Algae Microscopic plants, which continuously enter your pool from the air and particularly from rain. The most common colour is green but reddish brown and yellow strains (known as mustard algae) also occur. Algae can grow on pool walls and surfaces or float freely in the water
Aluminium Sulphate A flocculant which improves the clarity of pool water. See 'flocculant'.
Ammonia A chemical that forms from the breakdown of urea in urine and sweat.
Amperometric Pool water analysers that directly measures the concentration of chlorine (hypochlorous acid) using a platinum and a copper electrode. This is the most reliable method of controlling chlorine residuals in busy pools.
Backwashing A procedure to clean sand filters whereby the circulation through a filter is reversed to flow to waste, to flush away the accumulated contaminants.
Balance Tank A reservoir of water between the pool itself and the rest of the circulation system. It is used on deck level pools to maintain a constant pool water level when bathers enter the pool or water is used for backwashing.
Bather Load The number of people who use a pool or spa in a defined time period of 24 hours.
Bio-film The microbial growth which grows on the inside of water pipes particularly in spas and hot tubs which can provide a breeding ground for organisms such as legionella.
Bromine A disinfectant used in pools and particularly in spas.
Calcium Chloride A chemical which increases the calcium level in pool water.
Calcium Hardness The quantity of calcium dissolved in the water. High levels can promote scale and cloudy water. Low levels can lead to corrosion and grout loss. Calcium hardness is naturally present in hard mains water. It needs to be added in soft water areas either by adding calcium chloride or by using calcium hypochlorite. The minimum level suggested is 250 mg/l, although the preferred level is 500 mg/l.
Calcium Hypochlorite This is an unstabilised form of chlorine that is available in either a granular or tablet form.
Chloramines The result of a reaction between nitrogen compounds (derived from bathers) and chlorine chemicals. Chloramines exist as either monochloramine, dichloramine, or nitrogen trichloride. Nitrogen trichloride is an irritant and has a pungent smell, in high concentrations in badly run pools it has been associated with causing asthma or chlorine coughing. Chloramines have little disinfecting powers.
Chlorine The most common disinfectant used in swimming pool water to destroy or inhibit bacterial and algal growth in addition to oxidising unwanted organic matter. Available as a un-stabilised as a liquid or solid and stabilised chlorinated cyanurates.
Chlorine Demand The amount of chlorine required to destroy the organic contaminants in the water before an active chlorine residual can be achieved.
Clarifier A chemical used to improve the clarity of the pool/spa water. See 'flocculant'.
Coliforms Bacteria of the intestine of which E coli is a common example.
Colony Counts An estimate of the number of bacteria of all sorts in pool water.
Combined Chlorine A product of the reaction between free chlorine and contaminants in the water. It consists of mono-chloramine (which gives the distinct chlorine swimming pool smell), dichloramine and nitrogen trichloride (which is very irritating to bathers eyes and noses). Measured by the difference between the No.1 and No.3 DPD tests.
Comparator An item of testing equipment used to visually test pool water for parameters such as free chlorine, total chlorine, total bromine and pH.
Cryptosporidium Cryptosporidium is a unicellular parasite that is not easily killed by chlorine or bromine and that causes a gastro-intestinal illness.
Cyanuric Acid A stabiliser usually added combined in stabilised chlorine (dichlor or trichlor) to help prevent chlorine loss in sunlight. It can also be added in it's pure form as cyanuric acid. Too much cyanuric acid can cause chlorine to work less effectively as a sanitiser (see 'chlorine lock'). The only way of removing cyanuric acid from a pool is by dilution with fresh water.
Dechlorination The process of chlorine removal (usually by the addition of sodium thiosulphate) when too much chlorine has been added to the pool.
Deck Level A type of pool design with the pool water at the same level as the pool sides.
Dichlor A common name for Sodium Dichloroisocyanurate Dihydrate. A popular granular form of stabilised chlorine used as a sanitiser in a pool or spa. It contains 55% available chlorine. Also known as troclosene sodium.
Dilution Replacing pool water with mains water to reduce the soluble pollution and by-products which build up in the pool. For public pools it is recommended that 30 litres of pool water is replaced for each bather visit.
DPD An abbreviation for Diethyl-p-Phenylene Diamine. A chemical indicator, which measures the amount of free chlorine/total bromine in your pool or spa.
E.Coli A species of coliform bacteria spread from person to person and common in water.
Filter Media Material that makes up the filter element. Sand and glass are two main materials used.
Free Chlorine (Also known as Free Available or Free residual Chlorine) The amount of chlorine remaining after the chlorine added to the pool has reacted with all the contaminants in the pool. That is to say the amount which is available to kill bacteria and viruses; and oxidise other pollutants. Tested for with DPD No.1 test tablets.
Hard Water When rainwater passes the through soluble rocks (e.g. chalk and limestone), mineral salts such as calcium and magnesium carbonate dissolve into it. This is then carried into the water supply and in turn into the pool/spa water. The hardness of the water is expressed in parts per million (p.p.m) and anything over 250 p.p.m is classed as hard water. Medium to Hard water is good for swimming pools.
Humidity A measure of the moisture in the atmosphere.
Hydrochloric Acid An acid used (with care) to lower pool water pH value and alkalinity.
Hypochlorous Acid The active form of chlorine which kills bacteria and viruses as well as oxidising other pollutants.
Inlets The pipes that return clean water from the filters into the pool basin.
