Pool Chemistry

Maintaining the chemistry in your pool

(This information is also on laminated sheets inside your testing kit)
Your pool is 9.15 metres long, 5.2 metres wide, and 1 metre deep giving a volume of 47580 litres.
The temperature is maintained at 28 deg C by the heat pump.
When the pool is first filled it is important to adjust the hardness and total alkalinity of the water as this greatly assists in stabilising the chemistry and avoids large changes in pH and free Chlorine levels. Also when first filled it is important to shock dose the pool with chlorine to remove any initial bacteria.
When a pool has been balanced it is maintained by measuring and adjusting the free Chlorine and the pH levels on a daily basis (it is recommended to measure these 3 times a day if the pool is in heavy use as it is much easier to maintain a pool that is not too far away from the balance condition).


All chemicals should be treated with respect. They should be stored in a safe place away from other substances such as hydrocarbons (petrol, diesel, etc).
Dry chemicals should always be dissolved in water before being added to the pool, and liquids added to water to dilute them.
Acid must always be added to water NOT the other way round as it generates heat and could splash up into your face or on clothing.


The hardness of the pool is important as it also assists in maintaining the correct pH and Chlorine balance. Hardness is controlled by the addition of Calcium Chloride granules, which must be dissolved in water before use. The target hardness is 150 – 200 ppm.
Adding calcium chloride which has a pH of 8.5 will also raise the pH of the pool.
Calcium chloride does not get used up in normal pool operation so once it is set should remain for the term and requires measuring say once a month. The only reason the hardness level will decrease is if there is a significant loss of water and the pool is topped up with soft tap water.


The alkalinity of the pool is important as it controls how quickly the pH of the pool changes when other chemicals are added. Is it controlled by adding Sodium Bicarbonate which is sometimes referred to as “pH Buffer” . The target alkalinity is 100-150 ppm.
Adding Sodium bicarbonate (pH  9) will also raise the pH of the pool.



The pH is a measure of the acidity of the pool. A pH of 7.0 is neutral. Less than 7.0 is acidic and greater than 7.0 is alkaline. The target for the pH in the pool is slightly alkaline 7.2 to 7.6 is ideal.
The pH of the pool water is maintained by adding sodium bicarbonate (See note on alkalinity) which increases the pH.
Correcting the hardness by adding Calcium Chloride (see note on hardness) will also increase the pH value.
Both alkalinity and hardness should be changed gradually so that the pH can be monitored to ensure that it stays within the target range of 7.2 to 7.6. If it does exceed 7.6 then the pH can be reduced by the addition of Sodium bisulphate.


Two measurements of chlorine are important.

  • The free chlorine level (fCl) is a measure of the amount of chlorine available to sanitise the pool.
  • The total chlorine level (tCl) is a measure of both the free chlorine and the chlorine that has been used up in the sanitising process.

The used chlorine is chlorine that has been converted into Chloramines. It is the Chloramines that gives a pool a strong Chlorine smell and must be removed. Chloramines are removed by ADDING more chlorine to the pool.

As this pool is being heavily used the free chlorine levels should be kept high. Free chlorine is rapidly decreased by natural causes such as sunlight, and by the bather load.
The target free chlorine level is 4.0 – 6.0 ppm with the total chlorine being the same indicating that there are no chloramines present. The total chlorine level should be maintained within 0.2 of the free chlorine level. i.e. if the free chlorine level is 5.0 the total chlorine level should be no more than 5.2.

In the pool the normal free chlorine level is maintained by the inline chlorinator mounted on the trailer. This has a dial which can be set to control the amount of chlorine added automatically to the pool. Once set it should maintain the free chlorine level at the correct value for the daily bather load. The inline chlorinator contains TriChlor tablets which dissolve very slowly. These tablets also contain another chemical – Cyanuric Acid which is added to stabilise the free chlorine and inhibit the depletion of the chlorine by sunlight.
Cyanuric acid gradually builds up in the pool over time and should be less than a maximum of 100ppm. This will not be a problem in the pool as it is refilled at the start of each term.


Shock Dosing

When the Chloramine level is high the chloramines must be removed. This is done by shock dosing the pool with a high level of free chlorine. When shock dosed the pool should not be used until the free chlorine level has returned to normal values (4 – 6 ppm). Shock dose by adding 2 litres of liquid chlorine, or 400gm of Aquachlor, and waiting at least an hour.

It is important to make sure that the pH level of the pool is between 7.4 and 7.6 before shock dosing as it is within this range that the chlorine has maximum effect.


Occasionally the normal shock dose does not work. A high level of chlorine is added to the pool but the chloramines do not decrease. This can be caused by the pool chemistry being out of balance.

Each of the individual chemical readings can be within the recommended range but the combination of pH, Hardness, Alkalinity results in pool water which is either corrosive or will deposit scale, particularly in the filters.


Balancing your pool is discussed in a separate article - Balancing your Pool.