Technical Glossary
A
Acids: These are chemical compounds whose water-based solutions have a sour taste, turn blue litmus paper red, and can combine with metals to form salts.
Agitator: When used in connection with swimming pools, an agitator is either a manual paddle driven by a handle, or a motor driven device fitted on top of a mixing tank, to agitate chemicals when mixed with water prior to being pumped into the pool. Chemicals such as calcium hypochlorite, soda ash and alum are examples of those used in pool water treatment, but require effective mixing to prevent solids from clogging dosing-pipe runs to injectors.
Air Release Valves: These valves are usually fitted at the highest point on a water filter to release unwanted air in a system. They can be automatic - releasing air when a set pressure has been reached, or manually controlled by a valve.
Algae: This is microscopic aquatic plant life that can grow in water where there is a combination of warmth and light.
Algaecide: A chemical compound that is used to kill or inhibit the growth of algae.
Alkali: These are bases that dissolve in water.
Alkalinity: The alkalinity of a water is a measure of the alkaline salts dissolved in it. These are mainly in the form of carbonates and bicarbonates. The higher the alkalinity of the water, the more resistant it is to pH value change. Large fluctuations in pH value or "pH bounce" that take place when changes are made to the level of disinfectant content and pH correction, can be prevented by keeping the total alkalinity of the pool above 75 mg/l as CaCo3. (Note also that effective coagulation does not take place where the total alkalinity is below this level.)
Alum or Aluminium sulphate: This is a chemical that is used as a flocculant (the same as a coagulant) to aid filtration. It produces a floc that forms like a gel in the upper regions of a sand filter bed, and this traps particles that are smaller than the gaps between the sand grains.
Amperometric: This is a method used to determine chlorine levels in some automatic controllers. The chlorine content of the water tested is measured by checking the level of conductivity between two electrodes.
Anhydrous: A dry chemical that contains little or no water in its chemical compound.
Available chlorine: A measure of the oxidising power of a compound.
B
Backwash: This describes the process where filter beds are cleaned with pool water, by reversing the flow of water through the filters and dumping it to waste.
Bacteria: Microscopic organisms, some of which can cause disease. They are introduced to a swimming pool or hot tub by bathers, wind, dust, rain and surface drainage.
Balanced water: This is water which has been subjected to a series of tests and is considered to be chemically balanced for swimming. The Langelier Saturation Index is a series of tests that analyse the levels of alkalinity, calcium hardness, temperature and Total Dissolved Solids to calculate whether the water is aggressive or corrosive.
Balance Tank: or Displacement Tank: This is a vessel that is positioned near to a swimming pool and connected by pipework, to contain the water that is displaced by swimmers entering a pool, and returns to the pool when they leave it. This system is used to keep a pool at the same level, as with level-deck designs.
Bather Load: This refers to the number of bathers using a pool - and these are the the principal source of pool water contamination
Bathing Capacity: The optimum number of bathers that are allowed into a public swimming pool at any one time.
BCDMH: Bromo-chloroi-methyl-hydantoin. This is a bromine sanitiser, normally in the form of bromine sticks. Bromine is a gas of the Halogen group.
Bleach: An alternative name for sodium hypochlorite.
Breakpoint Chlorination: A method of chlorination, where sufficient quantities of chlorine are added to water to oxidise most of the ammonia and organic matter and still leaving a residual of free chlorine (that which is not neutralised.)
Bromamines: Bromamines are products of the reaction between bromine and ammonia, where bromine-based disinfectants are used.
Bund: This is an outer tank, built or installed around a vessel containing liquid chemicals, to contain overflows or spillages. Its capacity should be 10% greater than the total capacity of the tanks or vessels that it contains. Its inner surfaces should be painted with a covering that resists attack by the chemicals contained within it.
C
Calcium chloride flake: This is a common form of calcium chloride (CaCO3) as supplied by manufacturers to increase the calcium hardness of pool water.
