Conversion Table  Kilograms x 2.205 = Pounds Imperial gallons x 4.45 = Litres  Pounds x 0.454 = Kilograms Litres x 0.22 = Imperial Gallons  Ounces x 28.35 = Grams Imperial Gallons x 0.0045 = Cubic Metres  Grams x 0.035 = Ounces Cubic Metres x 220 = Imperial Gallons  Feet x 3.279 = Metres Imperial Gallons x 0.833 = U.S. Gallons  Metres x 0.305 = Feet U.S. Gallons x 1.201 = Imperial Gallons Further information on these topics can be found in the hth Pool Managers Handbook obtainable from us via the .... Acid. In swimming pools acids are used to reduce pH and /or alkalinity. Dry acid (sodium bisulphate), hydrochloric acid or carbon dioxide are the most popular types used. Aeration. A means of adding movement to water by blowing air into it. A spa is a typical situation where air is used. Agitator. Referring to swimming pools, the agitator is usually a motor driven mixing device for ensuring that chemicals dissolve when mixed with water. Usually mounted on the top of day tanks and used with calcium hypochlorite, soda ash, aluminium sulphate. Air release valves. Components placed usually at highest level of a system to allow unwanted air to be bled off either by an automatic component or by manual release. See also Automatic air release valve. Algae. Microscopic aquatic plant life that attaches itself to pool surfaces or floats freely in water. Green algae can be suspended in the water or like black and mustard can attach itself to pool surfaces. Algaecide. A chemical compound used to kill or inhibit the growth of algae. Alkali. A term applied to soluble carbonates and hydroxides. pH levels should be maintained slightly alkaline (pH 7.3 – 7.5) Also see base. Alum or aluminium sulphate. A chemical used as a flocculant to aid filtration. It produces a floc on the upper surface of the filter bed which entraps particles. See also PAC. Amperometric. A method of determining chlorine levels used in some automatic controllers. An electric current is used to measure the conductivity between two electrodes. Ancillary chemicals. A collective term for chemicals other than chlorine donors that are used to keep pool water in balanced condition. May also be referred to as PMP's or Pool Maintenance Products Anhydrous. A dry chemical that contains little or no water in its crystal structure. Less stable and more dangerous to use than hydrated. See also Hydrated. Automatic air release valve. A component that allows air to be released automatically as it collects inside a system. A ball which floats in water but reveals an air releasing aperture when air is present allows these devices to be most useful on the top of filters. Automatic controller. An electronic monitoring device that analyses pool water for either chlorine (amperometric), redox or both. The two types each have their own advantages and some countries must use a system that monitors both. pH can also be monitored by the units. They are capable of determining if the levels are above or below an operative determined set point and consequently activate the appropriate chemical to be introduced to the pool - chlorine to respond to falling chlorine levels and pH corrective chemicals if pH is moving away from the set point. Available chlorine. A measure of the oxidising power of a compound. [top] Backwash. Method of cleaning the media inside a filter by reversing the flow of water and removing accumulated dirt to drain. Bacteria. Microscopic organisms, some of which can cause disease. They are brought into the swimming pool by bathers, wind, dust, rain and surface drainage. Balanced water. Water which has been determined by a series of tests to be chemically balanced. The Langelier Saturation Index is reliable method to use for accuracy and incorporates the results of pH, alkalinity, calcium hardness, temperature and total dissolved solids (see TDS) tests to calculate whether the water is aggressive or corrosive. See also Langelier. Balance tank. Also known as a displacement tank. A sudden influx of many bathers would create an overflow of water. The balance tank takes up the displaced water whilst maintaining a constant level of water in the pool. Base. A chemical that has an effect of raising the pH when added to swimming pool water. Sodium carbonate (soda ash) and sodium hydroxide (caustic soda) are examples of bases. Bather load. The number of individuals using a pool in a 24 hour period and the pool's principle source of bacterial and organic contamination. Bathing capacity. The optimum number of bathers allowed into a public swimming pool at any one time. BCDMH. Bromo-chloro-dimethyl-hydantoin. A bromine sanitiser, normally in the form of tablets or sticks. Bicarb. See sodium bicarbonate. Biodegradable. When a chemical is added to pool water, if it breaks down into simpler products by means of a biological system it is biodegradable. Bleach. An alternative name for sodium hypochlorite. Bleaching out. An industry term used at times when excessive amounts of chlorine present in the pool water actually bleach away any colour in the colorimeter or photometer tests. The operator could be fooled into believing that the absence