poolschool-why add more chlorine

PoolSense asking for too much Chlorine?

People are questioning why the PoolSense device is telling them to add chlorine, yet the ORP reading does not change much when they do. Then when the pool water is tested with either a strip tester or at a pool shop the results show that they have a high level of free chlorine. The purpose of this article is to explain the differences between what we are measuring and what a test strip or pool shop will measure. We will also outline what the possible causes are and what steps a user needs to take to  resolve the problem.

Types of chlorine

The first part of understanding what the PoolSense reading is telling us is understanding what is happening when we add chlorine to the swimming pool. What we loosely term as chlorine can take many forms. In large municipal waterworks, gaseous chlorine is used to treat our drinking water. In large commercial swimming pools, liquid chlorine is added and for the most part, residential swimming pools use dry or granular chlorine.

Gaseous chlorine

Taking a brief closer look at each of these will help us understand why and how each different type of chlorine is used. The use of pure gaseous chlorine is the most effective method of producing clean, sterile drinking water. Gaseous chlorine is however extremely dangerous to use, store and transport so it remains in the realms of large municipalities and large volume sterilisation.

Liquid chlorine

The next type of chlorination is the addition of liquid chlorine. This is a slight misnomer as the liquid chlorine is not pure chlorine but rather chlorine bound to a few other elements that allow it to be stable in a liquid form. The most common form of liquid chlorine is Sodium Hypochlorite. This compound is written as NaOCl and from that, we can see that there are an additional sodium and oxygen molecule added to the chlorine. This form of chlorine is usually used in large swimming pools as it is very easy to implement an automatic dosing system that adds chlorine as needed to the water. The drawbacks of these systems are that the liquid chlorine is sensitive to light and heat so it will lose effectiveness if not stored properly. It also has a short shelf life so it cannot be stored for long periods.

Dry or granular chlorine

We now get to the chlorine we are most familiar with. The dry or granular chlorine we add to our pools through either a floater or simply throwing in a cup or two when needed. Again, this is a slight misnomer as the granular chlorine is not pure chlorine but rather chlorine bound to a few other elements to make a stable dry powder. For granular chlorine, the full name is Calcium Hypochlorite and the formula is written as Ca(OCl)2. We can see that it is similar to the Sodium Hypochlorite in the liquid chlorine.

How does chlorine sterilise water

So now we have looked at the different types of chlorine that are available and how they are added to water to sterilise it. We now need to look at how the chlorine actually goes to work to sterilise the water. This step is critical in understanding why we measure the different parameters in our pools.

When chlorine is added to water it is so reactive that it does not remain in an elemental state. As soon as chlorine comes into contact with water it reacts to form a compound called Hypochlorous acid. This is the chemical that we want in our water to perform the sterilisation. It reacts strongly with biological and chemical contaminants in the water.

Hypochlorous acid

The balancing act starts here. Hypochlorous acid forms a balance between itself and the hypochlorite ion in the water. These are two different forms of the same chemical and they are on each side of a balance beam. The balance point is controlled by the pH of the pool. The more acidic the pool the more hypochlorous acid is formed. The more basic the pool the more hypochlorite ion is formed. 

Hypochloous acid balance

Hypochlorous acid breaks down

Now that we know that we want the hypochlorous acid to do the sterilising work, we need to look at what breaks it down and removes it from the water, as well as what prevents it from doing its work in the water. Hypochlorous acid will break down when it reacts with a contaminant in the water or when exposed to UV light.

Contaminants decrease sterilising power

Contaminants can range from dust and leaves blew into the pool, to natural oils found on human skin, to metals and other inorganic elements added when topping up the pool water. This is usual and this leads to the slow decrease of sterilising power in the water.

UV light breaks down hypochlorous acid

The next method that breaks down the hypochlorous acid is UV light. This poses a serious problem as most pools are in a sunny patch to help keep the water nice for swimming. Simple exposure to the sun can lead to nearly 90% of the available chlorine being broken down during a single day. This is obviously not ideal or sustainable as we would have to continuously add large amounts of chlorine to maintain the desired sterilisation of the water.

Preventing Hypochlorous acid from breaking down

To solve the issue of UV light-driven break down a stabiliser or conditioner is added which is normally Cyanuric acid (or CYA). The CYA bonds weakly with the hypochlorous acid and prevents the breakdown that is usually caused by UV light. This weak bond, however, reduces the strength of the sterilisation effect of the hypochlorous acid. So adding too much stabiliser or CYA limits the ability to effectively sterilise the pool.

With the above knowledge in place we know that we want free chlorine or hypochlorous acid in the water, we want the water pH to be in the correct range, and we want enough stabiliser to make sure we don’t lose free chlorine to the sun.

Managing sterilising ability

So how much of each do we need? The traditional way of measuring the sterilising ability of the chlorine in the water is by measuring the free active chlorine or hypochlorous acid levels. This method, however, is inaccurate and fails when we have high levels of Cyanuric acid in the water. The weak bond between the Cyanuric and the Hypochlorous acids is enough to inhibit the sterilising action but when you test for free chlorine the bond can be broken easily by the test reagents. This means that the water can be weakly sterilising, but the free chlorine is off the scale.

PoolSense reads the sterilising ability

This is where the PoolSense device directly reads the sterilising ability of the water: the ORP measurement. This reading gives us a direct measure of how effective the chlorine we add is. Why not simply measure the free chlorine using a test strip? This is because the test strip reading cannot determine how much of the free chlorine is bound by the CYA in the pool.

If we continuously get low ORP readings but keep adding chlorine this tells us that we have far too much CYA in the water. We have a balancing act once again. We can’t have too much CYA in the water and we can’t have too little. Ideally, we want the CYA levels to be in the 20-30 mg/l or ppm range. We never want to be approaching the 50mg/l mark.

Sterilising and sunlight

What to do if the CYA level of the pool is too high

The only way to remove CYA from your swimming pool is to back-wash or empty the pool slightly and then top up with fresh water. Once the CYA levels have dropped we will then see the ORP rise higher when chlorine is added, and the pH is correct.

PoolSense also available from

PoolSense will be available in more stores very soon and those links will be added to the list below.