Glossary of Terms

Algae	Microscopic plants, which enter your pool either from the air, pool covers, or bathers. Their most common colour is green but yellow black strains occur too. Algae can cling in an unattractive manner on the pool walls and other surfaces.
Backwash	A procedure whereby the circulation through a filter is reversed to waste, to flush away contaminants.
Bather Load	The number of people who use a pool or spa in 24 hours.
Bromine	A sanitiser used in pools and spas.
Calcium Hypochlorite	This is an unstabilised form of chlorine that is available in either a granular or tablet form.
Calcium Hardness	One form of hardness in the water. This forms due to the high level of calcium salts in the water.
Chloramines	The result of a reaction between nitrogen compounds (derived from bathers) and chlorine chemicals.
Chlorides	By products in the water resulting from the use of any chlorine product.
Chlorine	The most common sanitiser used in pools. Available in many forms.
Chlorine Demand	The chlorine required to destroy the organic contaminants in the water.
Chlorine Residual (Free Chlorine)	The amount of chlorine remaining after the chlorine added to the pool has reacted with all the contaminants in the pool.
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).
Clarifier	A chemical used to improve the clarity of the pool/spa water.
Comparator	An item of testing equipment used to visually test pool water for parameters such as free chlorine, total chlorine, total bromine and pH.
Cyanuric Acid	A stabiliser either combined with dichlor or trichlor to help prevent chlorine loss in sunlight.
Dechlorination	The process of chlorine removal (usually by the addition of sodium thiosulphate) when too much chlorine has been added to the pool.
Dichlor	A common name for Sodium Dichloroisocyanurate Dihydrate. A popular granular form of stabilised chlorine used as a santiser in a pool or spa. It contains 55% available chlorine.
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.
Dry Acid	Sodium bisulphate, which is used to reduce the pH and alkalinity of pool water.
Hard Water	When rainwater passes the through rocks (e.g. chalk and limestone), mineral salts such as calcium and magnesium carbonate dissolve into it. This then continues 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.
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 amount of acid and alkalinity in the 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.
pH Increaser (pH +)	An alkaline substance (usually sodium carbonate) used to increase the pH of pool water.
pH Decreaser (pH -)	An acidic substance (such as sodium bisulphate) used to decrease the pH of pool water.
Photometer	An item of digital testing equipment use to test parameters such as pH, alkalinity and hardness.
Sanitiser	A substance, which kills and destroys bacteria and viruses in the water, keeping it clean and healthy.
Shock Treatment	Shock treatment is 'boosting the chlorine level'. When algae is present it may be necessary to increase the chlorine level from 1-3 p.p.m to 5-10 p.p.m to kill the algae. The amount of shock required depends on the amount of algae present. It may be necessary to increase the chlorine level to 30 p.p.m in some cases. This must be done when no one is in the pool and the pool should only be used again when the chlorine level has returned to normal.
Skimmer	A fixture fitted on the water level to remove contamination from the top level of water.
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.
TDS	Known as Total Dissolved Solids. This is a measurement of everything that is dissolved in the water. TDS is measured in p.p.m (or mg/l). The normal maximum is the incoming mains water TDS plus 1000 p.p.m.
Total Alkalinity	For pools it is a measure of the bicarbonate in the pool water.
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.
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.
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 llangellier 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.