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Thread: strange FC readings during shock treatment

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    strange FC readings during shock treatment

    Hi everyone,
    First time post here. I'm following all the steps for shocking with bleach. Have a 16,000 gallon inground Pebble Tech pool and have used liquid bleach solely (with an occasional trichlor tab or two and polyquat) for about 5+ yrs now. Usually keep a close eye on the readings throughout the year, so I really have never had a problem. Haven't had the need to drain or even shock since we did the Pebble Tech about 4 yrs. ago.
    However -
    Last week, I noticed a bit of green algae in a few spots around the pool.
    My readings at that time were;
    FC =2.0 (normally 3-4)
    Alk = 120
    PH 7.6
    CYA = 35-40
    Cal= 350
    temp = 68*

    So I began the shock treatment the night before last.
    Brushed all areas. Brought the FC level up to 14. Next morning was down to FC=5.0ppm, added more bleach, brushed etc. yesterday throughout the day and eve.
    This morning reading was 9.0 w/CC=0, so I added another 3 (96.oz 6%) jugs, which (after waiting about an hour to test) only brought the level up to 12ppm. So I added another two 96.oz jugs. After waiting another 1/2 hour or so, I checked again. New reading was back down to 11ppm. (CC=0).
    I'm somewhat confused at this point. The green algae is gone at least. I've used a total of 18- 96oz.6% jugs so far.
    I'm using the PS233 test kit (from pool solutions) (DPD powder etc.).
    The only thing that *I* can think of that possibly could be a prob is my DPD powder is from last summer, and it looks just a little off-white/gray, rather than the bright white it used to be and I suppose could be giving me inaccurate FC readings???
    Just wondering if anyone has any ideas, or has experienced this in the past.
    Thanks in advance!
    20,000 Gallon In-ground Vinyl Liner
    Hayward Sand Filter
    CompuPool CPSC48 SWG

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

    Re: strange FC readings during shock treatment

    Welcome to TFP!

    The DPD powder is normally good until it has turned dark black. If yours is only slightly gray then it is fine.

    My guess is that the bleach you are using has a lower strength than you are expecting. If you are using either 3% bleach, or bleach that has been stored for too long and lost strength, you would get results much as you describe.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3

    Re: strange FC readings during shock treatment

    Thanks Jason!
    I'll grab a new batch of jugs from a different store and see if that makes a difference.
    20,000 Gallon In-ground Vinyl Liner
    Hayward Sand Filter
    CompuPool CPSC48 SWG

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