Algae: Green, black or brown microscopic plant life which is nourished by sunlight.

Algicide: A chemical or process for killing algae. An algistat is an agent for preventing their growth.

Alum: A flocculating agent. Potassium and ammonium alum are the most common types used in the treatment of pool water. Aluminum sulphate is often used with gravity sand filters.

Amps: Electrical power.

Atmospheric Pressure: Sea level atmospheric pressure is approximately 14.7 PSI or 14.7 X 2.31 = 34 feet of head. This means, at sea level, the surface of the water in a pool supports a weight of air equivalent in head to 34 feet of water.

Automatic Chlorination: The feeding of a sanitizer to a pool or spa through a manual or electronic device.

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

Backwash Cycle: The time required to backwash thoroughly the filter medium and/or elements by the reverse flow of water through the filter.

Backwash Rate: The rate of flow of water through the filter during the backwash cycle, expressed in U.S. gallons per minute per square foot of effective filter area.

Body Feed: The continuous addition of small amounts of filter aid during the operation of a diatomaceous earth filter.

Bromide: A compound of bromine. Two of the salts, Sodium and Potassium Bromide, are sometimes used to produce a disinfectant or algaecide.

Bromine: Works as a sanitizer or disinfectant to kill bacteria and algae in pool and spa water. It does not eliminate swimmer waste unless it is combined with an oxidizer. It is very vulnerable to direct sunlight, therefore it's not efficient in outdoor applications. The ideal range is from 2.0 to 4.0 ppm for pools and 3.0 - 5.0 ppm in spas.

Calcium hardness: The total amount of calcium and magnesium in pool water. The ideal range is from 200 to 400 ppm in both pools and spas.

Calcium hypochlorite: A very common form of chlorine used in pools and spas. A granular or tablet substance usually dissolved in water just prior to adding it to pool and spa water.

Cartridge: A depth, pleated, or surface type filter component with fixed dimensions and designed to remove suspended particles from water flowing through the filter.

Cavitation: Occurs when atmospheric pressure is unable to push water to the pump as fast as the pump can discharge it.

Centrifugal Pump: There are two types: straight centrifugal (non self priming) and self priming centrifugal. Centrifugal incorporate an impeller which sling the water to create flow.

Chemical Feeder: Any device to feed chemicals, but usually one feeding alum, acid, chlorine, filter aid, algaecide, or soda ash. Included in this category are proportioning pumps, injector type feeders, pot type feeders, operating from a pressure differential, and dry type feeders.

Chloramines: An inefficient disinfectant formed when chlorine has combined with ammonia and nitrogen in pool and spa water. It has a foul, "chlorine" odor and can cause skin and eye irritation.

Chlorinator: A device to feed, regulate the flow, and measure the amount of chlorine gas introduced into the water being treated.

Chlorine: Works as a sanitizer or disinfectant in pool and spa water to kill bacteria and algae, and oxidizes ammonia and nitrogen compounds. The ideal range is from 1.0 to 3.0 ppm in pools and 1.5 - 3.0 ppm in spas.

Chlorine gas: The most pure form of chlorine and it can only be applied to pools by a trained professional. It is not an appropriate sanitizer for spas.

Circulation System: An arrangement of mechanical equipment or components, connected by piping to a pool or spa in a closed circuit. The function of a circulation system is to direct water from the pool or spa, causing it to flow through the various system components for purposes of clarifying, heating, purifying and returning the water back to the origin' body of water.

Colorimetric tests: A test for chlorine, bromine and pH where the reagent causes a change in color when reacting with the specified chemical.

Corrosion: The etching or oxidation of a material by chemical action.

Cyanuric acid: A stabilizer that works to keep a reserve of "free available" chlorine in pool and spa water, protecting it from direct sunlight. Because their exposure to sunlight is limited, it is not frequently used in spas. It is, however, present in some sanitizers such as trichlor and dichlor. The ideal level is from 30 to 100 ppm in both pools and spas.

Design Rate of Flow (Design Filter Rate): The average rate of flow in a system which is used for design calculation (usually the flow in gallons per minute divided by the effective filter area in square feet).

Diatomite: The filtering medium of a diatomaceous earth filter composed of microscopic fossil skeletons of the "diatom", a tiny freshwater marine plankton.

Discharge Head: The total head, including static head and friction head, on the discharge side of the pump.

Distributor (Top or Bottom): The device in a filter designed to divert the incoming water to prevent erosion of the filter media.