of colour suggests that no chlorine is present in the pool and so introduces more. Breakpoint chlorination. A method of chlorination in common use, involving the addition of chlorine to pool water to a point where most of the ammonia and organic matter is oxidised allowing the existence of a free chlorine residual. Bromine tablets. See BCDMH. Buffer. A chemical which when dissolved in swimming pool water will help the pH to resist change. Sodium bicarbonate is an example of this type of chemical. Bund. A chemical day tank should be placed inside a separate tank which is sized 10% greater than the volume of the day tank. The bund can be brick built or made from chemically resistant plastic. But must be non porous. Butterfly valve. A rubber lined valve which has become popular in replacing iron gate valves in pool circulatory lines. By their design they can be opened too quickly so must be used in a controlled manner. [top] Calcium chloride flake. A chemical in flake form that is used to increase the calcium hardness of pool water. Adds chlorides to pool also. Calcium differential. This is an industry term used to express the difference in calcium levels found in the mains water and that of the pool water. Is significantly important if the pool is not using cal hypochlorite or calcium flake. Calcium hardness. The quantity of mineral calcium dissolved in water. The minimum recommended level is 250 mg/l as CaCO3, although a preferred level is 500 mg/l as CaCO3. High levels can promote scale and cloudy water if the water is incorrectly balanced. Low levels can promote corrosion and also lead to grout loss. Calcium hungry. A term used to describe pool water that contains little calcium. In this state the water will seek calcium to reach a state of equilibrium. Erosion of tile grout is the most common indicator that the water is calcium hungry. Calcium layer. See Protective scale. Cal hypo. A shortened name when referring to calcium hypochlorite, a granular or tablet format, calcium based chlorine donor with typically 68% available chlorine. Calorifier. An alternative name for a heat exchanger used for indirectly heating pool water. It contains a series of tubes inside an insulated casing. The tubes carry hot water from the buildings general heating system. Pool water takes heat from the tubes by flowing over them on its way to the pool. Carbon dioxide (CO2). A gas used for pH correction. When introduced into pool water it forms carbonic acid and so lowers pH. It is now accepted that it is not cost effective or efficient to use carbon dioxide in pools where the mains water has a total alkalinity of more than 150 mg/l as CaCO3 or a calcium hardness in excess of 300 mg/l as CaCO3. Cationic Liquid. This positively charged chemical attracts negatively particles from pool water to provide improved clarity. Chelate or Chelating agents. See sequestrant. Chloramine. See Combined chlorine. Chlorides. Chemical salts from the addition of chlorine based products, hydrochloric acid etc. Main constituent of TDS. Chlorine. A disinfectant added to pool water to destroy or inhibit bacterial and algal growth in addition to oxidising unwanted organic matter. Available as a gas, liquid or solid. Chlorine demand. The amount of chlorine required to destroy bacteria, algae and/or other organic matter in pool water before an active chlorine residual can be achieved. Chlorine donor. A chemical which provides hypochlorous acid when dissolved in water. Chlorine efficacy. The amount of hypochlorous acid present as distinct from hypochlorite ion in a chlorine solution. The amount produced is dependent on pH value present. Chlorine lock. A condition where the residual chlorine has become ineffective because of excessive levels of cyanuric acid (stabiliser). Circulation pump. The usual name for the main pump that takes water from the pool and pushes it through the water treatment process that the pool plant provides. Clarifier. Compounds used to aid water clarity by coagulating small suspended particles so they may be filtered out. Clarity. Referring to pool water. Clarity is the clearness of the water. Good clarity is brought about by a combination of sanitisation and filtration. Cloudy water. The underlying cause can be one of several factors. Refer to trouble shooting chart. Coagulate / Coagulation. A process whereby negatively charged suspended particles are neutralised by a cationic liquid thus enabling them to join together and be removed by the filtration system. Combined chlorine. Formed by the reaction between hypochlorous acid and ammonia / nitrogenous based compounds such as urine or sweat. Combined chlorine has little in the way of disinfecting properties and is usually associated with a pungent odour and eye irritation. Measured by the difference between free chlorine and total chlorine tests. i.e. Total chlorine = DPD No 1. + DPD No 3 tests. Comparator. A method for testing pool water using reagents that determine the amount of chemical present by producing a colour in the test sample. Comparing this colour with standards give an indication to the user of chemical levels present. Conditioner. See