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Thread: Why Shock?

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    Why Shock?

    Alright this maybe a stupid question but here goes. If we maintain our chemicals at the proper levels and scrub once a week, is there any reason to shock?

    My Combined Chlorine has never exceeded .5 yet. I had my pool open for over two months now and only shocked once. I shocked, in conjunction with extra scrubbing, this one time only because the liner felt a little slimy. Other than that, I never saw any reason to do it. Am I missing something?

    Some of my veteran pool friends & family shock once every one to two weeks. As I do not entirely trust my veteran pool friends and family I figured I would post this question. When I ask these seasoned vets why, they cannot provide a plausible response other than, I've never had a problem with my water and because thats what other people & pool stores told them to do. Never a true reason to do it.

    It's interesting how little some veterans know.....my veterans seem to have a cattle mentality!
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    ktdave's Avatar
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    There are many people that I've seen on here that have never shocked their pool. There really is no reason to unless you have CC above 0.5 or you see evidence of an algae bloom.
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  3. Back To Top    #3

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    I've shock only once this year, when my water got a tad cloudy. Weekly shock is a conspiracy conjured up by the pool stores/chemical compaines. A conspiracy I tell ya
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    Ohm_Boy's Avatar
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    Shocking is a remedy prodecedure. Breakpoint chlorination is really only necessary to resolve a CC issue. Without a cc issue, there is no need to shock.
    If you let the FC drop too low, or have a nitrogen/ammonia spike (and we all know who THAT is ), then there may be cause for shocking.

    I think that it helps folks with high stabilizer levels (also from following pool store advice) to keep the algae at bay since their FC levels are not high enough for their 80 million or so CYA levels.
    [center:1kpalu48]Helpful Links: Pool School | CYA/Chlorine Chart | Pool Calculator[/center:1kpalu48]

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    I haven't shocked at all this year, although I keep my chlorine at a level many pool stores consider shock levels.

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    Thank You! So far pretty much the results I thought I would get.

    Ohm_Boy - You bring up another point I have been meaning to ask. Why do we need higher chlorine levels with higher CYA levels? I know that's the rule I just don't understand. Sorry but I am an anal CPA and need to know why.

    I believe I remember reading an excellent post by the Mermaid Queen (sorry if I am giving wrong kudos) where she gave a great analogy. She described CYA as sunscreen for chlorine. The CYA prevents the chlorine from being burnt up by the sun.

    I also understand that too much CYA makes the water cloudy. To use my own analogy and piggyback the Mermaid's, its very much like using too much sunscreen where it will not absorb in to your skin and leaves a white residue on your skin or in a pools case, cloudy water.

    Good analogy but why higher chlorine for higher CYA? Shouldn't the higher CYA preserve the chlorine more? Isn't the purpose of CYA to reduce the use of chlorine not increase it? There appears to be a fine line that once past becomes a nuisance, but what is that line and why?
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    Cya binds to the chlorine, effectively taking it "out of service" for the time being. Sort of like a time released thing. As the chlorine is "called into service", the cya releases some of it.
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    Poseidon gives an excellent analogy so CYA protects chlorine from the sun in two ways combined (the first being what you said): 1) the CYA absorbs UV rays from sunlight thereby protecting the chlorine at lower depths in the pool and 2) the CYA combines with chlorine to form new compounds that don't break down as quickly from the UV in sunlight. It is this latter effect that causes the "active" disinfecting chlorine concentration to be significantly reduced (and I mean significantly -- at a pH of 7.5, 30 ppm CYA roughly reduces "active" chlorine concentration by a factor of around 30, coincidentally). Due to the equilibrium chemistry, if you double the CYA amount, you need to approximately double the FC amount to end up with the same "active" disinfecting chlorine.

    The Free Chlorine (FC) test measures not only the "active" disinfecting chlorine, but also the chlorine that is held in reserve attached to CYA. This is because, as Poseidon points out, the chlorine attached to CYA is released as needed so the FC test ends up measuring it. Fortunately, it takes a very small amount of "active" disinfecting chlorine to kill bacteria and prevent algae.

