Free Chlorine and Your Swimming Pool

Many factors can affect how much free chlorine is in your swimming pool. These include:

  1. Free Chlorine vs Used Chlorine
  2. Sunlight
  3. Bather Load
  4. Method of chlorine delivery
  5. Water balance
  6. Organic Debris


Let’s work through them one at a time.

Free Chlorine vs Used Chlorine
Free chlorine is chlorine molecules which have not yet killed bacteria in your pool water. Once a free chlorine molecule has killed bacteria it becomes used chlorine, or a chloramines. High levels of chloramines in your pool can prevent free chlorine from working, thereby turning your pool green even though free chlorine is being added. The way to remove chloramines is by Shock Dosing. Shock Dosing involves suddenly pushing up free chlorine levels very high (above 5ppm). This can be done by the addition of liquid chlorine or a granular ‘shock pack’. This has the effect of burning off the chloramines, and allowing the free chlorine to return to being active. Consult a pool professional for advice on how much chlorine your pool would require for shock dosing.


Ultraviolet light destroys chlorine molecules. This is part of the reason why your pool requires more chlorine in the summer than the winter. The way to prevent sunlight reducing chlorine levels is to use a chlorine stabilizer – sometimes referred to as sunscreen or sunblock. Stabilizer is cyanuric acid, and it works by forming a weak bond with free chlorine which protects the chlorine from ultraviolet rays. Often, liquid or granular chlorine is sold as stabilized chlorine. This means the stabilizer has already been added. When using salt chlorination, adding stabilizer is essential to prevent chlorine loss from sunlight. See more about stabilizer under water balance. A good pool cover will also reduce chlorine loss from sunlight however this is not always practical in summer when you are regularly using the pool.


Bather Load
Bather Load is simply how many people are swimming and for how long. People still sweat while in the water, plus there are oils and fats coming of your skin. Plus there’s dirt when, for example, you get dirty feet walking across the grass to jump in the pool. All these things will reduce free chlorine dramatically. Free chlorine levels can drop to zero in a few hours when 3 or 4 people are using the pool.


Method of Chlorine Delivery
There are 2 ways to get chlorine into your pool – add it manually or use a salt chlorinator. When you manually add chlorine you will use either liquid or dissolved granular chlorine. This will bring the chlorine level up quite quickly when added, then the chlorine level will decrease as it is used to kill bacteria until you add again. This can mean adding chlorine daily in peak summer use. Salt chlorination uses a device, called a salt chlorinator, which produces chlorine gas directly into the water. It requires that salt is added to the pool and works by passing electrical current through an electrode, usually called a cell, immersed in the water. The electrode uses the salt content in the water to conduct the current, and the by-product of this process is chlorine gas. Salt chlorinators produce small amounts of chlorine but do it constantly, meaning they are an easier way to maintain safe levels of chlorine in the water. However they are dependent on the salt level in the pool, so periodic additions of salt is required. If the salt level is too low, enough chlorine will not be produced to maintain a clear swimming pool. Salt chlorinators usually have an internal clock allowing them to run automatically, and controls for setting the level of chlorine production. As a general rule, slat chlorinators will need to run for at least 8 hours a day at maximum output during the peak summer period to maintain safe free chlorine levels in your pool. It is important to note that salt chlorinator cells do not last for ever. Expected life varies between brands and can be anywhere from 2 to 7 years. You will usually know if a cell is worn when maximum output cannot be achieved even though salt levels are correct.


Water Balance
There are many parameters which constitute a healthy water balance. The ones which pertain to chlorine and its ability to kill bacteria are pH, stabilizer, organic phosphates and salt (when a salt chlorinator is being used).
  1. ph – Free chlorine will only be active and able to kill bacteria when the pH level is maintained correctly. This requires a pH level of between 7 and 8, where 7.4 is usually considered to be ideal. Chlorine has a very high pH, so adding it to your pool by any method pushes the pH up. This is why it is necessary to add Hydrochloric Acid to the pool which reduces the pH, bringing it back into the range where the chlorine is active.
  2. Stabilizer – Stabilizer levels should be maintained between 30 and 80 parts per million for effective sun screening. Generally speaking, lower levels in the winter and higher levels in the summer will be required. It is important to note that when stabilizer levels go over 100ppm this can produce an effect called chlorine lock where no matter what your free chlorine level is, the chlorine is unable to kill bacteria. This is a situation where your pool can turn green even though you have a good level of chlorine in the water.
  3. Organic Phosphates – These are introduced into the water through organic debris (leaves, branches and dirt) and run off from the garden. Phosphate levels above 0.5 (?) can deplete free chlorine VERY quickly. A product called Starver is used to reduce phosphate levels in the water.
  4. Salt – When a salt chlorinator is being used, salt must be present in the water for chlorine to be produced. The salt level required depends on the brand of salt chlorinator. As a general rule, most salt chlorinators will require a salt level between 4000 and 6000ppm. While salt is not used up in the chlorination process, whenever your water level is topped up it dilutes the salt content of the water. Regular checking of salt levels is a must when using a salt chlorinator.


Organic debris
Leaves, branches and dirt will all introduce bacteria into your pool. Regular scooping, brushing and vacuuming of your pool is necessary to adequately maintain safe chlorine levels.


Here is a check list for if your pool is looking murky, green or

the free chlorine levels are low:

  • Has the bather load suddenly increased?
  • Is there a lot of debris or dust in the pool?
  • Does your pool require shock dosing?
  • Is your stabilizer level between 30 and 80 ppm?
  • Is the pH correct?
  • Are phosphates present in the water?

If you are using a salt chlorinator:

  • Is the salt level correct?
  • Are you running it for long enough each day?
  • Is it set to maximum?
  • Is the cell worn out?

Would you like to view our range of water testing equipment or replacement salt cells?


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