Cyanuric acid. Conductivity. See also TDS. In pool water, the conveyance of a current between dissimilar metals allowing corrosion to take place. Continuously rated. The term given to an electrical motor which is designed to run continuously. Controller. See Automatic Controller. Copper. A metal which when present in pool water may cause water discolouration (blue-green) and / or coloured stains. Corrosion. The etching of metal in a destructive manner by chemical or electrochemical attack. Corrosion is greatly influenced by high TDS, low pH or low calcium hardness. High TDS levels can promote electrochemical activity and allows galvanic corrosion to occur. Creatinine. Urinary waste product which reacts with chlorine and can cause eye irritation. Cannot be removed by shock dosing. Cyanuric acid. A stabiliser added to pool water to protect the active chlorine against loss by ultra violet radiation from the sun. [top] Day tank. A plastic vessel used for mixing chemicals which are to be dosed to the pool. It is normally re charged daily to provide enough chemical supply for at least the remainder of the day. Also see Bund) Dechlorination. At times, either by operator error or equipment malfunction, the pool may have suffered from the addition of too much chlorine. Dechlorination is the procedure whereby the chlorine is chemically reduced by the addition of a chlorine destroyer such as sodium thiosulphate. Dilution of the pool is the recommended and quicker method of returning the pool to safe bathing conditions. Deck level. A swimming pool design where the surface of the water is level with the pool promenade. Popular with bathers who have difficulty climbing out of the pool. Unpopular with partially sighted bathers who find it difficult to anticipate where the pool edge is. Dedicated clean up equipment. The practice of using certain pieces of equipment such as a dustpan or sweeping brush for specific purposes only. e.g. a brush that has been used for sweeping up spilt dichlor should only ever be used for that purpose again. Defoamer. A chemical used mainly in spas to eliminate foaming by altering the surface tension of the water. Diatomaceous earth. An effective rechargeable filter media derived from microscopic shell life ground into a powder. Di-chlor. Short name for sodium dichloroisocyanurate dihydrate, a chlorinated isocyanurate with typically 55% available chlorine and containing cyanuric acid. Dilution. The introduction of fresh water to dilute unwanted pollution, salts, chemicals and chlorine by products. Disinfect. To kill or inhibit the growth of harmful bacteria and viruses in swimming pool water. Dichloramine (NHCl2). Dichloramine is a chemical produced when monochloramine has one atom of hydrogen replaced by chlorine. See also Breakpoint Chlorination. Disinfectant. A product which can be used to disinfect pool water. Dissipation. When a chemical is liberated from pool water by being spread over a large area. Sunlight will also cause dissipation. Dosing lines. The plastic tubing which is typically connected to the dosing pump and carries chemical solution to the swimming pool circulatory system. They should ideally be housed or sheathed in wider diameter tubing to prevent leaks from spraying personnel. Dosing pump. A pump that injects chemicals steadily into the pool water via a length of tubing and an injection point. DPD. Short for Diethyl-p-Phenylene Diamine. A chemical reagent which reacts with active bromine or chlorine and turns pink. The darker the pink colour the higher the chlorine content. Test kits using DPD are now preferred over other methods. Orthotolidine (OTO) should not be used as it is considered to be carcinogenic. DPD No.1 measures free chlorine. DPD No3. measures total chlorine when used with DPD No.1. Dry acid. More common name for sodium bisulphate or sodium hydrogen sulphate used to reduce pool water pH and total alkalinity. Care should be taken if using this product on a regular basis as sulphate levels can become unacceptably high, resulting in grout loss. Maximum recommended sulphate level is 360 mg/l. Dry chlorine. A chlorine donor available in granular or tablet form. This term when used is usually referring to calcium hypochlorite. Dye test. A method for determining and examining flow patterns in the pool tank. The addition of a food grade dye to the pool circulation system enables flow patterns to be observed as the dye enters the pool. [top] Erosion feeder. A device containing sanitiser tablets or macro granules. Water from the circulatory system is fed through the device, eroding the chemical and bringing it into solution. This solution is reintroduced to the pool water circulation system as it leaves the feeder. Erosion of grout. See grout erosion. [top] Filter. A pressure vessel containing a media that removes particulate matter as pool water passes through it. Filter design, size shape and type of media used is dependent on size of pool and type of use. Filter Cartridge. The element placed inside a small filter used generally in residential swimming pools and Spa's. Normally made of paper fibre, they are