Layman's Guide to Technical Terms


Algae/ Bromine/ Calcium Hardness/ Calcium Hypochlorite/ Chlorine/ Chlorine Demand/ Chlorine Donor/ Chlorine Lock/ Chlorine Residual/ Chlorine Stabiliser/ Combined Chlorine/ Cyanuric Acid/ Dichlor/ Hard Water/ Hypobromous Acid/ Hypochlorite Ion/ Hypochlorous Acid/ PPM/ pH/ Soft Water/ Shock Chlorine/ Stabilised Chlorines/ Superchlorination/ Total Alkalinity/ Total Chlorine/ Total Dissolved Solids/ Trichlor/ Water Balance/

Algae are microscopic plants which can literally transform your pool water from clear blue to the appearance of a stagnant pond in 24 hours. They are introduced into the pool by airborne spores, from make-up water, from covers which have been dragged over grass and moss and from vegitable matter. Onset is quite common after thunder storms, especially if the water temperature is high.

Algae take up dissolved carbon dioxide and therefore cause the pH to rise rapidly.

There are hundreds of different species of algae - some green, some yellow ("mustard algae"), others black or even pink. They can be the clinging type and colonise the pool surfaces, or float suspended in the water. Normally not a problem if chlorine levels are maintained.


Bromine, like chlorine is a member of the halogen family i.e it kills bacteria, viruses and algae and oxidises. Elemental bromine is a reddish brown colour and is liquid at room temperature. The form used in private and commercial pools is the tabletted version - BCDMH, or bromochlorodimethylhydantoin. (It becomes less daunting if you split it up into its component bits - bromo chloro di methyl hydantoin). BCDMH dissolves in water to release hypobromous acid and hypochlorous acid. The hypochlorous acid serves to regenerate the spent bromine. It is dosed into the pool by means of a circulatory feeder.

A dry chlorine donor available in the form of granules or tablets made by the absorbtion of chlorine in lime. It is unstabilised, making it suitable for regular sanitisation or for shock treatment, and produces 65% available chlorine. It tends to raise the pH, but this and the presence of calcium is beneficial for pools in soft water areas.

Calcium hardness is the amount of dissolved calcium (plus some other minerals like magnesium) in the water. The word dissolved is important - if you can see calcium scaling up the pipework or the surface of the pool, it is no longer dissolved - it has stolen a march on you. Too much calcium means cloudiness and scaling up, too little could lead to the water satisfying its appetite for calcium by taking it from your grouting.

An element of the halogen family. It dissolves in water to form hypochlorous acid (HOCl, or free chlorine - the principal water sanitiser) and hydrochloric acid.

The amount of chlorine needed to destroy pollutants in pool water such as bacteria, algae and other contaminants.

One of the many chlorine compounds available which when dissolved in water will provide chlorine or hypochlorous acid.

You are probably thinking this means that the chlorine is totally locked up and inactive. Perhaps you are meant to. It is a term coined by the critics of stabilised chlorines (dichlor and trichlor). Its not quite as dire as that.

What happens is that with insufficient water replacement the amount of stabiliser (cyanuric acid) in the water builds up over a period of months or years to levels where it becomes a bit of a problem. This makes it difficult for the dichlor or trichlor to react with the water and produce sufficient hypochlorous acid (free chlorine) to achieve a satisfactory kill time against bacteria and other micro-organisms.

The cyanuric acid level at which this occurs is a matter of debate amongst the experts. Some say it happens as low as 80ppm cyanuric acid, others go for a figure nearer 200ppm. In the experience of the author, you lose significant performance at around 160ppm.

However, if your pool dealer implies that the chlorine is completely inactive, he is misleading you (there is evidence that kill times actually start to improve at 400ppm!).

If you think your pool is over-stabilised, kill times can be restored by running the pool at raised levels of free chlorine (DoE guidelines give clearance for up to 4ppm). But the longer term solution is to re-invigorate the pool by introducing more fresh water. This can be done by bigger than usual backwashes, or if absolutely necessary by a partial water replacement. But do not let the water level fall too far in liner pools.

Chlorine residual is the amount of chlorine left over after the chlorine demand has been met.

The chemical name is cyanuric acid. Be reassured - cyanuric acid is a polymerised urea; it has nothing whatever to do with cyanide or isocyanates. It is also sometimes misleadingly called conditioner.