Calcium differential: This is the difference between the calcium levels found in a mains or supply water, and that of the pool water. It is generally more important in pools that are not treated by the addition of calcium hypochlorite or calcium chloride.
Calcium Hardness: This is a measure of the calcium salts dissolved in pool water. Calcium hardness is that part of the total hardness consisting of calcium salts, and is the measure that relates particularly to swimming pools. It should be checked weekly, using test tablets.
To raise the hardness of the water, calcium chloride added at the rate of 1.5kg for each 50m3 of pool water will increase the calcium hardness level by around 20mg/l. Where total hardness levels are too high, dilution is the only way to reduce them but only if the mains water is considerably softer.
Guidelines on hardness: Where the calcium hardness level is allowed to fall below about 40 mg/l, the water is likely to be corrosive to the fabric of the pool plant, a concrete tank and tile grouting. At levels above approximately 75mg/l a protective scale can begin to form, and as far as corrosion is concerned, there should be no reason to boost hardness levels beyond that. All elements of a pool plant and building should be able to withstand water at this level, though where where low hardness levels in some pools are blamed for the loss of tile grouting, operators may choose to boost hardness levels as high as 400mg/l - for example by using calcium hypochlorite instead of sodium hypochlorite.
The recommended level is for the calcium hardness of a pool water to be kept in the range 75 to 150 mg/l, as CaCO3.
Carbon Dioxide (CO2): This is a gas used for pH correction. When added to a pool water it form carbonic acid and so lowers pH. It is not considered cost effective to use carbon dioxide in pools where the mains water has total alkalinity of more than 150 mg/l as CaCo3, or a calcium hardness of more than 300mg/l as CaCo3.
Cationic Liquid: This chemical is a positively charged liquid that attracts negative particles within a pool water, and is used as a filtration aid to produce improved water clarity.
Chlorine Demand: This is the amount of chlorine that is required to destroy bacteria and organic matter before a residual of free chlorine is produced.
Chloramines: These are products of the reaction between chlorine and organic nitrogen. As well as reacting with ammonia, chlorine reacts with organic (carbon-based) nitrogen compounds that are derived from proteins in pollution from bathers. The resultant chlorinated organic amines contribute to combined chlorine readings. Though they are stable and promoted, rather than removed, by excess chlorine, only dilution significantly reduces them. However, swimming pools that are treated with ozone, carbon filtration or ultra-violet light can require a lesser degree of dilution.
Chlorine Donor: This is the name given to a chemical that produces hypochlorous acid when added to water.
Coagulant or Flocculant: Coagulants (sometimes called flocculants) enhance the removal of dissolved or suspended material by bringing it out of solution or suspension as solids (coagulation), and then clumping the solids together (flocculation), producing a floc that is more easily trapped in a filter. Coagulants help to collect bacteria generally, but are particularly important in helping to filter out the infective cysts of cryptosporidium and giardia, which are small and resistant to chlorination.
Coagulants are less effective where pH levels are above the recommended levels, and a minimum alkalinity level of about 75 mg/l as CaCO3 is required for effective flocculation.
Combined Chlorine, Free Chlorine, Total Chlorine: Combined chlorine is produced in the reaction between ammonia and excess chlorine, and the process of this reaction is chloramines, which are measured as combined chlorine in standard swimming pool tests.
Combined chlorine plus free chlorine is called total chlorine. Free chlorine is a measure of that which is available for disinfection.