cleaned by removal and rinsing by hose pipe. They have a limited life before total renewal is necessary. Filter media. The material placed inside the swimming pool filter to trap oxidised matter and debris from the pool water. Sand or diatomaceous earth, are types of filter media that are used depending on type of filter and pool usage. Weekly backwashing releases the dirt to drain allowing the filter media to continue its filtering process. Filtrate. The term used to describe water that has passed through the filtering process. Flocculant. Material used to aid filtration allowing the removal of particles that may not be removed by the filter media alone. Flow gauge. A visual indicator fitted in the filtration system that shows how much water per hour is being passed through the treatment process. Free active, Free available or Free residual chlorine. These are all names for 'hypochlorous acid' (HOCl) a very powerful disinfectant which has not yet combined with pollution in the water. [top] Galvanic attack. See Galvanic corrosion. Galvanic corrosion. The reaction caused when dissimilar metals in close proximity to each other corrode, with the pool water acting as an electrolyte. Generally speaking the higher the TDS in the pool, the more conductive the water and the more severe and quicker the corrosion. Gate valve. An opening/closing device which controls the path of water in pool circulatory systems. By design they can be opened and closed slowly. Grout. The material used between ceramic tiles in the pool area to ensure an even surface. Grout erosion. The 'leaching' of grout is a phenomenon that occurs when water is aggressively out of balance. Generally speaking it occurs when calcium levels are below 250 mg/l. The lower this level the faster the erosion happens. Erosion can also occur because of high sulphate levels. [top] Hand dosing. The manual addition of chemicals into the pool. The recommended method for introducing chemicals to pool water is via mechanical means e.g. a pump or feeder of some kind. Hand dosing can rarely be justified and should only be carried out as an emergency measure, and only when bathers are not present. Hard water. Usually refers to mains water which is high in both bi-carbonates (alkalinity) and calcium (hardness) salts. Two types of hardness exist. Temporary hardness can be reduced. Permanent hardness is more difficult to deal with. Difficult to form lather when washing. Heat exchanger. See also Calorifier. High chlorine level. See Maximum chlorine levels. High pH. The addition of too much alkali will result in pH levels rising above the recommended level of 7.6. Hi rate filter. A type of filter designed to handle water at around 50m3/m2/h. hth. Calcium hypochlorite with typical 68% available chlorine manufactured by Arch Chemicals Inc. hth stands for high test hypochlorite and is the world's best selling brand of dry chlorine. Hydrated. A dry chemical that is combined with water in its crystal structure. Hydrated products are more stable and safer to use than anhydrous products. Hydrochloric acid (HCl). An acid used to lower pool water pH and / or total alkalinity. Supplied in liquid form and must be handled with great care. In pool water, a weak solution is formed as a by product of chlorine. Hydrogen ion. The hydrogen ion is the atom which produces acidity in water. See also pH. Hypochlorous acid (HOCl). The main killing agent present in a chlorinated pool, it is a powerful bactericide and oxidiser. [top] Injection point. Usually refers to the point/s at which chemicals are introduced into the pool circulating system. Normally the chlorine is injected pre filter and pH correctant, post calorifier. Flocculant is also injected pre filter. Inlets. In a swimming pool, the inlets are usually sited at the shallow end and often have the purpose of allowing the water to be well distributed around the pool tank. Inorganic chloramines. Compounds resulting from the chlorination process. Removed by shock dosing with chlorine. Iron. A metal which when present in high quantity in pool water may cause coloured water (reddish brown) or coloured stains on the pool tank. See also Sequestrant. Isocyanurates. The collective term for the family of chlorine donor products which contain cyanuric acid – typically dichloroisocyanurate dihydrate (Dichlor) and trichloroisocyanuric acid. (Trichlor). [top] Liquid chlorine or liquid bleach. Often used to describe sodium hypochlorite or other chlorinated solutions. Strictly speaking liquid chlorine is liquified chlorine gas. Low chlorine levels. See Minimum chlorine levels. Low pH. The addition of too much acid will result in pH levels falling below the absolute minimum recommended of 7.2. [top] Mains water. Potable water or drinking water supplied to the pool. Make up water. The water used to top up the pool when levels fall after backwashing or through evaporation. Maximum chlorine levels. In the United Kingdom and Ireland, the maximum free chlorine level at which bathers can safely use the pool. There is no legal limit but industry accepted parameters are that at 5 mg/l of free chlorine dosing should