poolschool_understanding poolwater chemistry

Understanding your pool’s water chemistry

The purpose of this document is to outline the basics of pool chemistry and how you, as a customer, can get the most out of your PoolSense device. Pool water chemistry is a complex subject. The health of your pool depends on so many factors that affect the efficiency of the chlorine, which is the product that kills the bugs. 

chlorine below 1ppm

What makes bugs grow and your pool green?

Essentially the green in your pool consists of algae that multiply/blooms exponentially if the chlorine levels in your pool drop below a concentration of 1 ppm (one part per million). There are other factors that also influence the growth of this algae, temperature and in certain circumstances high levels of pH. The environment has a marked impact on the bacterial load your pool is subject to. Leaves and other debris are the biggest challenges you will probably face in maintaining your pool.

What “consumes” the chlorine you add?

Without going into the microbiology, the chlorine you add breaks down into two products, hypochlorous acid (HOCL) and hypochlorite ions (OCL-).

How does the chlorine kill the bugs?

Both products of chlorine kill bacteria and algae by breaking down the enzymes and cell structures, rendering them “oxidized” and harmless.

How do you keep the chlorine in your pool effective?

Chlorine is consumed in combating the bacteria in your pool, but in addition, UV rays from the sun have a marked effect on chlorine levels as well. It is one of the reasons we notify you in the evening as this is the best time to add chlorine as it has a full night of darkness with no UV to do its work. You can see this effect in the PoolSense app where the ORP rises in the night and drops during the day.


Over the past few years, manufacturers have introduced stabilisers into the chlorine products they supply. The common stabiliser used today is Cyanuric Acid and it helps shield the chlorine from the effects of UV from the sun. Cyanuric Acid or stabiliser is added to most commonly used chlorine products including granular and tablets (floaters).

Chlorine lock & Cloudiness

As with anything, it needs to be kept in balance between 30ppm and 40ppm in order to be effective. However, there are side effects of continually adding products with stabilisers in that should the concentration go above 100ppm (mg/l) then it has the effect of creating what is commonly known as chlorine lock, as you need more and more chlorine to do the same job of killing the algae. It can also lead to a higher risk of staining and cloudiness. In the normal course of maintaining your pool where you are backwashing regularly the effect of stabilisers may not be felt for a long time, as you are replacing water when you replace or top up your pool water from your tap. This has the effect of diluting the Cyanuric Acid and keeping the levels in the normal range. 
If your PoolSense app keeps asking you to add chlorine but it isn’t rising continuously the chances are that you have chlorine lock. We suggest you take a sample to your nearest pool shop and ask them to test it. If it comes back with excessive Cyanuric Acid/Stabiliser levels then you will need to partially drain and refill your pool with fresh water. Watch your PoolSense app and see if the ORP levels rise more easily after adding chlorine, repeat if necessary.

when to add chlorine
chlorine lock

What impact does pH have on the effectiveness 

of the chlorine in your pool?

Firstly, what is pH? pH is a measure of the acidity of your pool. In the chemical world, the measure of pH ranges from 0 to 14 where 0 is extremely acidic and 14 is extremely basic (non-acidic). A pH of 7.0 means the water is “neutral”, as a matter of interest your tears are typically a pH of around 7.0 so this is why you feel differences in pool acidity in your eyes, as too low or high a pH will have the effect of creating irritation. This effect may also be felt on your skin.

pH affects chlorine levels

pH affects chlorine levels, especially when it’s too high. When the pH level is too low, chlorine actually sanitizes more efficiently, but the pool water becomes corrosive. It can damage pipes, pool liners, metal components, the pool pump and floaties such as beach balls, that remain in the pool too long. However, if the pH is over 7.6 your chlorine no longer effective, in fact as you can see from this chart at a pH of 7.6 your chlorine is only around 30% effective and you will be wasting a lot of money as at that pH you need a lot more chlorine for it to be effective.

The chart below shows the level of active Hypochlorous Acid is in the water with a pH level of 6.0 all the way to 8.5.