It can be obtained as a granular product in its own right, but it is also formed when the stabilised chlorines (dichlor or trichlor) are dissolved in water - they dissociate (split up) into hypochlorous acid (free chlorine) and cyanuric acid (stabiliser).

Low levels of stabiliser are beneficial because they prevent wastage of free chlorine by the u/v waves in sunlight, but high levels are a disadvantage because they make it take longer for the chlorine to kill micro-organisms.

If you are using dichlor or trichlor, there should be no need to add extra stabiliser. However, it can be helpful to stabilise pool water with cyanuric acid if you sanitise your pool with sodium hypochlorite or calcium hypochlorite which have no inbuilt stabiliser. Danger - do not mix these chemicals in the dry state.

Also known as chloramines and made up of monochloramine (NH2Cl), dichloramine (NHCL2) and nitrogen trichloride (NCl3). They are formed when free chlorine (Hypochlorous acid) reacts with nitrogen compounds which are introduced into pool water by bather pollution, and which in turn break down into ammonium compounds (hence, chlor + amine).

They are comparatively ineffective as sanitisers, and nitrogen trichloride in particular is responsible for the stale chlorine smell associated in the old days with poorly maintained public pools. Chloramines must be broken down by raising the level of free chlorine.

Try this one out - the chemical name is sodium dichloroisocyanurate dihydrate. One of the stabilised chlorine donors - trichlor is the other. It is called dichlor because there are only two atoms of chlorine bonded to nitrogens on the molecule (sodium is bonded to the third nitrogen) - trichlor has three. Usually sold in the form of granules of 55% available chlorine. When dissolved in water, it dissociates (splits up) into hypochlorous acid (free chlorine) and cyanuric acid (stabiliser).

Water is considered hard if its calcium hardness is over 250ppm and its alkalinity is over 150ppm. Typically, hard water is found on the chalk uplands of S and SE England, but is by no means restricted to those regions. pH tends to be relatively stable in hard water conditions, but it may be best to use a mildly acidic chlorine donor such as trichlor tablets (or possibly dichlor granules) so as to achieve a 'natural' pH balance in the water.

Hypobromous acid or free bromine is the main disinfectant in pools on bromine or BCDMH. It is formed (a) by dissociation when BCDMH is dissolved in water and (b) by the reaction between hypochlorous acid and spent bromine (bromide ion). It is more expensive than the chlorine donors, but is effective over a much wider range of pH values. Free bromine levels should be maintained at 4 - 6ppm. in soft water areas.

The species of chlorine resulting from dissociation (splitting up) of hypochlorous acid (HOCl) into its constituent parts - H+ and OCl- (hypochlorite ion). This happens if the pH is too high - if it is too low the hypochlorous acid dissociates into molecular chlorine (CL2). Hypochlorite ion is a poor disinfectant because the negative charge creates an obstacle to penetrating the wall of the cell. Hypochlorous acid is 100 times faster than hypochlorite ion in killing a micro-organism.

Also known as free chlorine. It is formed when calcium hypochlorite, dichlor, trichlor or chlorine gas are mixed with water and dissociate. This is the main pool water disinfectant.

Hypochlorous acid acts (a) as a sanitiser killing potentially harmful bacteria and micro-organisms (it can enter a cell's wall and upset its protein and enzyme function), (b) as an oxidising agent eliminating organic and inorganic impurities by a process similar to combustion e g it burns out pollution introduced by bathers such as sweat and urine (yes, I'm afraid people do).

Useful amounts of hypochlorous acid can only be obtained if the pH is within certain limits [see pH], or if the stabiliser level is not too high.

Equivalent to milligrams per litre (mg/l). The standard way of quantifying the amount of chlorine or minerals in the water. To give some idea of the scale, 1ppm (or 1mg/l) is equivalent to 3 inches on top of Mount Everest.

The pH scale runs from 0 to 14 and is a measure of the acidity or alkalinity of a solution. This is a very important concept, you should refer to pH explained for a comprehensive account. You will never get the best out of your pool unless the pH is correct.