cease. At 10mg/l swimming should cease. This may vary in other countries. Maximum pH value. The maximum pH at which favourable bathing conditions can be maintained is around 7.6. Medium rate filter. A filter designed to handle pool water at a rate of around 25m3/m2/h. Metering pump. See Dosing pump. Milligrammes per litre (mg/l). Used as units of measurement for pool water tests, e.g. chlorine, alkalinity, TDS etc. Same as parts per million (ppm). Minimum chlorine levels. In the United Kingdom and Ireland, the minimum level of free chlorine for safe bathing is 0.5 mg/l. This is generally accepted as the industry standard. Minimum pH value. The minimum pH at which favourable bathing conditions can be maintained is around 7.2. Mixing tank. A tank used to pre mix a chemical before transferring the solution to a separate day tank from where it is injected to the pool. Monochloramine (NH2Cl). A chemical that is produced in pool water when ammonia has one atom of hydrogen replaced by chlorine. See also Breakpoint chlorination. MSDS or Material Safety Data Sheet. The information sheet that should accompany every chemical purchased. It allows operators to use chemicals safely by making them aware of hazards and risks associated with products. Often referred to as the COSHH sheet. Muriatic acid. An American term for hydrochloric acid. [top] Negative index. When using the Langelier Saturation Index to assess pool water condition, a negative index would suggest that the pool water is in a corrosive state. Neutral water. This term usually refers to the pH value being neither acid nor alkali i.e. – pH 7. Non return valve. A device placed in a flow line which allows a liquid to move in only the intended direction. Non-scaling. The inability of water to be protective laying down protective scale. [top] Organic chloramines. Nitrogen bearing compounds such as urine, saliva, 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. Organism. Examples of organisms are bacteria, viruses and algae. ORP. Oxidation Reduction Potential or Redox is a measure of sanitiser activity. It measures the potential activity of the sanitiser rather than quantity present. OTO (o-Tolodine). A chemical reagent formerly used in pool water test kits for measuring chlorine residuals. This compound is now considered to be a health hazard and should no longer be used. Overstabilised. A condition in pool water when excessive levels of cyanuric acid (stabiliser) cause 'chlorine lock' whereby the chlorine becomes ineffective. Green water is one of the signs that this is possibly occurring. Oxidation. The process of chemically 'burning u' contaminants by causing them to combine with free available chlorine. Oxidiser. A chemical substance which destroys organic matter - bacteria, viruses and algae. Ozone (O3). A powerful bactericide gas. It is generated on site and used as a pool water disinfectant. It is removed from the pool water before coming into contact with bathers by a de-ozonation process. [top] Parameters. The upper and lower limits for levels of chlorine and other chemical compounds in pool water. Particulate matter. Microscopic debris being collected by the filter. Parts per million (ppm). Used as units of measurement in pool water tests e.g. chlorine, alkalinity TDS etc. Same as milligrammes per litre (mg/l). Peak demand. The demand on chemical dosing at the busiest time when bathing load is at a maximum. Personal Protective Equipment (PPE). In the United Kingdom, by law, PPE refers to the safety wear and apparatus which must be supplied to operatives when handling chemicals. pH. The pH scale is the negative logarithm of the hydrogen ion content of water. In normal swimming pool terms the pH is a measurement of the relative acidity or basicity (alkalinity). The scale runs from 0 (maximum acidity) to 14 (maximum basicity - alkaline), with a neutral point at 7. In pools the accepted pH range is ideally 7.3 - 7.5. pH correction. When the pH is adjusted to an acceptable value by the addition of a chemical. An acid to lower, or soda ash to increase. The ideal range for swimming pools is 7.2 – 7.6. Phenol red. A chemical reagent used in the measurement of pH. The pH range measured using this method is from 6.8 – 8.4. pH minus. A common term for a chemical which is of acid type and is used in swimming pools to reduce pH of pool water. Dry Acid is a typical example. pH plus. A common term for a chemical that is alkali and used to raise the pH in pool water. Soda ash is a typical example. Photometric colorimeter. An electronic device that uses reagents to produce a range of colours of varying intensity to measure the concentration of a chemical. Pollution. Impurities brought into the pool by bathers, pool chemicals and airborne matter. Poly aluminium chloride (PAC). A flocculation chemical which has become more popular over the last few years as an alternative to Aluminium sulphate. Pool liner. Some pools instead of being lined with conventional ceramic tiles, may have a plastic liner which contains the water in the pool 'tank'. They are usually placed in position by a specialist company and have a limited life. Pool