Conversion Table
| From |
Multiplier |
To |
| Kilogrammes |
2.205 |
Pounds |
| Pounds |
0.454 |
Kilogrammes |
| Ounces |
28.35 |
Grammes |
| Grammes |
0.035 |
Ounces |
| Feet |
3.279 |
Metres |
| Metres |
0.305 |
Feet |
| Imperial Gallons |
4.54 |
litres |
| Litres |
0.22 |
Imperial Gallons |
| Imperial Gallons |
0.0045 |
Cubic metres |
| Cubic metres |
220 |
Imperial gallons |
| Imperial Gallons |
0.833 |
U.S.Gallons |
| U.S.Gallons |
1.201 |
Imp. Gallons |
Chlorine-Free: This refers to a range of products or systems that a pool owners can use to treat swimming pool water, instead of chlorine:
- Bromine
- Persulphates (known also as Active Oxygen)
- Ozone
- Copper and Silver
- Polymeric Biguanides (sold as Baquacil and Revacil)
Chlorine Lock: This is a condition where the residual of chlorine has become ineffective due to excessive levels of cyanuric acid (stabiliser).
Clarifier: Compounds used to aid water clarity by coagulating small suspended particles so that they can be arrested in the filter, and subsequently removed by backwashing.
Combined Chlorine: This is formed by the reaction between hypochlorous acid and ammonia/ nitrogenous based compounds such as perspiration. Combined chlorine has very poor disinfecting properties. It is measured by subtracting the figure from a free chlorine test from the figure from the total chlorine test.
Conductivity: This refers to the ability of a water to convey a current between dissimilar metals. The higher the level of Dissolved Solids, the greater the level of conductivity. This is significant as it allows corrosion to take place.
Corrosion: This describes the reduction of metal by chemical or electrochemical attack. Corrosion is greatly affected by high TDS levels, low pH or low calcium hardness. High TDS levels can promote electromechanical activity and allow galvanic corrosion to occur.
Cyanuric Acid: A stabiliser added to pool water to protect the active chlorine against loss by ultra violet radiation from the sun.
D
Dechlorination: If it is necessary to reduce the chlorine residual in a pool due to accidental overtreatment, one can use sodium thiosulphate or hydrogen peroxide. (If the combined chlorine levels of a pool are high, it is better to dump pool water and add fresh water to dilute concentration levels).
Peroxide has the advantage that it is a good oxidiser, has some disinfectant potential and breaks down harmlessly to oxygen and water (though it is more hazardous to handle).
Sodium Thiosulphate can cause cloudiness and raise sulphate levels.
Defoamer: A chemical additive used mainly in the treatment of spa water, used to eliminate foaming by altering the surface tension of the water.
Diatomaceous Earth: An effective rechargeable filter media that is derived from microscopic shell-life that is ground into a powder - often abbreviated to 'D.E.'
Di-Chlor: A short name for sodium dichloroisocyanurate dehydrate, a chlorinated isocyanurate with typically 55% available chlorine and containing cyanuric acid.
Dichloramine (NHCl2): Dichloramine is a chemical produced when monochloramine has one atom of hydrogen replaced by chlorine.
DPD: Short for Diethyl-p-Phenylene Diamine. It is a chemical reagent which reacts with active bromine or chlorine and turns pink. Test kits using DPD have replaced OTO (orthotolidine) which is considered to be carcinogenic.
Dry Acid: A more common name for sodium bisulphate or sodium hydrogenate sulphate, which is used to reduce the pH and total alkalinity levels in a pool.
Dry Chlorine: A chlorine donor available in granular or tablet form. This term usually refers to calcium hypochlorite.
E
Every Day! Check your pool or spa at least every day, and more frequently if it is in use! Regular checks make maintenance tasks quicker and easier, and enable you to deal with any mechanical breakdowns before they get more involved.
F
Filter: A vessel containing a media that removes particulate matter as water passes through it. Filters for water take a range of forms, from gravity filters as used in water works and older outdoor pools, to low, medium and high pressure sand filters and pre-coat or diatomaceous earth filters in modern systems.
Cartridge filters are often used in small domestic pools and spas, and are normally made of paper fibre, pleated to offer a greater surface area. They are cleaned by rinsing carefully under fresh running water, and have a limited life before replacement is required.
Filter media: This is the material used inside the filter to trap oxidised matter and debris. Backwashing releases the dirt by reverse-flowing water through it, and leading it off to drain.
