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Thread: Larger FCl/TCl mismatch after shock?

  1. Back To Top    #1

    Join Date
    Jan 2011
    Roseville, CA

    Larger FCl/TCl mismatch after shock?

    Last night I checked my pool chemistry, using my LaMotte ColorQ, and found the following:
    • Free Chlorine = 2.89 PPM[/*:m:2nyweghm]
    • Total Chlorine = 3.26 PPM[/*:m:2nyweghm]
    • pH = 7.6[/*:m:2nyweghm]
    • Alkalinity = 99[/*:m:2nyweghm]
    • Calcium Hardness = 110[/*:m:2nyweghm]
    • Cyanuric acid = 48 PPM[/*:m:2nyweghm]

    As there was a lot of swimming by many last weekend and looking forward to it again this coming weekend and the difference between Free and Total chlorine was larger than normal, I decided to shock the pool. My pool is about 12,500 gallons and I shocked with 2 pounds of 53% Calcium Hypochlorite at about midnight. I just checked it (11 hours later) and see the Free Chlorine is 6.29 PPM and Total Chlorine is 7.47 PPM... that is a much larger difference between free and total chlorine than before shocking... is this normal? I'm still fairly new at pool maintenance, I bought the house in February and just started swimming in it about 3-4 weeks ago. This week, solar has maintained the temp between 82-75F. The water is very clear and inviting.

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Larger FCl/TCl mismatch after shock?

    You aren't using enough chlorine. With CYA around 50, shock level is 16 to 20. You only raised the FC level up to about 13. You need to maintain shock level until CC falls to 0.2 or lower. (Note that Pool School says CC of 0.5 or lower, but you are using a different CC test than that recommendation was designed for, so you need to get CC down to 0.2 or lower.)

    CC is formed when organic debris combines with chlorine. Subsequently, additional chlorine combines with the CC and further breaks it down. When the FC level is fairly low and there is a lot of organic debris in the water, not all of the chlorine will combine with the debris to form CC. As you add more chlorine, additional CC forms and some of the existing CC is broken down. If you keep adding chlorine, this process continues until all of the organic debris is finally broken down.
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