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Thread: Shock value discussion

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    BoDarville's Avatar
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    Shock value discussion

    march2012 wrote:[quote:3dkqwbdi] duraleigh wrote: I have seen several posts saying that the shock value suggested is a "minimum". That is incorrect because, by definition, there would then be no maximum. The shock value is a suggested constant. It certainly has some margin on both sides but it should be considered a target at which you should always aim.
    the problem is that if your pool is consuming 5 ppm an hour and you put it at 20 and leave for half the day, then you havent kept it at shock level[/quote:3dkqwbdi]

    I had the same thought as march2012 about the FC dropping below shock level if you had to leave the pool for half the day. Now, I haven't needed to shock since going BBB but I want to make sure that I do it right should the need arise.

    1. Is the point that the shock level is a target and not a minimum?
    2. In the Pool School article, Shock Your Pool, step 2 states: Add enough chlorine to bring FC up to shock level (or a little higher).

    Aside from using extra chlorine, what is the harm in overshooting the target shock level by 10-20% if you must leave the pool unattended for half the day as insurance that the shock level does not drop significantly below the target by the time you can run a follow-up test? Until you get a better idea of how much chlorine is being consumed during the shock process, it seems to me that it makes more sense to err on the side of being slightly above the target (especially if you must leave the pool unattended for several hours) as opposed to risking dropping significantly below the target leading to improper shocking and wasting chlorine. Am I missing something?
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    Re: No idea what I'm doing

    It certainly has some margin on both sides but it should be considered a target at which you should always aim.
    Repeating for emphasis. Sure 10% over is probably fine but where do you draw the line? There are some problems in overdosing pools with almost every additive. If there were not then the suggested level would be limitless.

    Whether or not you can monitor your pool and keep the FC at that level, the shock value is a target, not a minimum.
    Dave S.
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    JasonLion's Avatar
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    Re: Shock value discussion

    Mustard algae shock level is an appropriate upper bound. That is just about the highest level that is safe to use in all pools at all CYA levels. In some pools you can go much higher, but for the recommendation to apply universally that is about the highest you should go.
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    BoDarville's Avatar
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    Re: Shock value discussion

    Thanks Jason. That answers my question.
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    Re: Shock value discussion

    What a timely post, I was thinking along these same lines. If I may ask a related question, I'm sure that it has been asked and answered a number of times. In one of the posts I read that "Chlorine Lock" is a myth. This is what the pool stores drill into us, you must 'shock' (noun) at a level to break the CC bonds. Total BS? If I understand the process correctly, keep the chlorine at shock level and when the contaminates are removed from the pool the CC will take care of itself. Correct?
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    Re: Shock value discussion

    Total BS?
    Yes, it is.
    Dave S.
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    Re: Shock value discussion

    Shocking is simply the raising of the active chlorine level to a higher level to make all chlorine chemical reactions go faster. The most common Combined Chlorine (CC) in outdoor residential pools gets handled at normal chlorine levels so there is usually no need to shock and most pools measure <= 0.5 ppm CC (my own pool usually measures <= 0.2 ppm CC using a 25 ml sample size). You generally only need to shock when there is an unusual event such as algae growth (usually if you let the chlorine level get too low relative to the CYA level) or a bunch of leaves and pollen got in the pool and you're registering higher CC (and are impatient) or there was a dead animal or a fecal accident.
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