Filtration Rate: The filtration rate is the volume of water that passes through a filter bed in litres per square metre per hour. As the filter bed becomes blocked with debris the flow rate slows down and the volume of filtered water reduces. This can have a significant effect on water quality in a busy pool, as it also affects the speed with which disinfection chemicals are distributed in a pool.
Flocculant: Material used to aid filtration, by trapping smaller particles than can be arrested by the filter media alone.
Flow Rate: The flow of water in litres per minute, or gallons per hour, at the point where it is measured. This can be at the circulation pump or the rate of flow through the filters.
G
Galvanic corrosion: The reaction caused when dissimilar metals in close proximity to each other corrode, with the pool water acting as an electrolyte. Higher conductivity levels in a pool (from high TDS levels) generally accelerate the rate of corrosion.
H
Halogen: This is the chemical family that covers the four gases - chlorine, bromine, iodine and fluorine.
Hard and Soft water: The total hardness of a water is a measure of all its calcium and magnesium salts, such as carbonates, bicarbonates, sulphates and chlorides.
Temporary hardness is that part of the total hardness which precipitates from the water on boiling, and consists of calcium and magnesium carbonates and bicarbonates.
Permanent hardness is the part which does not precipitate from the water on boiling; it consists of other calcium and magnesium salts such as sulphides and chlorides.
Hydrated: A dry chemical that is combined with water in its crystal structure. Hydrated products are more stable than anhydrous products.
Hydrochloric Acid: (HCl): An acid used to lower the ph and/or total alkalinity. It is generally supplied in liquid form and needs caution in handling. Hydrochloric acid is also formed when sodium hypochlorite is added to water to release chlorine for disinfection.
Hydrogen Ion: The hydrogen ion is the atom that produces acidity in water.
Hypochlorous acid (HOCl): The main killing agent present in a chlorinated pool, it is a powerful bactericide and oxidiser.
I
Inorganic Chloramines: Compounds resulting from the chlorination process. These are removed by shock dosing with chlorine.
Isocyanurates: The collective term for the family of chlorine donor products that contain cyanuric acid, typically as dichlor and trichlor.
J
Just Do It! Check it daily, regularly! Check the water quality every day - every few hours if the pool or spa is in use. If it is ignored it will detriorate - guaranteed! Treat it like your pedigree cat, dog or horse - look after it carefully, daily, without fail!
K
Keep it clean! A dirty pool or spa attracts more dirt and debris from users who think that the operator doesn't care. YOU set the standards and others will envy your pool, your commitment and your interest! Show them all!
L
Look, look and look again! In other words - visual checks! Your eyes can tell you a great deal about the water in a pool! At the waterline - is there a build-up of grease or dirt? This harbours germs, looks unsightly and is an extra distraction for the water treatment chemicals' main task - so remove it as soon as you see it!
Is there debris in the pool or on the pool bottom? - just remove it! Is the strainer box blocked with leaves - look and clean it? Does the water flow at the inlets look good? - backwash the filter as it only takes a few minutes! Is the water quality alright? Carry out a water test - look at the results and act accordingly! You don't need to be a chemist to keep your pool or spa in great condition!
M
Make-up water: Potable water or drinking water that is supplied to the pool for filing and topping up as necessary.
Milligrammes per Litre - mg/l: This is the same measurement as Parts Per Million (ppm), and is the standard measure of the chemical content of pool water.
Monochloramine (NH2Cl): A chemical that is produced in pool water when ammonia has one atom of hydrogen replaced by chlorine.
Material Safety Data Sheets: (M.S.D.S.'s): These are the information sheets that should be available with every chemical purchased, to enable users and shippers to know how to use chemicals safely by making them aware of hazards and risks associated with products.
Muriatic Acid: An American term for Hydrochloric Acid.
N
Negative Index: When using the Langelier Saturation Index to assess the condition of a pool water, a negative index indicates that the pool water is corrosive.
