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Thread: Question on FC rising higher than expected

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    Question on FC rising higher than expected

    I've been fighting the past few weeks with my pool.

    To keep the story short, I'll focus this one thread on a specific question related to FC and the amount of 12.5% liquid chlorine required to raise my above ground pool to Shock level.

    My pool is roughly 7600 gallons or so (18ft above ground vinyl). According to the poolcac, adding 8oz of 12.5% liquid bleach is enough to raise the FC by 1. Yesterday my FC was 0.5, and my shock level is 16. The pool calc shows that 128 fl oz of 12.5% should raise the FC by 16 in 7500-8000 gallons.

    So, I poured in 128 fl oz. A few hours later my FC was at 25, way above the expected 16. This happened to me last week as well one time. How can the FC raise so much in a known amount of water. The dilution is a known value based on the gallons in my pool. What else would have a huge effect on my FC level that instead of raising by roughly 16 I raised it by 25?
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    Texas Splash's Avatar
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    Re: Question on FC rising higher than expected

    The only things that should effect your target result after dosing with a chemical are:
    - Pool volume incorrect
    - Poolmath Calculator info not entered properly (I confirmed what you did - 124 ounces of 12.5% to increase FC by 16 in a 7,600 gal pool)
    - Strength of product stronger/weaker than advertised
    - Product not allowed to mix well enough
    - Water/testing sample pulled from different locations (not consistent); water circulation differences in the pool
    - Testing error

    Those are some things I would think of at the moment.
    Pat (a.k.a. Texas Splash) ~ My Pool: Viking Fiberglass; 17,888 Gal; Waterway Supreme 2-sp/2-hp pump; Hayward Ctg filter; TF-100 w/ Speed Stir
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    needsajet's Avatar
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    Re: Question on FC rising higher than expected

    All I can think of is two possible things. Maybe testing error and pool volume. Chlorine can be stronger than the listed % when very fresh, but that's small.

    Testing... 10 ml sample, 1 drop = 0.5 ppm FC, TF100 or K-2006 test kit?
    Pool volume. Is 48" the water depth or pool wall height? Is the 18' measured?

    Chlorinator? Floater? Someone sneaking in with bonus chlorinating liquid? Amazon drone delivery?
    12k IG salt; glass beads in plaster; K-2006C, K-1766, CCL, and Aussie 4in1 (HTH); Pentair Eco800 1.2HP VS; Zodiac SWC 1.3 lb/day (25 g/hr); 25" filter recycled glass; OKU solar panels; 1/2 HP solar pump; Rebel (Warrior) pool cleaner; FlowViz; prior pool AG 10k | Read Before Posting to get the best possible advice | ... and this helped me a lot!: TFPC for Beginners

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    Re: Question on FC rising higher than expected

    Ok, thanks. I pool filter/pump circulates pretty strongly, and I did the original check roughly 3 hours after adding the bleach when I had the original FC 25 reading. That was last night, and this morning after the pump running all night the reading was still FC 19. So I have to imaging that is plenty of time for the product to mix in. I do sample from only 2 locations in the pool, but I'll vary it up going forward and also try changing the direction of my return spout to alter the flow pattern in the pool.
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    needsajet's Avatar
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    Re: Question on FC rising higher than expected

    Yeo, I missed circulation, but it sounds like you have that part under control
    12k IG salt; glass beads in plaster; K-2006C, K-1766, CCL, and Aussie 4in1 (HTH); Pentair Eco800 1.2HP VS; Zodiac SWC 1.3 lb/day (25 g/hr); 25" filter recycled glass; OKU solar panels; 1/2 HP solar pump; Rebel (Warrior) pool cleaner; FlowViz; prior pool AG 10k | Read Before Posting to get the best possible advice | ... and this helped me a lot!: TFPC for Beginners

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    Mr Bruce's Avatar
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    Re: Question on FC rising higher than expected

    When I put your pool dimensions into Estimating Pool Size I get 7100 gallons.
    32K gallon Plaster - 1hp Hayward 2 speed Super Pump - Hayward S200 Sand Filter - TF100XL
    Test Kits - Pool Math - Chlorine/CYA/Target/Slam Chart

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    Re: Question on FC rising higher than expected

    Quote Originally Posted by needsajet View Post
    All I can think of is two possible things. Maybe testing error and pool volume. Chlorine can be stronger than the listed % when very fresh, but that's small.

    Testing... 10 ml sample, 1 drop = 0.5 ppm FC, TF100 or K-2006 test kit?
    Pool volume. Is 48" the water depth or pool wall height? Is the 18' measured?

    Chlorinator? Floater? Someone sneaking in with bonus chlorinating liquid? Amazon drone delivery?
    Pool is 18ft measured. I rechecked water depth this morning to try to keep my sanity and it measured to 47" depth so just an inch under 4ft exactly. By my calculations an 18ft pool at avg depth of 4ft is roughly 7600 gallons.

    I use the TF100 kit. Just ordered some refills that hopefully will come this week so I could rule out any faulty/old product causing issues. 10 ml water in the tube, and count number of drops till clear and then take half of that number to get the FC value.

    No chlorinator or floater, only chemicals I add is the liquid bleach manually.

    Thanks for the response and help.

    - - - Updated - - -

    Quote Originally Posted by Mr Bruce View Post
    When I put your pool dimensions into Estimating Pool Size I get 7100 gallons.
    I'm taking my estimation from the poolcalc here on this website: http://www.troublefreepool.com/calc.html

    It says 18ft round pool at 4ft deep is 7600 gallons. Even still, at 7100 gallons, using 128 fl oz should only raise FC to 18, still way less than the 25 reading I got.
    18' round AG vinyl | avg. 3'9" deep | ~7,600 gallons
    Hayward Pro Series Sand Filter, Model S166T | Hayward 1hp Pump

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    duraleigh's Avatar
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    Re: Question on FC rising higher than expected

    It is not the chemistry.........It would not be faulty and measure too high....too low, maybe, but not too high.

    It is not the mixing of your pool water. 30-45 minutes is gracious plenty unless your circulation is absolutely horrible.

    It is not the chlorine. 12.5% is the strongest you can get.

    I don't know what the problem is, but it is not the above.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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