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What is a Chlorine "Lock?"

Updated: Jul 13, 2019

What is a chlorine "lock?"

If you've ever had your water tested and been told you have a lock in your chlorine levels, this is what that really means. Your water test report should have three different chlorine readings: Free Chlorine, Total Chlorine, and Combined Chlorine. Free Chlorine is your available, working chlorine in the pool. You want your Total Chlorine reading, the sum of your Free and Combined Chlorine readings, to be the same as your Free Chlorine reading, meaning all the chlorine in your swimming pool is working to sanitize the water. If your Free and Total Chlorine readings are not the same, and have a bigger difference than 0.5 ppm, we consider this a chlorine "lock," also known as your Combined Chlorine level. All that means is that some of the chlorine in your pool is not working to sanitize your water and needs to be brought back to working condition. Ideally, your Combined Chlorine reading should be zero.


Many people associate chlorine odor, itchy skin or burning eyes with "having too much chlorine" when really it means you have Combine Chlorine and the pool needs shocked.

Example of No Chlorine Lock

Free Chlorine: 3 ppm

Total Chlorine: 3 ppm

Combined Chlorine: 0 ppm

Example of Chlorine Lock

Free Chlorine: 3 ppm

Total Chlorine: 5 ppm

Combined Chlorine: 2 ppm

How do you fix a chlorine lock?

Super Chlorination

If you do not have a Free Chlorine reading of at least 0.5 ppm, we will need to "super shock" your pool. Breaking a chlorine lock will take more than you weekly dose of chlorine. Typically it takes 3X the regular amount and maybe more. The extra amount of chlorine needed will depend on the size of the chlorine lock and the number of gallons of water in your pool.

If you have any questions about chlorine locks, please call or come in the store and ask any retail employee. We are more than happy to help!

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