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Water Balance

Why is it so important to correctly maintain the
correct chemical balance of your pool water?



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.

If the water is corrosive, it could have a damaging effect on metal fittings in the circulation system and even on the fabric of the pool itself. If the water is highly scale forming, there would be a danger of scaling and eventual blockage in the pipework, and of calcium deposits on the pool surfaces.

The aim is to have water which is slightly scale forming. This will enable a hard, thin layer of calcium to be deposited on vulnerable areas of the circulation system to afford a measure of protection from corrosion without the ill effects of serious scaling up.

There is a formula to test whether a pool has water which is corrosive or scale forming called the Langelier Water Balance Formula.



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. 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 pool surface or grouting.



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

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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.

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Water is considered hard if its calcium hardness is over 250ppm and its alkalinity is over 150ppm.

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Water is considered soft if it has a hardness of under 50ppm as calcium carbonate and an alkalinity of under 30ppm as calcium chloride.

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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.

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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.

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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

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pH Explained


The scientific definition is 'the negative logarithm of the Hydrogen ion concentration'. OK - don't give up Ė itís a very important concept. Letís get through to that definition in every day terms.

First accept pH is a scale measuring the acidity or alkalinity of a solution. Squeezed lemon and vinegar are sour or acidic. If we drank them we would take something alkaline like bicarbonate of soda or magnesia to neutralise the acidity in our stomachs - in other words raise the pH. The pH scale runs from 0 (highly acidic) to 14 (highly alkaline) with distilled water being neutral at pH 7.0.

Now instead of sour, letís use the term 'hydrogen ions', and instead of alkaline letís use the term 'hydroxyl ions'. Vinegar has many more hydrogen ions than hydroxyl ions. Conversely, soda ash and bicarbonate, being alkaline, possess more hydroxyl ions than hydrogen ions. In summary, acids produce hydrogen ions, alkalis produce hydroxyl ions. pH is the power (German 'potenz') of a solution to yield hydrogen ions [H+].

One more step to go. The scale between 0 and 14 is logarithmic (pH 8.0 is 10 times more alkaline than pH 7.0 and pH 9.0 is 100 times more alkaline than pH 7.0)

Now we are back to the scientific explanation of pH as 'the negative logarithm of the hydrogen ion concentration'. Negative because the more hydrogen ions, the lower the pH.


Why is pH so important?

  1. The pH value affects the amount of hypochlorous acid (free available chlorine) that is formed, and therefore determines the effectiveness of the chlorine as a killer of bugs.

    • At pH 6.5, 90% of the chlorine will be hypochlorous acid

    • At pH 7.5, 50% of the chlorine will be hypochlorous acid

    • At pH 8.0, 20% of the chlorine will be hypochlorous acid

Unfortunately you cannot run your pool at pH 6.5 - it would be acidic enough to corrode the metal fittings in your pool circulation system and it is too far from the human body's pH of 7.4 to be comfortable to bathe in. The compromise is 7.2 to 7.6, preferably midpoint of 7.4. Remember, if you let the pH drift out of this range, you will have to use more chlorine to get adequate disinfection.

  1. Bather comfort. At high pH, the water will make your eyes sting and possibly give you a sore throat

  2. At high pH there are two dangers.

  3. (1) The danger of scale forming on your pool surfaces, pipework and fittings. This is because at a pH of around 8.0, the calcium in the water combines with carbonates in the water. Result? Calcium carbonate or scale.
    (2) Calcium carbonate can form into tiny particles and float around in the water giving it a cloudy, turbid appearance.

  4. A low pH can corrode metals, eating away at copper fittings and heat exchangers leaving metal oxides to stain pool surfaces. Under certain conditions the precipitated (particulate) metals can tint your hair, giving you a rather dated appearance in these post-punk times!

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