There are numerous terms referred to when it comes to treating and testing pool water but to the beginner these can sometimes be confusing. The following gives the meaning of the most common terms:

Acid: A chemical compound that can donate hydrogen ions to the pool water to lower the pH. Sometimes referred to as a pH decreaser. Common compound used in pools sodium bisulphate (dry acid).

Algae: Simple form of plant life that contain chlorophyll, thrive in sunlight and make the pool look unsightly and green. Common form green, black and mustard.

Algicide: Specialist chemical that aid the killing, controlling and prevention of algae formation.

Alkaline: Chemical compound with a pH of above 7 that can neutralise acid and increase the pH of the pool water. Sometimes referred to as pH increasers.

Alkalinity: See total alkalinity.

Backwash: A term used to describe one way of cleaning filter media. In backwashing, the flow of water in the filtration system is reversed so that trapped dirt is lifted out of the filter bed and sent to waste.

Bacteria: Single celled microorganism that can be either pathogenic (disease producing) or non-pathogenic. Most are ‘killed’ by free chlorine.

Bather Load: The number of swimmers in the pool at any given time. Most pools have a recommended maximum bather load and this should be mentioned in the normal operating procedure.

Balanced Water: Water is in balance if it is either scale forming or corrosive. It is normally determined by using the Langelier Saturation Index, which uses the parameters pH, total alkalinity, calcium hardness, total dissolved solids and temperature to calculate whether the water is balanced.

Bromine: A disinfectant sometimes used as an alternative to chlorine.

Calcium Hardness: The amount of calcium dissolved in the water. Normally measured once a week with the readings being expressed in mg/l (ppm) CaCO3. As water likes to have some calcium, the level should be around 200 mg/l (ppm).

Calcium Hypochlorite: See hypochlorite.

Chloramines: See Chlorine (combined).

Chlorine: Most common chemical used for sanitising swimming pool water. Acts as both a disinfectant and oxidiser of swimming pool water killing bacteria, algae etc. and oxidising waste products.

Chlorine (free): Sometimes called available or residual chlorine it is the active, unused chlo-rine that is available to oxidise organic material and kill bacteria. Measuring using DPD No. 1 tablets. Desired levels of free chlorine should be in your normal operating procedure.

Chlorine (combined): Chlorine that has been ‘used’ and combined with another product such as ammonia or nitrogenous waste. Sometimes referred to as chloramines. Combined chlorines are not wanted and cause irritation and the strong chlorine smell found in some pools. Combined chlorines have no disinfection ability and levels should be kept as close to zero as possible.

Chlorine (total): The total amount of chlorine in the pool made up of free and combined chlorine. Measured using DPD No. 3 tablets. By measuring the total chlorine and free chlorine values, the combined level can be calculated as follows:

Total Chlorine – Free Chlorine = Combined Chlorine

Coilforms: A type of bacteria found in waste matter can cause sickness.

Comparator: Device for measuring chemical levels in pool water. Used with coloured discs and reagents to do a visual match of colours.

Copper: A metal found in some water supplies, pipework, fittings and contained in some algaecides. Can cause staining and blue / green water if levels are high.

Cyanuric Acid: A stabiliser added to some chlorine donors to extend the life of the chlorine and protect it from dissipating in strong sunlight. High levels and cause ‘chlorine lock’ and give the water a green tinge. Levels should be kept below 100 mg/l (ppm).

DPD: Harmless indicator, which reacts with chlorine to give a red colour, which can then be measured so levels can be calculated. DPD stands for N,N-diethyl-p-phenylenediamine. Can also be used to measure bromine and ozone.

Filter / Filteration: Method for removing particles from the pool water.

Flocculant: Chemical added to the pool water, which encourages particles to stick together. These larger ‘clumps’ are easier for the filter to remove.

Hardness: See calcium hardness

Hypochlorite: Chemical used as a source of chlorine in pools. Common types are sodium and calcium hypochlorite.

Hypochlorous Acid (HOCl): Produced when any chlorine type reacts with water. Also known as free chlorine.

Indicator: A reagent that reacts with a chemical to give a colour. DPD and Phenol Red are two commonly used in pool water analysis.

Iron: Metal that is present in some water sources. Can cause staining at high levels.

Langelier Saturation Index: See balanced water.

Microorganism: A microscopic plant or animal that (also) lives in the water. Examples bacteria and algae.

Milligram per litre (mg/l): Measurement of levels of chemical in the pool water. Sometimes referred to as parts per million (ppm).

pH: Measure of how acid or alkaline the water is. The pH scales runs from 0-14. A level of pH 7 is neutral, below 7acidic and above 7 alkaline. As the pH scale is, logarithmic small changes in reading are actually larger changes i.e. a pH of 6 is actually 10x more acidic than a reading of 7. Likewise, a pH of 5 is 100x more acidic than a reading of 7. pH is measured using Phenol Red tablets and an ideal level in pools is 7.2-7.

Phenol Red: Indicator used for measuring pH levels. Changes colour from orange to pink in the range 6.8-8.4

Phosphate: Chemical found in some drinking water supplies. Encourages algae growth by acting as ‘food’ and therefore levels in pool water should be as low as possible. Phosphate removers are readily available.

Photometer: Electronic device of measuring chemical levels in pool water. Used with reagents which form a colour which the meter then measures and converts into a reading.

Potable Water: Water safe for drinking.

Part per Million (ppm): Measurement of levels of chemicals in the pool water. Same as milligrams per litre (mg/l).

PPM: Parts Per Million

Reagent: Used for chemical testing. Usually in tablet or liquid and also in powder format.

Saturation Index: See balanced water.

Sodium Hypochlorite: See hypochlorite.

Sodium Thiosulphate: Chemical added to pool water to neutralise (remove) chlorine in the event of over dosing.

Stabiliser: See Cyanuric Acid.

Sulphate: Chemical that can cause issues if levels are too high. Attacks concrete and other pool fittings. Also, adds to TDS levels.

Tablet Count: Test method for specific parameters where tablets are added one by one until there is a colour change. Tests include total alkalinity and calcium hardness.

TDS: see Total Dissolved Solids

Test Kit: A device used to monitor specific chemical in pool water.

Temperature: Measurement of how hot/cold the pool water is. Expressed in either Centigrade (⁰C) or Fahrenheit (⁰F).

Total Alkalinity: The total amount of carbonates, bicarbonates and hydroxides in pool water. Acts as buffer to stop the pH swinging erratically. Levels of 80-140mg/l (ppm) are ideal.

Total Dissolved Solids (TDS): A measure of the total amount of dissolved matter in the water. Normally measured using an electronic meter. Levels should be no more than 1500 mg/l (ppm) or 1000 mg/l (ppm) above the value of the feed water. TDS cannot be removed by filtration and if levels are high then fresh water needs to be added (dilution).

TDS Pen: Device for measuring Total Dissolved Solids (TDS) levels.

Turbidity: The cloudiness of the pool water. Cloudy water isn’t safe. Measured in Nephlomeric Turbidity Units (NTU) levels ideally should be 0.5 NTU or better.

Turnover Rate: The amount of time it takes for most of the water to pass through the filter.

Ultra Violet Light (UV): Secondary non-chemical disinfection system.