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Thread: Why did my FC drop from 3.5 to zero in 18 hrs?

  1. Back To Top    #1
    Her pool boy's Avatar
    Join Date
    May 2016
    St Catharines, Ontario, Canada

    Why did my FC drop from 3.5 to zero in 18 hrs?

    Still new to this method so am curious about why my FC would drop so drastically as up until this morning it was maintaining FC and would only need to move it about .5 to 1 ppm per day. But not today.

    There are a few factors that have changed:

    1. I was increasing the hardness of the pool by 25 ppm by adding 3.5kg of Calcium Chloride
    2. It rained over night

    reading from yesterday were

    FC 3.5
    CC <.5
    TA 100
    CH 100
    CYA 32

    I did raise the CH to between 125 and 150 overnight.

    I have added bleach to raise the FC back to 3.5. Just wondering if what I experienced is normal?

    16' x 50' - 95K L (25000 gallon) I.G. Vinyl liner 12.5 Liquid Chlorine
    Wicor Pantera P24 Sand Filter Pentair Superflo Variable Speed Pump
    Zodiac 3510WD Robotic Cleaner Raypak R336A Heater. ​Taylor K2006c test kit

  2. Back To Top    #2
    Mod Squad Texas Splash's Avatar
    Join Date
    Jun 2014
    South-Central Texas, Marion/San Antonio

    Re: Why did my FC drop from 3.5 to zero in 18 hrs?

    Hello Steve. It's not unusual for a healthy pool to lose anywhere from 2-4 ppm of FC per day. I myself have to replenish about 2.5 ppm per day - more if we have a lot of swimmers. Given you are already at a fairly low FC of 3-3.5, you could conceivably drop down below 2 with good sunlight, swimming, and/or simply by normal FC expenditures each day. Sunlight and organics will consume FC, so it's one of those. You may simply need to increase FC a little more. Your ideal target is 4, so you may need to try 5 and see if that holds a little better through the day until you add more. If you find that FC is simply dwindling too fast, you may need to perform an overnight (OCLT) test just to confirm there is nothing wrong. If you pass the OCLT and are still losing FC too fast, you may need to increase CYA to 40 to protect your FC better from the sun.
    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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  3. Back To Top    #3

    TFP Guide

    cowboycasey's Avatar
    Join Date
    Jul 2013
    Fletcher, OK

    Re: Why did my FC drop from 3.5 to zero in 18 hrs?

    you are all over it that is why we test every day and see what your pool is doing, it could have been pollen, dust, a bird, who knows.. something got in there and used up your FC, did you guys swim yesterday?

    just watch it, you can test a couple times today and see if FC is going down fast or you can do what I do anytime I see something like that, I take it to SLAM level and let it drift back down slowly to target level, you can still swim and it won't hurt anything...
    Pool: Intex 16x32 15000 gal, 2 speed 340042, Pentair CC320 Filter, CircuPool SJ45 Salt System, Intermatic PE653RC; Hot Tub: 650 Gal SWG Megachlor
    links: pool school * Recommended-Levels * SLAM * CYA chart * Test kits * How To Post Pictures * Poolmath * OCLT ** Support your website if we helped you :) **

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