    Richard
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    Excellent responses! I'm almost there, but let me continue my ignorance.

    If the CYA combines with FC to "time release as necessary" why would we need to increase chlorine to compensate? Shouldn't the CYA time release the FC and viola we have proper disinfecting levels?
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    JasonLion's Avatar
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    The CYA binds to some percentage of the chlorine, taking it temporarily out of service. The remaining chlorine level may not be enough to sanitize the pool unless we raise the total amount of chlorine. Say we are aiming for a FC of 1, but CYA is binding to 80% of the chlorine, that means we need a FC of 5 in order to have 1 unit of chlorine active in the pool.
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    Okay. I think I got it. In one lame ol' rookie sentence:

    The CYA is smart enough to hold free chlorine to time release when needed, however, it is not smart enough to know exactly when to release, hence the concern of raising CYA too high.

    Do any of you professors want to give this statement two thumbs up?
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  12. Back To Top    #12

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    The rate of chemical reactions is based on the instantaneous concentration of the "active" species. It's a probability thing. Imagine lots of "active" molecules bouncing around running into all kinds of organics, bacteria, and algae. When it hits something (in the right way -- with the right orientation and enough energy), it can react with it thereby killing it (if it's a pathogen) or oxidizing it (if it's an organic that doesn't grow) and there are other intermediate reactions, but let's keep this simple. The chlorine that is attached to CYA (as unique compounds) also bumps into these substances, but doesn't react (for a variety of reasons I won't get into here).

    In addition to the above, there is an equilibrium between the chlorine attached to CYA and the chlorine that is active and unattached to CYA -- they are going back and forth as well. So if some of the active unattached chlorine gets used up, then there is less of it to get attached to CYA and there is more of it already attached so the net result is that the amount of chlorine attached to CYA will drop somewhat and the amount of chlorine that is active and unattached will increase somewhat to roughly stay in the same proportionate balance. Unless there is a big change, roughly speaking the active amount stays at a constant small proportion of the total amount (the amount measured as Free Chlorine).

    So while the chlorine attached to the CYA is clearly available as a "reserve", it is not "active".

    You can think of an analogy of people fighting a war where you have a group of "active" soldiers on the front lines fighting hand-to-hand combat with an enemy. You have many more soldiers in the rear that are not directly fighting and are in "reserve". When some soldiers in the front lines get killed or injured, you can replace them with some from the reserve, but the rate at which you will be able to wound or kill the enemy is only dependent on the number of soldiers you have on the front lines doing the hand-to-hand combat. It doesn't matter how many you have in reserve. The amount in reserve only tells you how long you can continue to fight -- not the RATE at which you can fight effectively.

    So the key is to have enough soldiers on the front line to kill the enemy faster than the enemy is able to reproduce (double in size) which is what bacteria and algae do. So only the "active" front-line fighters determine whether you can prevent bacteria and algae from growing. The "reserve" is just there so you don't run out until the next time you add more chlorine to beef up both the reserve and the front lines.

    Technically speaking, if you could ensure that there was 0.1 ppm Free Chlorine (FC) everywhere in the pool at all times, then that's all you would need to kill bacteria and algae faster than they can reproduce. You wouldn't use any CYA in that case. The fact is that you simply can't maintain such a low chlorine amount as it gets used up locally and circulation isn't good enough to "get replacements" and the chlorine spends a lot of time getting used up combining with organics that are not pathogens (from sweat, leaves, etc.). So to ensure that the chlorine doesn't run out, we use CYA, not only to protect the chlorine from sunlight, but to keep the actual chlorine level low and hold most of it in reserve.

