Stabilised or un-stabilised – what’s the difference?

Chlorine comes in many forms, some are stabilised, some are not - but what is stabiliser?  And when do you need it?

Sunlight, whilst perhaps the most sought after feature of a British summer can also have an effect on the sanitiser level in your outdoor pool.  The ultraviolet light can degrade the chlorine disinfectant - but this can be fixed by including a stabiliser as part of your pool water treatment.

Stabiliser (or cyanuric acid) can be added to pool water as a stand alone product, or you can choose to use a sanitiser that already includes this – a stabilised chlorine product.  Chlorifix, Chlorilong Classic, Power 5 and Ultimate 7 are all stabilised chlorine products designed as regular sanitisers with different functions but all include the benefit for outdoor pools of protecting the chlorine residual from degradation by sunlight.  Stabilised chlorine granules such as Chlorifix Shock can also be used as a shock dose to rapidly increase the chlorine level when required.

Cyanuric acid needs to be over 20ppm for an outdoor pool in order to protect the chlorine in the water from degradation.  It should not be allowed to exceed 100 as higher levels can ‘lock’ the chlorine in the water causing the chlorine to be ineffective.  Stabiliser levels are reduced by dilution and usually a good backwash regime coupled with the inevitable British rain provides enough water replacement to keep the stabiliser within the recommended parameters.  However, it should be checked regularly with a test kit to ensure that this is the case.

Shock Dosing

A swimming pool put to good use can experience water quality issues such as green or cloudy water as bather loads increase and stormy weather brings algae spores to life.  In these instances a shock treatment is required to quickly increase the active chlorine content which will destroy microorganisms that are responsible for turbid water as well as the algae that turns the water green. 

Un-stabilised chlorine is primarily used as a shock treatment in case of water problems, high heat or high pool usage as it does not cause over stabilisation of the water.    With a shock treatment you are aiming to quickly increase the chlorine level to 10ppm with the filtration operating to ensure that the shock treatment reaches all parts of the system.  The filtration should be allowed to operate for at least 12 hours.  Note that the pool should not be used until the chlorine level has dropped below 3mg/l.

Chloryte calcium hypochlorite granules contain 70% active chlorine and provide an immediate, effective shock treatment without the risk of over-stabilization.  Available in a 5kg pail it is easy to handle and can be added directly to pool water. 

Sodium hypochlorite or liquid chlorine is another un-stabilised option for shock treatment though it has a much lower active chlorine content (10-15%) and in its more awkward 20 litre container can splash when handled unless a hand transfer pump is used. 

Don’t forget pH

In order for chlorine to work effectively the pool water should be balanced prior to the addition of chlorine with pH in the region of 7.0 – 7.4.  In addition, the type of chlorine used will have an effect on the pH of the pool water, so when you add chlorine you may also need to subsequently correct the pH.