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Thread: chlorine/CYA question

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    chlorine/CYA question

    I see that there are two different charts on here... on in pool school and one in this forum. which one would be better to use?

    also, I read that it is not safe to swim if your chlorine is above 4ppm, but based on the chart in pool school it says that my target should be 6ppm for a CYA of 40... is this still safe to swim in?

    what is the absolute max the chlorine should be to get in and be safe?

    thanks!
    Intex above ground 16x42, vinyl liner, metal sides, round, filter it came with

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    Re: chlorine/CYA question

    The typically touted "safe chlorine limit" of 4 (or 5, or 3, I have heard different numbers) does not consider the stabilizing effects of CYA and is only realistic if CYA is zero.

    It is safe to swim up to SLAM level for any given CYA. With that information it is, in theory, safe to swim in water with hundreds of ppm FC as long as the CYA is high enough.

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    Re: chlorine/CYA question

    By the chart on this forum I assume you mean this Chlorine/CYA Chart. The chart in the Pool School (Chlorine / CYA Chart) is a simplification, mostly rounding, of the chart in this forum. They are not inconsistent. It's simplest and easiest to follow the Pool School chart numbers, but that's up to you.

    As for safety, we generally say that you can swim up to the SLAM (shock level and maintain) level though you will notice the chlorine more at that level. The active chlorine level in the pool at shock level is roughly the same as a pool with around 0.6 ppm FC and no CYA so less than the amount of chlorine found in indoor and some outdoor commercial/public pools. The high FC level when CYA is present is only an issue if one were to drink very large quantities of pool water regularly. The EPA limit of 4 ppm FC is for drinking water at 2 liters per day every day for a lifetime.

    Your example of 6 ppm FC with 40 ppm CYA is not in any chart I could find. Did you mean 50 ppm CYA? Then the target is 6 ppm FC in that case (minimum is 4 ppm FC). 6 ppm FC with 50 ppm CYA has the same active chlorine level as only 0.1 ppm FC with no CYA. Over 98% of the chlorine is bound to CYA and is effectively inactive, doesn't react significantly, doesn't outgas, has no skin absorption, etc. This is why you shouldn't be looking at solely the FC number in isolation to determine the effectiveness of chlorine or whether it is safe. It is the FC/CYA ratio that is most relevant as that relates to the active chlorine level that does the actual disinfection killing pathogens, prevention of algae growth, oxidation of bather waste, oxidation of swimsuits, skin and hair, and outgassing. The recommended level we set is that which is sufficient to prevent green and black algae growth regardless of algae nutrient (phosphate, nitrate) level. It kills pathogens very quickly, but is not so high as to be harsh or noticeable.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: chlorine/CYA question

    super informative! thank you!
    Intex above ground 16x42, vinyl liner, metal sides, round, filter it came with

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    Re: chlorine/CYA question

    Anecdotal, but helpful information from the commercial side of things. The health authorities around here will shut down a pool at 10ppm, no matter what the CYA. (Although CYA is also legally limited to a range of 30-80). I personally operate with nominal ranges of 2-4 or 3-5 depending on the pool (4-6 in certain pools with bad paint jobs and constant algae problems).
    * Responsible for the operation of 21 semi-private pools and spas at 11 locations, varying from 1,900L to 20,000L spas and from 13,000L to 200,000L pools.

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    Re: chlorine/CYA question

    The pool industry itself seems either unable or unwilling to grasp the FC/CYA relation, I don't expect local health officials to either. With commercial pools you just need to do the best you can within the rules you must follow. Just as a small amount of CYA is beneficial in an indoor pool, it is not allowed in many places. Commercial pools just gotta do what they gotta do.

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