    This is the main reason why I believe outdoor pools do not have the problems of indoor pools with regard to disinfection by-products that cause asthma and respiratory illnesses. Indoor pools do not have CYA, yet have chlorine levels of 2 ppm FC or so and therefore have 10-20 times the amount of "active" disinfecting chlorine and therefore degrade swimsuits, skin and hair 10-20 times faster and produce disinfection by-products including trihalomethanes (including chloroform) and chloramines (including nitrogen trichloride) 10-20 times faster. It's true that a lack of good air circulation and a lack of sunlight are other factors, but I believe the 10-20 times faster reaction rates and presumed 10-20 times higher airborne concentrations are more important. I am currently working with the CDC and the top expert on the chlorine/asthma indoor pool issue in England so hopefully we'll be able to see if using some CYA in indoor pools will solve this problem. The CDC is concerned about lowering disinfection levels, but I pointed out it's no worse than what is already done in outdoor pools and I've suggested alternatives to handling the protozoan cysts/oocysts (Giardia, Cryptosporidium) that are hard to kill with chlorine (I'm suggesting using chlorine dioxide which can be produced in a CYA pool easily using sodium chlorite, but the dosages must be pretty precise).

    Richard
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    Ohm_Boy's Avatar
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    We country-boy idiots think of it as buffering the chlorine - keeps it from dissipating as quickly, but it also 'dulls the edges' of it, so it can't 'bite' as fiercely. So it's kindof a double-edged sword. CYA protects the chlorine from UV, but also protects everything else from the chlorine.
    Since it accumulates and will not go away, if you are using a stabilized form of chlorine, it'll continuously rise, which will reduce the effectiveness of the chlorine, requiring higher levels of chlorine, which add more stabilizer.... you get the idea. Ultimately, you need to reduce the CYA, or shock the pool, to get effective sanitization. Oh wait - there are other answers... mineral treatments and phosphate removers... aww heck, I can't even type that with a straight face.

    This is why bleach is so highly advocated here. No side-effects.
    [center:1kpalu48]Helpful Links: Pool School | CYA/Chlorine Chart | Pool Calculator[/center:1kpalu48]

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    These are great explanatons and really do help us newer folks......but how do you know if you have algae in the first place. I check my ph and chlorine every few days, I rarely have any CC and I haven't actually had to add anything in a few weeks. funny when I have my water checked at the pool store, I always need to mess with the TA or the CH or something but lately it has stayed very stable according to my Taylor testing. I understand if i do get some CC then something is brewing - but when it is just starting what would I notice - what would it look like?
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    The first sign of algae I usually notice is the water getting a little "dull", not having the sparkle it usually has. After that FC starts droping more quickly than I would expect. Then the water gets actually murky.
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  16. Back To Top    #16
    Ohm_Boy's Avatar
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    CC doesn't always mean algae, as Combined Chlorine simply means that the formerly-Free Chlorine has bound to something else, and I believe that it is usually nitrogen, to form chloramines. Chloramines would be what is responsible for the "chlorine smell" and most of the fabled eye irritation that people hate so much, often erroneously attributed to high chlorine. While I'm sure that various biologicals can be a source of nitrogen in a pool, I believe that people are the biggest cause. Sweat, urine, suntan oils, etc. seem to be the bulk offenders. Good chlorine reserve not only sanitizes the water by killing the organisms, it will oxidize the residue, oils, etc. and will free the combined chlorine. This is how shocking reduces CC levels. That's another reason to choose chlorine as a sanitizer over some more expensive alternatives, as they don't have the oxidization capability.

    At any rate, algae begins unobtrusively, then rapidly blooms. Naturally, you want to maintain an environment which keeps it from happening at all. Keep the chlorine at levels recommended by the best guess chart according to your CYA level, hold the pH around 7.2-7.6 or so, use liquid chlorine for a chlorine source to avoid jockying the CYA and/or calcium around, and you're well on your way. You can watch as your neighbors shock their pools and wait on the CL levels to drop, and they can watch you as you enjoy your pool from within the water.
    [center:1kpalu48]Helpful Links: Pool School | CYA/Chlorine Chart | Pool Calculator[/center:1kpalu48]

  17. Back To Top    #17

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    By George I think I got it! Thanks Chem Geek, Ohm_Boy, JasonLion, Posseidon and KTDave!
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