DPD: A common chlorine or bromine test using color change as an indicator. It allows for separate free available and combined chlorine testing.

Effluent: The water that flows out of a filter, pump or other device.

Electrolysis: Decomposition of metal due to flow of electrical current.

End-point reaction: The resulting color change in a test sample created when you add drops of a given reagent. The number of drops correlates with a measurement.

Equivalent Length of pipe: Friction loss in valves and fittings expressed in equivalent length of stright pipe in feet.

Feet of Head: A basis for indicating the resistance in a hydraulic system, equivalent to the height of a column of water that would cause the same resistance (100 feet of head = 43.3 psi, 1 foot of head = .433 psi).

Filter: A device that removes undissolved particles from water by recirculating the water through a porous substance( filter medium or element).

Filter Aid: A type of finely divided media used to coat a septum type filter, usually diatomaceous earth or volcanic ash. (Note: Alum, as used on the bed of a sand filter, is also referred to as filter aid.)

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 or spa. A filter element usually consists of a septum support, or a cartridge.

Filtration Flow: The designed rate of flow, in volume per time (gpm, gph) through the filter system.

Filtration Rate: The rate of filtration of water flowing through a filter during the filter cycle expressed in U.S. gallons per minute per square foot of effective filter area.

Flocculating Agent: A chemical substance or compound that promotes the combination, agglomeration, or coagulation of suspended particles in water.

Free available chlorine: The killing, active form of chlorine.

Freeboard: The clear vertical distance between the top of the filter medium and the lowest outlet of the upper distribution system in a permanent medium filter.

Friction Loss: The friction or resistance between the inner surfaces of pipe and the water flowing through it.

Gallons per minute: Expressed as gpm, refers to a quantity of water moving in a given period of time.

Lithium hypochlorite: A granular form of chlorine used in pools and spas. It is known for its tendency to dissolve quickly.

Muriatic acid: A liquid acid that is most commonly used to reduce pH and total alkalinity levels. It tends to be very strong, and is not recommended for use in spas.

OTO: A common chlorine or bromine test using color change as an indicator. This test will not separate free available chlorine from combined chlorine.

Ozone: A powerful gas that acts as a sanitizer and oxidizer, but is highly unstable and cannot be used to create a sanitizer residual.

pH: A level determining the acidic or basic quality of pool and spa water. The ideal range is from 7.4 to 7.6 in pools and 7.2 to 7.8 in spas.

Parts per million (ppm): A unit of measurement used for chlorine, bromine, total alkalinity, calcium hardness and total dissolved solids.

Phenol red: A common pH test that uses color change as a indicator.

Scale formation: This usually occurs on the walls of the pool or spa when there are high levels of calcium hardness, total dissolved solids, pH and total alkalinity.

Shocking: Adding a large dose of chlorine or other chemical's to quickly increase the amount of free available sanitizers compared with the less effective, "combined" form of the sanitizer.

Soda ash: A substance used to raise pH and total alkalinity. It is not recommended for use in spas.

Sodium bicarbonate: Otherwise known as household baking soda, this substance is used to raise pH and total alkalinity levels in pools and spas.

Sodium hypochlorite: Otherwise known as common household bleach, this is the most common form of liquid chlorine used in pools and spas.

Test strips: Chemically treated strips that have the appropriate amounts of reagents on them. Simply dip them into the water and read the reactions. These strips can test free available and total chlorine, bromine, pH, calcium hardness, total alkalinity and cyanuric acid.

Titration test: A test used for acid and base demands, total alkalinity and calcium hardness. It creates an end-point reaction by adding drops of the reagent to elicit a change in the sample's color. The number of drops required correlates with the measurement.

Total alkalinity: Works in a buffering capacity, protecting the water from dramatic pH changes. The ideal range is from 80 to 140 ppm in pools and 80 to 120 ppm in spas.

Total dissolved solids: The total amount of dissolved materials in pool or spa water. The ideal range is from 1,000 to 2,000 ppm in pools and 1,500 ppm above the start-up TDS in spas.

Turbidity tests: Used to test the amount of cyanuric acid in the water. The reagent causes a cloudiness of the test water. Measurement is obtained by comparing the test water's visibility (the degree of clarity of a dot in the test vial) to the test kit manufacturer's chart.

Water balance: This balance is reached when all elements (pH, total alkalinity, calcium hardness and total dissolved solids) are within their proper ranges.