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Thread: Newbie: To shock or not to shock... That is the question

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    CarlD's Avatar
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    Default Re: Newbie: To shock or not to shock... That is the question

    Here's some updated info as best I remember it from the guys who work on SWCGs.

    1) Pool maintenance is pool maintenance. The SWCG makes things easier and SOME recommended numbers change, but all in all it's the same.
    2) Shocking is what you do when you have a problem or suspect you have a problem. Most systems can gen higher levels of chlorine, but (and I'm guessing here), not enough to clean up a major algae bloom.
    3) Bleach/liquid chlorine is the ideal adjunct method for adding chlorine. Bleach is made from salt-water, ie brine, and you had to add salt to the pool for the SWCG to work.
    4) Manufacturers generally prefer higher levels of stabilizer (CYA) than expected. 70-80ppm is pretty normal.
    5) according to Chem_Geek, our resident chemistry geek (get it?) you need to keep FC at 5% of cya, so that's 3ppm when CYA=60ppm and 4ppm when CYA=80. It's also much lower than the Best Guess table, but the SWCG puts a steady flow of chlorine into the water.
    6) According to our other pros, a low Total Alkalinity works very well. 60-80ppm is what they tell us, far lower than the 80/90 to 120/130. I'm guessing it has to do with the tendency many SWCGs have to raise pH. A lower T/A may buffer that.
    7) for reasons I don't understand, even a vinyl pool may need Calcium with a SWCG.

    Hope this helps.

    Carl
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    Default Re: Newbie: To shock or not to shock... That is the question

    So, I am to assume that I only need to shock my pool when I have a problem... correct? I don't need to shock it on a regular basis just because some pool dealer told me I needed to, is that correct? Sorry for the repeated question... I just want to make sure I completely understand the purpose for shocking a pool, and the regularity that it needs to be done.

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    madwil is offline Registered+ Widget Weaver madwil 0
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    Default Re: Newbie: To shock or not to shock... That is the question

    you do not need to shock, unless you have a problem- I did not shock at all last year, after completed opening...
    Shocking is to kill an algae bloom, or oxidize CC, or oxidize/sanitize organics; if you do not have a high swimmer load, and maintain FC at levels indicated by best guess chart, you won't have to shock either.
    Happy Swimming!

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    Default Re: Newbie: To shock or not to shock... That is the question

    You only need to shock if you let your chlorine level drop below the range that you are supposed to keep it in based on your CYA level (see Best Guess chart below in my signature), or if you have a really big swimmer load, or if you have a CC reading above 0.5. I also sometimes shock if I have a big storm that throws a lot of debris in my pool. Other than that, you don't really need to shock. I rarely shock my pool because I maintain my chemistry where it needs to be. I'm guessing other than at start-up, I probably don't shock my pool more than twice a summer and maybe not even twice. Of course pool stores tell you to shock weekly. They want your money!

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    Default Re: Newbie: To shock or not to shock... That is the question

    Quote Originally Posted by Watermom View Post
    You only need to shock if you let your chlorine level drop below the range that you are supposed to keep it in based on your CYA level (see Best Guess chart below in my signature), or if you have a really big swimmer load, or if you have a CC reading above 0.5. I also sometimes shock if I have a big storm that throws a lot of debris in my pool. Other than that, you don't really need to shock. I rarely shock my pool because I maintain my chemistry where it needs to be. I'm guessing other than at start-up, I probably don't shock my pool more than twice a summer and maybe not even twice. Of course pool stores tell you to shock weekly. They want your money!
    +1 for me exactly. I didn't go to shock levels at all last year except on opening. I will run the pool at the highest end of the operating level or even a little higher after heavy bather loads, storms, heat waves or other weirdness. That has been adequate.
    33,000ish gallon, 20X40 IG Vinyl Liner, no heater. Full sun, Sand filter. Been pool stored more times than I can count, not anymore! Thanks PF!

    20'x40' rectangle 33K gal IG pool; Bleach; Hayward sand filter; AO Smith 1HP pump; 14hrs; Taylor K2006; utility water; summer: ; winter: ; iPhone; PF:3.6

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    Default Re: Newbie: To shock or not to shock... That is the question

    According to Taylor you want to achieve Breakpoint chlorination (10 x the CC level) any time your CC reaches .2ppm I have been trying this at .5ppm and it seems to be working ok. I really don't think of it as "shocking" the pool when I'm only raising it to say 7ppm when I have a cc of .5 but I guess technically it is. My normal target range is 5-9 so you see I can achieve Breakpoint as a matter of my normal maintenance. Just another way of saying the same thing as the others perhaps.

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    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: Newbie: To shock or not to shock... That is the question

    The 10x rule is wrong when applied to CC. The rule originally came from the amount of chlorine needed to oxidize ammonia where the molar ratio is 3 chlorine to 2 ammonia. However, chlorine is measured in ppm Cl2 units while ammonia is measured in ppm N units and there is a factor of 5.06 difference between these units. So the ratio in chlorine and ammonia units becomes (3/2)*5.06 = 7.59 where side reactions and getting over the dichloramine hump has actual quantities required be in the 8-10 range and that's where the 10x rule was born for the chlorine oxidation of ammonia in their respective units.

    CC as monochloramine already has 1 of the 1.5 chlorine needed to oxidize it and CC is measured in chlorine ppm Cl2 units and not in ammonia ppm N units so there is no factor of 5. So 1 ppm CC only needs somewhere in the 0.5 to 0.7 ppm FC amount to get fully oxidized. Now there are other forms of CC including monochlorourea that need more chlorine, but still on the order of 2-3x the CC level. Besides, breakpoint chlorination is continuous so long as there is ANY measurable FC in the water and nothing gets "stuck" if you don't use enough FC -- you simply add more and it picks up where it left off (more or less).

    If you are measuring 0.5 ppm CC using a 10 ml sample size in your FAS-DPD chlorine test, then that is really <= 0.5 ppm and you should use a 25 ml sample size instead to see if it's really <= 0.2 ppm which is more likely.

    Now, all that said, it is true that a higher active chlorine level (FC/CYA ratio) will oxidize CC faster, but just don't think there is anything magical or special about "10x".

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