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Thread: Chlorine level quickly drops in days

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    bobhatfield's Avatar
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    Chlorine level quickly drops in days

    New at this. My chlorine levels seem to drop too fast, using 6% Clorox bleach from Costco and recommended amount from the calculator web site recommended here. Chlorine levels stay good for a day or two, then drops fast. Poring in 2-3 128oz bottles per week. Please advise!

    History: used chlorine tablets for several years until discovering this site. Pool has always been crystal clear and still is.

    TF-100 test kit:
    FC 0.5
    CC 0
    TC 0.5
    TA 170
    CH 800
    CYA 100+

    K-1000 basic test kit:
    pH 7.8

    Thank you for your help!
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
    SWG installed - but not working, needs new cell

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    Smykowski's Avatar
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    Re: Chlorine level quickly drops in days

    Have you read through Pool School yet?

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    Mod Squad jblizzle's Avatar
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    Re: Chlorine level quickly drops in days

    Read Pool School. That use sounds pretty normal if not on the low side for a pool your size. You should be adding chlorine everyday. A clean pool will use 2-3ppm of FC per day.

    Your CYA is way too high. And your FC is way too low for your high CYA. You have a ticking time bomb about to turn green.

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    Re: Chlorine level quickly drops in days

    As Jason has already stated, all your numbers are skew whiff, and need correction. CYA, CH, TA & pH all need corrected, such high levels pose a high risk of Calcium Scaling to your pool. 2.5 Gallons of Bleach per week is reasonable for 30,000 US Gallons, with 1 jug raising your FC by around 2ppm. Do you know what your source water levels are? It is essential to know what it will do to your water, you need to start there and work out how much water you need to replace in your pool, the CYA test can be done as a dillution test to get the correct amount - for example 1:1 pool water/tap water gives the CYA value as 50% of the actual level. With a maximum CYA level of 70 for SWG Pools (I note your is not working) you will need to get your CYA level down.

    Regards
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    Stuart Murray
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    Re: Chlorine level quickly drops in days

    Nascent not-yet-visible algae growth often first shows up as an increase in chlorine demand. Though 2-3 ppm FC per day is fairly typical in pools exposed to sunlight, with your low FC and very high CYA I'll bet your chlorine loss is algae starting to take hold. If the water starts to get dull/cloudy, then you'll have a full-fledged bloom on your hands before you know it. Nip this in the bud. Find out your true CYA and dilute your pool water to lower it. Raise your FC high in the meantime, at least to 10 ppm until you know what your CYA level truly is.

    If the pool didn't have something like nascent algae growth consuming chlorine, then at 100 ppm CYA one usually loses around 15% of the FC per day so you'd go from 9 ppm FC down to 7.6 then the next day down to 6.5. The fact that the chlorine seems to be dropping slowly at first and then more rapidly dropping is an indication that when the chlorine is higher it is preventing algae from growing, but once it gets too low then algae growth accelerates which then consumes more chlorine. It sounds like your pool is "on the edge".
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    Re: Chlorine level quickly drops in days

    Thank you all. Yes, read and still digesting info in pool school.

    To lower CYA: Do I mix test amounts of tap and pool water and test to get the right CYA number then drain that calculated amount and refill? For example, if a 50/50 mix of pool/tap gets me in range, do I drain 1/2 the pool and refill? (Seems obvious but want to be sure with that quantity of water. $$)
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
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    Re: Chlorine level quickly drops in days

    Yes, that's right. You double your reading of a 50/50 mix of tap water and pool water. So if that reads 80 ppm, then your CYA is really 160 ppm. In that case, you'd want roughly a half drain/refill to get to 80 ppm. If instead it reads 60 ppm, then that's a CYA of 120 ppm and you only need a 1/3rd partial drain/refill. This assumes you want to get to 80 ppm which is usually where people in sunny areas end up as reasonable. Just note that at this CYA level even after you've killed off algae you'll be at around 25% per day FC loss from sunlight (so 2 ppm FC loss if you start at 8 ppm and end up at 6 ppm). A few people keep the CYA at 100 ppm to have a lower loss rate, but it's obviously harder to manage IF any problem occurs.