O
Organic chloramines: These are Nitrogen bearing compounds such as saliva, urine, perspiration, body oils and sun tan lotions that are continually being introduced into pool water by swimmers. Organic chloramines cannot be shock dosed or filtered out, and therefore must be removed by dilution or ultra-violet treatment,
ORP: The Oxidation Reduction Potential or Redox value is a measure of sanitizer activity. It measures the potential activity of the sanitizer rather than the quantity present.
Oxidation: The process whereby contaminants are burned up by causing them to combine with free available chlorine.
Ozone (O3): Ozone is a powerful bactericide gas. It is very aggressive and cannot be stored easily, so it is generated on site and used as pool water disinfectant. In swimming pools, it is removed before coming into contact with bathers by passing water through charcoal filters to remove the active ozone.
P
Parts per Million (ppm): Used as a unit of measurement is pool water tests, such as chlorine, alkalinity, TDS etc. It is the same unit as mg/l which is the metric equivalent.
PPE: This is the abbreviation used for Personal Protective Equipment, as laid down by Health & Safety legislation, defining the type of protection that is needed for people when involved in hazardous operations.
pH: pH is the measure of the hydrogen ion concentration in a solution. A pH of 1 is a strong acid with many hydrogen ions, and a pH of 14 has few hydrogen ions and is a strong alkali. A pH of 7 is a neutral solution.
pH minus: A common term used to describe a chemical that is used to lower the pH of a pool water ? dry acid such as Sodium Bisulphate is a typical example.
pH plus: A term used to describe a chemical that is alkaline and used to raise the pH of pool water. Soda ash is a typical example.
Phenol Red: This is a chemical reagent used in the measurement of pH. The pH range measured using this method is 6.8 to 8.4.
Pollution: This describes the impurities that are brought into the pool by bathers, pool chemicals and airborne matter.
Poly Aluminium Chloride (PAC): A flocculant chemical that has become popular as a liquid alternative to alum
Poly Aluminium Silicate Sulphate (PASS): Another flocculant chemical that has become popular as a liquid alternative to alum.
Positive Index: When using the Langelier Saturation Index, pool water tests that produce a positive index suggest that the pool water is not in a corrosive state.
Precipitation: A condition where solids that were previously dissolved in solution,fall out of solution and become visible in pool water.
Pressure Differential: This is the difference between the pressure reading before filtration and after filtration, and indicates the level of dirt that is blocking the filter bed and therefore requires backwashing. A freshly washed filter bed should offer minimal resistance to flow through the media.
Q
R
Raw water: The same as Mains water
Redox: See ORP: Oxygen Reducing Potential.
Residual: The amount of a compound existing in pool water, expressed as either milligrammes per litre (mg/l) or parts per million (ppm).
Residual Chlorine: The free chlorine element in a water that is still available for disinfection after the demand for chlorine has been satisfied. The normal free chlorine level that is safe for pools is in the region 0.5 to 2.0 mg/l. Chlorine dosing should cease at 5.0mg/l and swimming stopped if levels reach as high as 10 mg/l.
S
Sanitizer: A chemical added to pool water to act as a disinfectant, e.g. chlorine or bromine. Its purpose is to kill water borne organisms that could be a health hazard to people swimming in it.
Saturation Index: This usually refers to the Langelier method of testing water to keep it balanced for swimming pool use. With this test regime, one should aim for a result between +0.1 and +0.5.
Scale: This is a mineral deposit on surfaces, usually made up of calcium carbonate. It is usually unsightly and can interfere with normal pool operation. In a controlled situation it can be used to inhibit corrosion.
Set Point: The chlorine or pH level that is set on an automatic pool controller as the desired target to operate at.
Shock Treatment: The process of introducing enough sanitizer to a body of water to bring about the chemical destruction (oxidation) of excess inorganic matter or high combined chlorine levels.
Slow Rate Sand Filter: Usually large 'open bed' filters that operate at the filtration rate of 7.5m3/m2/hr.
Soda Ash: Sodium Carbonate.
