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Thread: Any downside to using sodium thiosulfate to lower cl level?

  1. #1
    Senior Member kodiak1120's Avatar
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    Sep 2011
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    Any downside to using sodium thiosulfate to lower cl level?

    Hi everyone,

    Still new to this... my pool pump motor died a week ago. I finally got a replacement on Monday, installed it Monday night and ran the filter overnight. Suprisingly, the water was still crystal clear with no signs of algea, even after having the pump off for almost a week. I did put several chlorine pucks in the floaters, but that was it. When I started the pump, I immediately noticed that "chlorine smell" that I know is caused by the chlorine reacting to stuff in the water. Also the water in the skimmer boxes had a bit of slime around the edges. We are having a get-together this weekend, so I didn't want to take any chances. I dumped in 5 gallons of 12% liquid sodium hypochlorite just to be safe.

    Now the weather is going to be almost 100 degrees and my chlorine levels are way up. As of this morning, my levels were: FC 23; CC 0. With a CYA of 40, the CYA chart says shocking level is 16. I've read that anything above shock level is not safe for swimming. I wouldn't care, but I don't want my wife or two-year old son in the pool unless it is safe.

    Anyway, I've read that you can use sodium thiosulfate (sold at Leslies; $12.99 for 3 lbs) to lower chlorine levels. From what I've read, 2.5 ozs. of sodium thiosulfate for every 10k gallons of pool water will lower chlorine levels by 1 ppm. My pool is 30k gallons, so if I added one container (3 lbs), it should lower my chlorine from 23 ppm down to roughly 16 ppm. Also, we're supposed to get full sun today, so that should lower the levels as well.

    Any downside to using this stuff? Also, do my calculations look right?

    Kevin
    ---
    30,000 gallon vinyl pool
    Hayward DE Filter
    Hayward Super Pump
    AO Smith 1 1/2 HP Pump (27GPM)
    Polaris 380 Pool Cleaner

    Check out the app I created to control my pool equipment from my smartphone:
    i-created-an-app-to-control-pool-equipment-with-smartphone-t61606-20.html

  2. #2
    Senior Member kodiak1120's Avatar
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    Re: Any downside to using sodium thiosulfate to lower cl lev

    Also, is there any way to figure out how much chlorine I will loose to sunlight, taking into account my CYA level?

    Kevin
    ---
    30,000 gallon vinyl pool
    Hayward DE Filter
    Hayward Super Pump
    AO Smith 1 1/2 HP Pump (27GPM)
    Polaris 380 Pool Cleaner

    Check out the app I created to control my pool equipment from my smartphone:
    i-created-an-app-to-control-pool-equipment-with-smartphone-t61606-20.html

  3. #3
    Member
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    May 2011
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    Ft. Lauderdale, Fl
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    Re: Any downside to using sodium thiosulfate to lower cl lev

    I have the same approximate values for my pool since Ive been shocking for mustard algae for a week or so now. I've been IN the pool everyday scrubbing nooks and crannies for at least an hour at a time and the only consequence has been that my hair dye has gone from dark brown to a lighter brown with an orangey tint. (yuck) I make sure I rinse out my bathing suit well so that the chlorine doesnt sit in it and weaken the material. I think that between the sun, bather load, and time you'll be surprized how quickly your FC will drop and will be happy it was high to begin with. I wouldn't go thru the extra expense just to add another chemical that you'll be swimming in. I don't think there's an issue swimming in shock level chlorine....
    anyone else with an opinion???
    32x16 -16,500 gallon plaster pool with connected spa and spillover, Hayward RS 1 HP pump, 2 Hayward Cartridge filters. 10 hours of direct sun a day and a first time pool owner!!

  4. #4
    Member
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    Ft. Lauderdale, Fl
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    Re: Any downside to using sodium thiosulfate to lower cl lev

    You're calcs look right, but I wouldn't add anything to the pool. I get full sun on my pool all day and with a 40 ppm CYA I lose about 2-3 ppm FC a day on average. So I would guess you'd be about the same depending on organics and bather load. If your party is this weekend, then over the next 3 days you'll drop anywhere between 6-9 ppm anyway. Wait till Friday and check your FC then...if you're still concerned then...maybe. But I usually boost my FC if I know I'm going to have a few people in the pool anyway just to ward off potential problems...organics, pee, suntan lotion, perfumes, beer, etc. LOL
    32x16 -16,500 gallon plaster pool with connected spa and spillover, Hayward RS 1 HP pump, 2 Hayward Cartridge filters. 10 hours of direct sun a day and a first time pool owner!!

  5. #5
    Administrator JasonLion's Avatar
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    Silver Spring, MD
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    Re: Any downside to using sodium thiosulfate to lower cl lev

    You will probably lose more than 50% of your starting FC level to sunlight each day. Your FC level will be down to something reasonable by sometime probably early this afternoon, so no point in using chemicals to lower the FC level. The only real problem with thiosulfate is that it is very easy to use way too much and drive the FC level down to zero accidentally. But it is generally un-needed. Why get involved in extra complexity you don't need to worry about.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    TFP Admin. Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  6. #6
    Senior Member kodiak1120's Avatar
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    Re: Any downside to using sodium thiosulfate to lower cl lev

    Thanks for the respones... I love this site. I think I'll just keep the wife and kid out for today and then check it tonight when I get home. Hopefully it will drop to something reasonable by tomorrow when the temperature is supposed to be 100 degrees.

    Kevin
    ---
    30,000 gallon vinyl pool
    Hayward DE Filter
    Hayward Super Pump
    AO Smith 1 1/2 HP Pump (27GPM)
    Polaris 380 Pool Cleaner

    Check out the app I created to control my pool equipment from my smartphone:
    i-created-an-app-to-control-pool-equipment-with-smartphone-t61606-20.html

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