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Free Chlorine, Combined Chlorine, and Total Chlorine

Updated: May 6



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Chlorine Tabs

Chlorine, the staple sanitizer for most swimming pools, has 3 different readings when it comes to being tested: free, combined, and total.


Free chlorine is the amount of chlorine in your pool that is currently working to actually sanitize. Most pool test kits and test strips check for this reading. This is the reading most pool owners are familiar with.


Combined chlorine is the chlorine in your pool that is NOT working because it has combined with other impurities in the pool instead of sanitizing, therefore it has been rendered useless.


Total chlorine is simply the total amount of chlorine in the pool. It is the sum of free and combined.


Easy way to see it: Total chlorine-free chlorine + combined chlorine


Why are these 3 chlorine readings important? Because you may have a lot of chlorine in the pool that is not working, and last time I checked, chlorine is not cheap!


Have you ever put shock in your pool, tested for chlorine, and not get a reading? Where did the chlorine go? The answer is: its in there, it's just tied up with other impurities - it has combined with those impurities instead of being "free" to sanitize. Combined chlorine causes a lot of issues: cloudy pools, pools that have a strong chlorine smell, green pools - you name it.


Here's how chlorine works: when chlorine is introduced into a pool, it runs around destroying impurities in the water. When there are too many impurities in the water (whether you can visibly see them or not), they outnumber the chlorine molecules and instead of the chlorine destroying them, it combines with them.


So how do you turn this combined chlorine into free chlorine? The answer is - more chlorine! You see, once there's enough chlorine to overpower those impurities, it destroys the impurities and the bond that caused the combined chlorine.


Think of the combined chlorine as captured soldiers. They went in to do their job, but were overtaken. How do you save those soldiers? By sending in enough reinforcements to destroy the enemy and free those captive soldiers. Now you've got ALL your soldiers back!


If your test kit or test strips tell you free chlorine and total chlorine levels, the difference between the two is your combined chlorine.


Example: Free chlorine: 3ppm / Total chlorine: 5ppm


In a good reading, you want your total chlorine and free chlorine to be the same number. Here, the total chlorine is 5 and the free is 3, meaning there's 2ppm of combined.


How do you break that combined? Here's how: For every 1ppm of combined chlorine, you need to add 1lb of 68% shock per 10,000 gallons of water. So if your pool is 10,000 gallons, you would need 2lbs of 68% shock in the above example to break that 2ppm of combined chlorine and make it free chlorine. If your pool is 20,000 gallons in the above example, you would need 4lbs of 68% shock to break that 2ppm of combined.


So in a nutshell, if you have problems getting a free chlorine reading when you KNOW there's chlorine in the pool, then you've got combined chlorine. Bring a sample in and we'll straighten it out for you.


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