Water is considered soft if has a hardness of under 50ppm as calcium carbonate and an alkalinity of under 30ppm as calcium chloride. In the British Isles, soft water is common in Scotland, Wales, the extreme SW of England and parts of Ireland. The pH can be rather unstable in soft water areas, but alkaline chlorine donors such as calcium hypochlorite will help to increase hardness as will the addition of calcium chloride.

Usually a short-hand way of referring to sodium hypochlorite or calcium hypochlorite which can be dosed into the pool at high rates of addition without increasing levels of stabiliser (cyanuric acid).

The purpose of shock dosing is to overcome a problem such as algae growth or unpleasant chlorine smells. It does this by satisfying chlorine demand i.e. by killing bacteria, algae, and other micro-organisms, and breaking down accumulated organic impurities to leave a chlorine residual.

Sometimes also referred to as isos, or chlorinated isocyanurates. They are the chlorine compounds which when dissolved in water dissociate (split up) into hypochlorous acid (free chlorine) and cyanuric acid (stabiliser).

Dichlor and trichlor are stabilised chlorines (isos) but calcium hypochlorite and sodium hypochlorite are not. WARNING - Never mix concentrated chlorines in the dry state.

Much the same idea as shock dosing with chlorine, except that the term is used for routine (usually fortnightly) treatment where chlorine levels are raised to around 10 ppm to prevent proliferation of bacteria or algae infestation.

Closely related to pH, but the two must not be confused. Total alkalinity is a measure of the amount of alkaline materials in the water. This alkalinity will usually be present as bicarbonates, but with a very high pH carbonates and hydroxides can be present as well.

The relevance to pH is that the amount of alkali (hardness) in the water will determine how easy it is for changes in pH to occur.

If the alkalinity is too low (below 80ppm) there can be rapid fluctuations in pH - i.e. there is insufficient 'buffer' to the pH. High alkalinity (above 200ppm) will result in the water being too buffered - it will make it difficult to adjust or correct the pH.

By way of analogy, picture a sphere resting on a flat surface. If the sphere moves along the surface to the left, it is analogous to lowering the pH; if it moves to the right, it is equivalent to raising the pH. If the sphere had the bulk density of a balloon, very little force would be required to move it left or right. It would require a much greater force if the sphere had the bulk density of a cannon ball. In this example, the increase in bulk density between the balloon and the cannon ball is analogous to an increase in alkalinity. The increase in force required to move the balloon and the cannon ball is analogous to the idea of increased 'buffering'.

It is important to create the correct amount of buffering in the pool such that the pH can be adjusted if necessary but rapid fluctuations are prevented. High alkalinity and high pH can lead to cloudy water and scale formation. Low alkalinity can result in corrosion and discomfort to bathers.

Free chlorine plus combined chlorine. Hence chloramine levels can be worked out by the formula: Combined chlorine = total chlorine (from DPD no 3 tablet) - free chlorine (DPD no 1 tablet).

This apparent contradiction in terms refers to conductive chemicals that can accumulate in the pool particularly when the water evaporates, or when the pool is not 'diluted' with sufficient fresh water. You cannot see them because they are dissolved, but this does not stop them corroding metal parts (pumps, pipework, filters) on account of their conductivity. They are mostly made up of chlorides and sulphates. Chlorides can accumulate with long term use of sodium hypochlorite. Regular addition of alum based clarifiers (aluminium sulphate) and dry acid (sodium bisulphate) can increase sulphate levels. Periodic backwashing and water replacement are the best ways of controlling TDS

Short for trichloroisocyanuric acid - a bit easier to say than the chemical name for dichlor. Another stabilised chlorine donor. It is called trichlor because there are three atoms of chlorine bonded to the nitrogens on the molecule. This makes it stronger than dichlor which only has two.

Usually sold in the form of slow dissolving tablets of 91% available chlorine. When dissolved in water, trichlor dissociates (splits up) into hypochlorous acid (free chlorine) and cyanuric acid (stabiliser).

Water balance takes into account such factors as pH, total alkalinity, calcium hardness, total dissolved solids and pool water temperature to see whether there is a corrosive or scale forming tendency in the water. A mildly scale forming situation is preferred as a thin coating of calcium will protect the metal fittings in the circulation system.

There is a formula to test whether a pool has water which is corrosive or scale forming called the Langelier Water Balance Formula. A file containing a Java applet to provide a fully interactive facility to carry out a Langelier Water Balance Test is available on this site.

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