tank. A general term used to describe the fabric of the swimming pool itself – walls, floor, tiles etc. Pool volume. The total capacity of the pool tank when measured in litres, gallons or cubic metres. Positive index. When using the Langelier Saturation Index, pool water tests that produce a positive index suggest that the pool is in a non corrosive state. Planned preventative maintenance. A system that ensures all pieces of equipment receive a service on a regular basis thus minimising unexpected breakdowns. Recording of all work ensures system is optimised. Plate heat exchanger. A deign of heater that takes up less space than a conventional calorifier and consists of a series of plates that separate heating water from pool water allowing an exchange of heat to take place. Precipitation. This is a condition where solids that were previously in solution become suspended and visible in pool water. Pre cleanse procedure. The routine that bathers are encouraged to adopt before using the pool. Toilet, showers etc. Pressure differential. The pressure differential is used as a guide to determine when a filter requires backwashing. It is calculated by deducting the pressure reading after the filter from that showing before the filter. The 'difference' is an indication of dirt collected on the filter media. Pressure Side. See Secondary Side. Primary side. Referring to the circulating pump, the primary side is the network of pipework coming from the pool on the suction side of the pump. Progressive dilution. The theory of, this says that 66% of the pollution in any pool water is removed each turnover. For example, if 100 swimmers entered a pool and left after one hour. The pool then remained unoccupied. After one turnover 66% of the pollution would be removed, after a further hour 66% of the remaining 34% would be removed leaving approx 10%. A further turnover would remove 66% of the 10% and so on. Protective clothing. The safety gear which operatives must wear when handling pool chemicals to protect them from chemical spills, splashes, breathing hazards and mishandling. Protective scale. A micro thin layer of calcium scale that is formed on the inside of plant when pool water is maintained in a 'balanced state'. This scale becomes protective against corrosion. Pool water is said to be in balance if water tests for pH calcium hardness, alkalinity, temperature and total dissolved solids produce a positive result. [top] Rapid gravity filter. A filter designed to handle pool water at a rate of around 12.5m3/m2/h. Raw water. See also Mains water. Reagents. Chemical used to test concentrations of specific compounds in pool water. Redox. See ORP. Residual. The amount of a compound existing in pool water, usually expressed as milligrammes per litre (mg/l) or parts per million (ppm) e.g. free chlorine residual. Residual chlorine. The chlorine residual remaining in the pool water in a free state after the chlorine demand has been satisfied. The normal free chlorine residual found in pools is 0.5 - 2.0 mg/l. Chlorine dosing should cease at 5 mg/l and swimming stopped at 10 mg/l. Care should be taken when measuring chlorine residuals over 10 mg/l as tablet bleaching may occur resulting in an inaccurate result. [top] Salvage drum. A container that is used specifically for dry contaminated or waste chemical inside. No other kind of chemical should be placed in the container except the one that the salvage drum is designated for. Sample Water. In an automatic controller the sample water is the water taken from the pool circulatory system for analysis by the device. The term may also be given to the water sample taken from the pool by operators about to use the water for analysis. Sanitiser. A chemical added to pool water acting as a disinfectant e.g. chlorine, bromine. To kill water borne micro organisms and algae. Saturation index. Usually refers to the Langelier method of water balance tests. The Index is used to assess the corrosive or scale forming tendencies in pool water. A result of between +0.1 and +0.5 should be aimed for. Scale. The mineral deposit or precipitate on floors, walls, pipework, filter systems etc., usually calcium carbonate. It can be unsightly and interfere with normal pool operation. Under controlled conditions it can be used to inhibit corrosion. Scaling. The effect of pool water when calcium carbonate precipitates and forms scale. Screed. The concrete render upon which the ceramic tiles in the pool area are placed. Scum channel or scum trough. The earliest of the commonly found systems for removing pollutants from the pool surface. The channel or trough also serves as a place for bathers to hold onto when resting. Secondary side. Referring to the main circulating pump, the secondary side is the network of pipework on the pressure side of the pump where the water is pushed away and towards the filter. Septum. The name given to a flexible screen which is housed inside a pre-coat filter to allow retention of dirt. Sequestrant. Specially formulated chemical compounds used to hold metals e.g. iron, copper or calcium in solution to prevent scaling or staining. Set