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Thread: Testing for FC after Shock

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    Testing for FC after Shock

    Even though I've had the pool for 10 years, I just discovered this site and decided to go beyond my simple Ph/Chlorine test kit, pool store tests, and advice to "shock weekly." It's taken a little study to understand the relationship between CYA and FC, the difference between stabilized and unstabilized chlorine, and the shock regimen one follows to kill algae.

    I am trying to kill some persistent green algae which always appears on the steps and a few other places within days of shocking the pool. Obviously, I have not been shocking properly.

    Here are the test results: CH 225, TA 100, pH 7.5, CYA 51. At the time of this test, there had been no shock for some time and FC was 1.61. I've studied the tables and understand that based on my relatively high CYA, I should be maintaining more FC. OK. But the real question concerns checking FC after shock using my K-2006 Test Kit.

    To determine the level of FC if you use only a 10ml sample of water, you add R-0871 drop by drop until the sample is clear. Let's say I nail the FC level with my first addition of shock (20 for my type pool); the test would require 40 drops (1 drop = .5 ppm) to clear the water in the test vessel and confirm that I've got 20 ppm FC. Is this correct?

    Then the procedure is to maintain the shock at least for the first day (until the overnight test can be done to see if any chlorine consuming contaminants remain..), so I would check in the PM --- let's say FC has dropped some, so only 30 drops of R0871 are needed. Then there is the test in the AM --- another 20 or 30 drops, etc.

    Am I describing this correctly? It just seems one is going to go through a LOT of R-0871, and I read other posts where people say they use this test daily or every other day to measure FC.
    14,000G Gunite pool, 1-1/2 HP Challenger Pump + 1HP Booster Pump, Waterfall feature, Pentair 60 DE Filter, Polaris 360 Pool Sweep
    Manassas, VA

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    Mod Squad zea3's Avatar
    Join Date
    Jul 2009
    Houston, Texas

    Re: Testing for FC after Shock

    Yes, during the shock process you will run through a lot of testing reagent, depending on how long the pool needs to stay at shock level. Some people test daily, some every other day, some only weekly. After you have been testing the pool for a while you get a feel for how much chlorine the pool uses on a daily basis and dose it accordingly.
    TFP Moderator
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    Re: Testing for FC after Shock

    I have to admit I skimmed parts of your post, but I do have an answer for the drop test. You are correct, 1 drop = 0.5 ppm for a 10ml sample. (I'm assuming adding the powder first is implied) My CYA is around 90-95 so I ordered 3 more bottles of 871 from - I don't think I will need that many but I didn't want to order again for 1 item, plus I bought more cya reagent.

    I end up testing 2-3 times weekly, I use a spreadsheet to record my results and make educated guesses the rest of the time.

    At first I tried to test every day.
    [ Vogue Vectra 24' AG | Hard plumb: Hayward Power-Flo Matrix 1HP 2-speed, 27" sand filter & Pentair MiniMax 100 NG | Taylor K-2006 | Central IL ]
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    Mod Squad Bama Rambler's Avatar
    Join Date
    Jun 2009
    SouthWest Alabama

    Re: Testing for FC after Shock

    From what I can tell, you're understanding correctly. While you are shocking you do use a lot of reagents but normally you only test once a day (or less with the FAS-DPD) and with a FC of 4 to 8 (for a CYA of 50) you only use 8 to 16 drops of R-0871.

    Once you're completely done shocking, if you never let your FC get below the min for your CYA, you should never have to shock again, barring some unusual event.
    Dave J. TFP Moderator
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    Join Date
    Aug 2010
    Dallas, TX

    Re: Testing for FC after Shock

    I only use my FAS-DPD once a week or so (not shocking). I can use the DPD test and it's accurate enough for me for daily testing. The FAS-DPD is necessary for going through the shock process though, IMO there's no substitution. I do like to use the FAS-DPD occasionally for ongoing maintenance because it's FAR more accurate at measuring FC and CC than any other method. It gives me peace of mind knowing there's no CC in the pool.
    Built in 1957 44,000 gallon in-ground, Wet Edge Primera Stone in Sky Blue, Intelliflo VF Pump, 600 lb. Pentair Triton II TR-100 Sand Filter, CircuPool RG 60 Plus SWG, TF-100 test kit
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