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Thread: Low chlorine reading

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    Low chlorine reading

    This is my third season as a pool owner. Last two seasons i seemed to be doing ok with the chlorine levels.
    This year my Combined chlorine reading and Free chlorine reading both frequently read low.
    I have been reading the pool school FAQs and this is my suggested action plan. just wanted someone's expert opinion before i dump a lot of dichlor into the pool.
    I dont want to increase the CYA to a too high level by making a mistake.

    I have a mesh cover and I had a lot of debris from leaves at the beginning of the season. The water was cloudy for a long time. Pool store told me i had 100ppm phosphates. I kept shocking the pool and it has cleared up now, but the chlorine readings are still both low. I have only been adding 3-4 lb's at a time.
    I turned up the setting on my off-line chlorinator and dumped a lot of thrichlor pucks in it. Still the chlorine readings wouldn't come up.
    Now my CYA has increased, it was 30 ppm before now it looks like 60 ppm.
    My understanding in where i went wrong is i didn't add enough dichlor to perform superchlorination, i was just adding to the combined chlorine, process never got completed. In order to increase the FC i have to add more dichlor. According to the pool calculator i should add 7lb's 13 oz to achieve a shock level of 20 for FC. Now if i understand correctly as the CYA increases so does the shock FC.

    Does the 7lb 13 oz sound correct?

    I have a 26500 gallon pool
    Vinyl Liner
    CC is 1ppm
    FC is 0.5ppm
    pH is 7.6
    Alkalinity is 120 ppm.
    Hardness is 150ppm
    CYA is 60ppm

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    Richard320's Avatar
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    Re: Low chlorine reading

    Don't use any more dichlor. As you noted, CYA is already maxed out.

    If that CH reading is correct, you'd be a perfect candidate for Calcium Hypochlorite powder. And then there's always plain ol' bleach, which doesn't leave anything behind but salt. pool-school/types_chlorine_pool
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    Re: Low chlorine reading

    Quote Originally Posted by Richard320
    Don't use any more dichlor. As you noted, CYA is already maxed out.

    If that CH reading is correct, you'd be a perfect candidate for Calcium Hypochlorite powder. And then there's always plain ol' bleach, which doesn't leave anything behind but salt. pool-school/types_chlorine_pool
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    duraleigh's Avatar
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    Re: Low chlorine reading

    mflarsen,
    Welcome to the forum Read How to "Shock your Pool" up in Pool School. Follow that process carefully. YOu did not holod your chlorine high enough long enough....it is a continual process, not a one time dose.

    USE ONLY CLOROX. Continued use of dichlor will make your pool unmanageable.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    Mod Squad Bama Rambler's Avatar
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    Re: Low chlorine reading

    One thing I noticed is that you say the FC & CC reading are both low. If you meant TC (total chlorine) instead of CC (combined Chlorine) that's ok but if you really meant Conbined Chlorine then it should always be very near zero.

    If you really have a CC of 1 you should follow the shock advice.
    Dave J. TFP Moderator
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    Re: Low chlorine reading

    did the readings again this morning.
    i meant total chlorine not combined chlorine.
    I understand that the free chlorine is 0 because i never added enough, but shouldn't the total chlorine be greater than 0 when i have the chlorinator running?

    Total hardness 150ppm
    Total chlorine now 0 was 0.5 yesterday
    Free chlorine 0
    pH 7.4
    Total Alkalinity 120ppm
    CYA 80 ppm

    Now if i go with your suggestion of adding calcium hypochlorite how much should i add?
    When using the pool calculator to figure out how much calcium hypochlorite i should use should i use the target FC value or the shock FC value from the CYA/chlorine chart?
    pool-school/chlorine_cya_chart_shock

    Target would be 9 = 3lb 1oz
    Shock FC would be 31 = 11lb

    If i add all that calcium hypochlorite i would probably need some muriatic acid too right, to bring down the pH as it will probably rise?

    thanks

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    Re: Low chlorine reading

    that's if i use 65% calcium hypochlorite

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    Mod Squad Bama Rambler's Avatar
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    Re: Low chlorine reading

    How much Cal-hypo do you have?

