susceptible to gel-coat osmotic blistering if water chemistry is not maintained. Fiberglass pools represent one of the 3 primary pool construction types alongside vinyl liner and concrete. The implications of surface type for service protocols are addressed at pool surface types and service implications.
Vinyl Liner: A flexible PVC membrane that forms the pool interior. Liners are subject to fading, tearing, and separation at bead