Sodium Bicarbonate: NaCO3, also known as baking soda, it is used to increase the pH level of pool water.
Sodium Bisulphate: (NaHSO4): Often supplied in granular form and known as Dry Acid, it is used to decrease pH levels in pool water.
Sodium Thiosulphate (Na2S2O35H2O) This is used to neutralise excessive chlorine levels in pool water and should be used with caution.
Soft Water: This is water that contains few calcium and magnesium salts.
Stabiliser: see Cyanuric Acid.
Strainer or Strainer basket: This is usually a steel or plastic mesh (cylindrical or flat) that is located just before the circulation pump of a pool or spa. Its purpose is to arrest physical debris such as plasters, hair, leaves, and any other items that have been drawn into the pool's pipework by the suction from the pump. Such debris could become tied around the pump's impellor, and would also block the media bed within the filter, thereby slowing flow rates through the sand-bed and reducing filtration capacity.
Strainers should be removed and cleaned sufficiently frequently to maintain good water flow rates through the filter bed.
T
TDS: The abbreviation for Total Dissolved Solids:
TDS meter: a device to measure the electrical conductivity of water, which is usually caused by the accumulation of salts in the water.
Total Alkalinity: A measurement of the total amount of alkaline (basic) chemicals such a s bicarbonates, carbonates and hydroxides in the water. Alkalinity buffers the pH and prevents sudden changes caused by the addition of chemicals. High total alkalinity may result in difficulty adjusting the pH.
Total Chlorine: A measurement of all the active and inactive chlorine compounds in pool water as measured by either DOD NO 4 or a combination of DPD NO 1 and DPD No 3 tablets.
Total Dissolved Solids -TDS: This term refers to salts dissolved in pool water, derived from the chemicals used. High TDS levels can promote electromechanical acyivity and hence corrosion. The recommended maximum level for pool water is Mains water + 1000 mg/l.
Trichlor: The short name for trichloroisocyanuric acid. This is a slow dissolving chlorine sanitizer containing 90% available chlorine.
Trichloramine (Nitrogen Trichloride NCl3): Nitrogen Trichloride is a chemical compound produced when dichloramine has all of its atoms of hydrogen replaced by chlorine
Turbidity: This occurs where particles are suspended in solution and cause a loss of clarity.
Turnover Period: This is the time taken for a volume of water equivalent to the total volume of the pool, to pass through the treatment plant once
U
Ultra-Violet: Ultra-violet is part of the electro-magnetic spectrum which includes visible light. Concentrated sunlight contains U-V which is a powerful natural disinfectant. U-V kills bacteria, viruses, moulds and their spores, thus reducing the risk of transmission of stomach, skin and respiratory tract infections. It also has an important secondary action - it initiates photo-chemical and photo-oxidation reactions which destroy chloramines. Such compounds are responsible for unpleasant irritation and smells in a pool water.
Under-drain system: This refers to the network of outlets situated in the base of a filter vessel, which are used to collect the water that has passed through the filter bed and is being returned to the pool.
V
Venturi: A device that uses the flow of air or a liquid to create a suction or vacuum which in turn can be used to pull more air or liquid into the flow pipework.
Virus: Submicroscopic infective agents that are capable of growth and multiplication only in living cells and which cause infections in man or other animals.
W
Water This is what it's all about! Water for drinking, swimming, washing - we love it in every form - so let's not waste it!
X
Xpert By the time you have reached this point in the glossary - you should be an eXpert! Keep working at looking after your pool or spa, and it will give you back much fun, pleasure, enjoyment, and the chance to relax with nature itself!
Y
Yoghurt This is a living product, resulting from an action by a bacteria. It is thick and often lumpy, with a funny taste - and this is what your pool will turn into if you don't look after it! Have Fun!!
Z
Zeolite: Zeolite materials are marketed as products to replace the sand in filters, with the main claims that zeolite removes ammonia and offers better quality water, producing savings as a result of backwashing less frequently and for shorter durations.
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