point. The chlorine or pH level set on an automatic controller by the operative as the desired target for the unit to adhere to. Shelf Life. Referring to pool chemicals, shelf life is the recommended period that the chemical can be stored without deterioration before the performance of the product is impaired. Shock treatment. The process of introducing significant quantities of a sanitiser to the pool to bring about the chemical destruction (oxidation) of excess inorganic matter or high combined chlorine levels. See also Inorganic chloramines. Skimmer. A device located at the perimeter of the pool at surface level. It removes debris floating on or near the surface of the pool which is taken via an anti siphon pipe to the filter. Some larger commercial type pools have several of these units placed strategically around the pool edge. Sight glass. The section of clear pipe which can be plastic or glass that allows the operative to observe the pool water being run to waste during the backwash process. When the water appears to become clear it indicates to the operator that the collected dirt on the filter bed is removed. Slow sand filter. Large open bed filters which filter water at around 7.5m3/m2/h. Soda Ash. See Sodium carbonate. Sodium bicarbonate (NaHCO3). Also known as baking soda. Used to increase the total alkalinity level of pool water. Sodium bisulphate (NaHSO4). Also known as dry acid. Used to decrease the pH level by trickling in a 10% solution round the pool perimeter or for total alkalinity reduction by dumping a 10% solution in one spot in the deep end of the pool. Sodium carbonate (Na2CO3). Also known as soda ash. Used to increase the pH level in pool water. Sodium thiosulphate (Na2S2O35H2O). Used to neutralise excessive chlorine residuals in pool water. Should be used with great caution. Soften. In relation to water, it is the removal of water hardness. The pre - softening of water to be used in a swimming pool is not recommended. Soft water. Water that contains few calcium or magnesium salts. Easy to lather. Soluble compounds. Chemical entities which totally dissolve in water. Source water. See Mains water. Spa. A small tank of water that may seat from two to twelve people. Water is heated to near body temperature, mixed with air and introduced to the spa to give the bather an invigorating bath. Stabiliser. See Cyanuric acid. Strainer or Strainer basket. The strainer basket is a cylindrical mesh component housed inside a strainer pot that is sited just before the main circulating pump. It protects the pump from damage by collecting hard and soft articles such as coins, hairgrips and lint from costumes. Strainer pot. The casing in which the strainer basket is housed. Suction side. See Primary side. Sulphates. Chemical salts introduced into the pool via sulphate based products such as Dry Acid and Alum. Surface draw off. The system for removing pool water from the surface of the pool where large amounts of debris accumulate. [top] TDS. The abbreviation for total dissolved solids. TDS meter. An electrical device which measures conductivity in water and displays the result as a TDS level. Test Kit. Equipment for testing pool water. Many types are now available from basic comparator type to digital display photometric. Total alkalinity. A measurement of the total amount of alkaline (basic) chemicals such as 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 DPD No.4 or a combination of DPD No.1 + DPD No.3. test tablet/s. Total dissolved solids (TDS). Term used in referring to salts dissolved in the pool water, derived from the chemicals used. High TDS levels can promote electrochemical activity and hence corrosion. The recommended pool water maximum is mains water TDS + 1,000 mg/l. Trichlor. Usual name for trichloroisocyanuric acid. A slow dissolving chlorine sanitiser containing 90% available chlorine. Available in tablet and macro granular form. Trichloramine (Nitrogen trichloride – NCl3). Nitrogen trichloride is a chemical produced when dichloramine has all of its atoms of hydrogen replaced by chlorine. Trichlorinator. An erosion type feeding device for dosing trichloroisocyanuric acid tablets into the pool circulating system. Turbidity. Where particles are suspended in solution and cause a loss of clarity. Turnover. The time taken for the total volume of pool water to pass through the treatment plant once. [top] Underdrain system. The network of outlets on the base of a filter that allows pool water having been filtered to return to the bathing tank. The nozzles that are incorporated into the system allow water through but retain sand in the filter. [top] Venturi. A device that is uses the motive power of the main circulation pump to create a vacuum within itself which then can be utilised to aerate or introduce a fluid to pool water. Virus. Sub microscopic infective agents capable of growth and multiplication only in living cells and which cause various infections in man or other animals. [top] Water Sample. The water taken from the pool circulation or from the pool direct for the means of analysis to determine chemical content or pH value.