Langelier Index A measure that indicates if pool water is corrosive to the pool systems or is likely to cause scaling. Originally developed for industrial heat exchangers and of limited value for pool water.
Legionella Bacteria that can cause respiratory infections. The full name is Legionella pneumophila.It is transmitted in droplets which are breathed in from water sprays. It is a more common problem in spas which run at a temperature ideal for its growth. Legionella breeds under the layer of biofilm which can grow on the hidden pipes, particularly in spas. There are proprietary treatments to remove this biofilm when the spa is emptied.
Nitrogen Trichloride The most irritant of the chloramines. Causes asthma like symptoms when present in high concentration. Also causes corrosion of stainless steel.
Outlets The ducts that carry water from the pool basin.
PAC - Polyaluminium Chloride A coagulant (flocculant) used on commercial pools that enhances the removal of suspended solids. It should be dosed continuosly (ideally with a peristaltic pump) before the filters at a rate of 0.1 ml per m³/hr of circulation rate
Palin Index An expression of the scaling or corrosivity of water calculated from the pH, calcium hardness,alkalinity, TDS and temperature of pool water. (see 'Langelier index')
Parts per million (p.p.m) A unit of measurement used to represent a small amount of substance evident in a mixture or solution. For example 2 p.p.m available chlorine in water. It is the same as mg/l (milligrams per litre).
pH A scale used to measure the acidity or basicity of water. The scale runs from 0 to 14 with 7 being neutral. Anything less than 7 is classified as being acidic and anything greater than 7 is more alkaline. Pools using chlorine as the sanitiser should maintain a pH between 7.2 and 7.8 (ideally 7.4 to 7.6). Pools using bromine should maintain pH between 7.2 and 8.2.
Photometer An item of digital testing equipment use to test parameters such as pH, alkalinity and hardness.
Pseudomonas A bacterium resulting from heavily contaminated pools. Can cause folliculitis and ear infections. Killed by chlorine.
Redox see 'ORP'.
Saturation Index Usually refers to the Langeller method of water balance testing.
Sanitiser A disinfectant which kills bacteria and viruses in the water quickly, keeping it clean and healthy and preventing diseases being passed between bathers in the pool water.
Scaling The deposition of usually calcium carbonate on pool walls, pipework etc.
Sequester or Chelate Specially formulated chemicals used to hold metals, e.g. iron, copper, or calcium in solution to prevent scaling or staining.
Shock The process of significantly boosting the quantities of a sanitizer in the pool water to bring about the chemical destruction (oxidation) of excess organic matter (such as algae) or high combined chlorine levels. (see 'Superchlorination')
Shock Chlorine A misnomer for calcium hypochlorite
Skimmer A fixture fitted on the water level to remove contamination from the top level of water, which contains most of the pollution.
Sodium Bicarbonate Also known as baking soda. A chemical which increases the alkalinity level in pool water.
Sodium Bisulphate Also known as Dry Acid or pH minus. An acidic substance used to decrease the pH and alkalinity of pool water.
Sodium Carbonate Also known as Soda Ash or pH plus. An alkaline substance used to increase the pH of pool water.
Sodium Hypochlorite A liquid form of chlorine.
Sodium Thiosulphate A chemical used for removing excess chlorine from the pool water. The correct amount (calculated from the chlorine concentration) must be added carefully and accurately.
Soften In relation to pool water it is the removal of water hardness. The softening of swimming pool water is not recommended.
Soft Water Water, which has less than 50 p.p.m of hardness is normally considered to be soft.
Stabilised Chlorine Chlorine donors such as 'Dichlor' and 'Trichlor'. They contain cyanuric acid, which prevents the rapid break down of chlorine in sunlight.
Superchlorination Adding an extra large dose (5 to 10 ppm) of chlorine to the water to destroy ammonia, nitrogen and swimmer waste, which can build up in the water.
Total Alkalinity For pools it is a measure of the bicarbonate in the pool water.
Total Bromine A measurement of all the active and inactive bromine compounds in pool water as measured by DPD No. 1 test tablets.
Total Chlorine The total amount of combined chlorine and free chlorine in the water. Measured by addition of a DPD 3 tablet to the normal DPD 1 test.
Total Dissolved Solids - TDS This is a measurement of everything that is dissolved in the water. TDS is derived mostly from the chemicals used in the pool. TDS is measured in p.p.m (or mg/l). The normal maximum is the incoming mains water TDS plus 1000 p.p.m. TDS can only be reduced by dumping pool water and replacing with fresh water.
Trichlor A commonly used name for trichloroisocyanuric acid. This is a popular stabilised chlorine donor for domestic pools. Commonly sold in slow dissolving tablets, trichlor provides 90% of its weight as chlorine.
Trichlorinator A flow erosion device for feeding trichloroisocyanuric acid tablets into the pool circulating system.
Tri- Tabs Term for trichloroisocyanuric acid tablets - a chlorinated isocyanurate with 90% available chlorine.. Tablet sizes available are commonly 20g (mini-tabs) and 200 g (maxi-tabs).
Turn-Over The time taken to theoretically circulate all the water in the pool once through the filters (pool volume divided by water circulation rate).
Water Balance The overall condition of the water, taking into account all parameters such as alkalinity, pH, total hardness etc. Balanced water is said to be neither corrosive or scaling. It is measured using Palin index or Langelier index.
Water Volume The amount of water in your pool or spa. The volume of your pool or spa can be calculated by using our dosing calculator.