With a pH level of 6.0, your chlorine will be highly effective with 97% of your chlorine available to kill bacteria and algae. However, at 6.0 your pool water would be extremely acidic and unsafe to swim in. On the flip side, if your pH is at 8.5 only 9% of your chlorine will be available to kill bacteria, algae and other pathogens. 
Your PoolSense App guides you very effectively and all you need to do is maintain the pH within the green band to get the best use out of the chlorine you have added.

acid measurements - too alkaline
active hypochlorous acid chart

Adjusting pH and keeping this within the range advised

The optimal band for pH is 7.2 to 7.6. When your pool is balanced and in what we refer to as “Double Green” as per the image it is relatively easy to maintain it in this balanced state. When your chlorine is consumed it is what is known in chemistry as a “Basic Reaction” which means that generally your pH will tend to increase over time and most of the time you will be asked to add some acid in order to reduce the pH.

Adding Acid has the effect of reducing the pH and adding Soda Ash or “pH Up” has the effect of increasing pH. However if you are asked to add “pH Up” to your pool, by the PoolSense App, and the pH is not rising your Total Alkalinity (TA) may be too low.

Total Alkalinity (TA)

In simple terms, TA is defined as the water’s resistance to change in pH, or the water’s ability to “buffer” the change in pH. For example, if you have a low TA 1 cup of acid may drop you from 7.5 to 6.8. With high TA 1, a cup of acid may drop you from 7.5 to 7.3. So high TA resists the change in pH, however, TA has no impact on sanitising power of the chlorine in the water. It simply plays a role in trying to keep your pH more stable. Adding a product like “Alkalinity Up” will generally also increase your pH and assist in getting you into “Double Green”.

ORP- Oxidation Reduction Potential

As previously discussed, you kill the bad stuff like pathogens/bacteria/algae in the pool by Oxidising them. ORP is the real measurement of how strong the water’s intrinsic ability is to Oxidise the bad stuff. You can improve the ORP by adding chlorine and keeping pH in range, and this is one of the basic measurements we use to assist and guide you in maintaining your pool water.

  • ORP should be above 650mV to ensure that the chlorine will work in pools.
  • More chlorine (ppm) – increases ORP
  • High pH (alkaline water) – reduces ORP
  • Low pH (acidic water) – increases ORP and the sanitising power of the water.

There is little to no impact if the ORP / ppm chlorine is too high. Even on the comfort level of the water.

Note adding granular chlorine to the water will generally increase your pH.

24h wait time
1. Wait 24 Hours

Leave your device to run for 24 hours before taking any action.

2. Get pH first

The key is to focus on getting your pH first. If your pH is well below the green band then the PoolSense app will be telling you to add pH Up. So follow the instructions and monitor the effect. You should see the pH increase fairly quickly, say within a few hours. If it starts to increase and the App tells you to add more –into the green band and the pH indicator on the App goes green. However if after 2 or three attempts of adding pH Up and you are not seeing a difference it may mean that your Total Alkalinity is too low and the water is resisting the change. In this case, we recommend taking a sample to your pool shop for testing. You may need to add a product like Alkalinity Up in order to increase the TA, which will then make the adjustment using acid more effective.

3. Focus on ORP

Once you have achieved the green state for pH, focus on the ORP levels (chlorine effectiveness). If the ORP levels are low the App will be asking you to add Chlorine, just follow the instructions. However, like pH, if after two to three days the very little effect is seen you may have too much Cyanuric Acid that is creating a chlorine lock effect as described above. Again, should this happen we would suggest you take a sample to the pool shop and ask them to test your water. The only way to really sort this out is to drain some water by backwashing and to fill the pool with tap water. However your pool shop professional may suggest other methods to assist in reducing the Cyanuric Acid or Stabiliser levels in your water.

So how do you get your pool into the “Double Green” state using PoolSense?

When you first drop your device into the pool, you will probably not believe the results. Your pool may look great and PoolSense may be telling you to add chlorine, or a product to adjust the pH. Do not panic, first read and understand the basic chemistry in this document and follow the simple steps below.

PoolSense also available from

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