    [s:3nvplkxa]I only get that you need to add 7 lbs to get to shock level.[/s:3nvplkxa] Make sure you're entering the correct numbers in the Now and Target columns of the Pool Calc.

    You need to add the chlorine, then wait an hour and test again and add what the PC says to given the test, then repeat until you pass the OCLT.

    I didn't account for the difference between the PC and the chlorine/CYA chart.
    Dave J. TFP Moderator
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    Re: Low chlorine reading

    thanks bama rambler

    i also noticed that the cya/chlorine chart and the pool calculator gives different values for FC shock.
    i put 0 in the FC now column. what should i use for the other column 9 for the target fc or 3 for the shock fc?

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    Re: Low chlorine reading

    9 is the upper range of normal FC for 60 CYA. Your shock level is 18, but 24 as stated on the chart won't hurt since your FC is going to drop pretty quickly at first.
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    Mod Squad Bama Rambler's Avatar
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    Re: Low chlorine reading

    As Robbie eluded to, you'll put the shock number in the Target column. Then you'll put whatever you test at in the Now column when you go back and test in an hour. Hopefully it'll be more than zero.
    Dave J. TFP Moderator
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    Re: Low chlorine reading

    Thanks Bama and Robbie

    Now i use trichlor tablets in my chlorinator someone told me never to mix calcium hypochlorite and trichlor, it may cause a chemical reaction leading to an explosion?
    Should i just turn off my chlorinator prior to shocking with the calcium hypochlorite or do i have to now use a different type of chlorine in the chlorinator?

    Thanks

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    Mod Squad Bama Rambler's Avatar
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    Re: Low chlorine reading

    You don't ahve to turn it off. Just don't physically mix them. Once they're in the pool water it doesn't matter. You can't put cal-hypo in a trichlor feeder but you're not going to do that anyway. You're going to broadcast the C-H in the pool.
    Dave J. TFP Moderator
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    Re: Low chlorine reading

    cool thx

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    Re: Low chlorine reading

    Like Dave says, you can mix the two. When I shocked my pool i used Cal Hypo and had trichlor tabs in floaters to help me maintain FC (and add come CYA, I was at 0) and I'm still here..
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    Re: Low chlorine reading

    shock with calcium hypochlorite last night. now FC is 3 and total chlorine is 3. took a water sample to pool store, they said my CYA was 30 which is lower than what i got.
    so i guess i am good now. the total chlorine will go up on its own now right with just running the chlorinator? I dont have to superchlorinate again?

  17. Back To Top    #17
    Mod Squad Bama Rambler's Avatar
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    Re: Low chlorine reading

    Did you hold it at shock till you passed the OCLT?
    Dave J. TFP Moderator
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  18. Back To Top    #18

    Re: Low chlorine reading

    i just dumped all the calcium hypochlorite in at once. i tested the water 4 hours later and both the total chlorine and the free chlorine were maxed out on the test strips. both 10.
    when i checked it again this morning they were both 3. now at 5 pm they are both 0.5. what does that mean? that i have algae using up my chlorine?
    I understand the concept that the algae eats up the free chlorine and it binds to create chloramines/combined chlorine. but shouldnt i have some sort of reading on the total chlorine?
    I just dumped 78oz of calcium hypochlorite in.
    where am i going wrong here?

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    frogabog's Avatar
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    Re: Low chlorine reading

    You need a good test kit.

    Loosing all your chlorine means something's eating it up and none is being added. Sunlight, bather load... algae... chlorinator not chlorinating...

    Test kit would answer this question.
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    Re: Low chlorine reading

    the water is crystal clear. i dont think it has ever been cleaner

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