    It also sounds like you may not be adding chlorine every day or two. You really need to do that. If you want more convenience, then there are options such as The Liquidator or a peristaltic pump or a saltwater chlorine generator. This is why Trichlor pucks/tabs are so seductive -- the convenience of continuous dosing. It's just that they increase CYA levels that ultimately lead to algae growth unless one spends more money with algaecides or phosphate removers.
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    Re: Chlorine level quickly drops in days

    Ok. My 50/50 mix shows a 50 ppm, doubled = 100 ppm. To get it to 80 ppm would I only need to drain/fill 20%?
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
    SWG installed - but not working, needs new cell

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    Butterfly's Avatar
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    Re: Chlorine level quickly drops in days

    Quote Originally Posted by bobhatfield
    Ok. My 50/50 mix shows a 50 ppm, doubled = 100 ppm. To get it to 80 ppm would I only need to drain/fill 20%?
    Yes, that is a good guestimate. But remember that the CYA test is +/-15% accuracy for regular testing and would lose more accuracy with the 50/50 dillution test.
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    Re: Chlorine level quickly drops in days

    Are you closing your pool soon? I ask because it may be better to wait till closing before a partial drain. Also you should test your fill water for CH, TA and pH so ou understand what effect it will have.
    Stuart Murray
    Scotland UK
    UK NPPOC

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    Re: Chlorine level quickly drops in days

    Not closing, in Phoenix, AZ. Good point re testing tap water.
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
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    Re: Chlorine level quickly drops in days

    Quote Originally Posted by bobhatfield
    Poring in 2-3 128oz bottles per week. Please advise!
    3 128 fluid ounce bottles of 6% bleach per week in 30,000 gallons is only 0.9 ppm FC per day so very low. You probably aren't adding enough chlorine or aren't adding it regularly enough (i.e. every day). A 30,000 gallon pool is larger than most and needs a larger amount of chlorine as a result.
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    Re: Chlorine level quickly drops in days

    I add 1 bottle of 8.25% (121oz) per day during the heat of the summer. I have a CYA of ~50 and a 28,000gal pool. I lose about 2.5-3ppm per day and the new size and strength bottle are perfect for me. Now that it's cooling off I am sure I will have to adjust down a bit.
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    bobhatfield's Avatar
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    Re: Chlorine level quickly drops in days

    Drained and now filling 20%...

    Wow, a jug/day? I had no idea - seems like justification for fixing my SWG (new cell). No?
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
    SWG installed - but not working, needs new cell

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    bobhatfield's Avatar
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    Re: Chlorine level quickly drops in days

    Ok, CYA now 60! But, CH is 550. I'm going to search for others with high CH and will post a new post with the numbers if I can't find answers.

    Thanks again.
    30K Gallons | Cartridge | In Ground | Plaster | WhisperFlo 2HP | Filter Flow Rate: 150 GPM
    SWG installed - but not working, needs new cell

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    Re: Chlorine level quickly drops in days

    Quote Originally Posted by bobhatfield
    Drained and now filling 20%...

    Wow, a jug/day? I had no idea - seems like justification for fixing my SWG (new cell). No?
    Yes

    My pool is 17K gallons and I use about 2/3 gallon per day.
    17,800 gallon, IG pool, 595 sq feet (17x35x4.5 avg rectangle), Hayward 525 cartridge filter, diamondbrite, 2 hp 2 speed hayward tristar filter, Stenner 45mhp10 automatically chlorinating

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    Re: Chlorine level quickly drops in days

    Quote Originally Posted by march2012
    Quote Originally Posted by bobhatfield
    Drained and now filling 20%...

    Wow, a jug/day? I had no idea - seems like justification for fixing my SWG (new cell). No?
    Yes

    My pool is 17K gallons and I use about 2/3 gallon per day.

    I guess I am lucky. With my 17,000 gal pool, that gets sun almost all day long, I use about 32oz a day, give or take. My CYA is around 40 though. Pool is closed now, so nothing to do now but wait till spring.

    -dave
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