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Thread: Post-bathing shock

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    Post-bathing shock

    Ok, so I've seen chem geek's suggestion that 1-bather hour requires aproximately 5 oz bleach to oxidize waste. Given my hot tub is a mere 290 gal, if my wife and I soak 1 hour per day, that's 10 oz 6% bleach or +17 FC, which is 1.5x shock level at 30 CYA. So basically I'm super-shocking every day. Is this right?

    I was running 20 CYA and could NOT maintain FC. I'd dose with 2 oz 10% bleach at night, in the morning I'd have 5+FC (OTO test) and the next evening it would be 0-.5 FC. That's when I upped it to 30 CYA, which seems to have helped.

    Does it make sense to run 40 CYA in a small tub in order to increase FC buffering? That way 17FC is right around shock level.

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    Mod Squad jblizzle's Avatar
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    Re: Post-bathing shock

    I am not sure he meant 5 oz of bleach for 1 man hour (maybe for a specific case). Usually I have seen it stated a ppm level per man-hour ... since as you say the volume matters.

    From the Sticky:
    When you use your tub (this applies to when you are still using Dichlor or after you switch to bleach), the rule of thumb is, you'll use approximately 7 ppm FC per person per hour in a 350 gal tub. Now this will depend on a lot of things, water temp, cleanliness of the users etc. If it's just two people, after taking showers, using no swimsuits, with the temp at 98 you may only need 2 ppm FC/person/hour. However, if you have 6 people in their clothes, drinking (spilling) beer, with a temp at 102 causing everyone to sweat, you may need 20 ppm FC/person/hour. So this is where you may need to make a judgment call and/or experiment. Remember, the Chlorine has to do two things, Oxidize Waste, and Kill Bacteria. If your tub is being subjected to a lot of waste (sweat, beer etc), your FC will be used up and not be able to kill bacteria. Not good! So I'll give you two scenarios and how you might go about running an experiment. Remember, all this up front work is only necessary in the beginning, until you learn how much FC is being used in your tub. Once you know, it becomes easy to maintain. The rule of thumb is, assuming CYA is 20-30 ppm, FC should normally be between 3-6, with a min of 1 and shock to 12 once a week.
    Is your spa covered or in the sun? If covered, the only reason your FC would have dropped would be due to something in the water.
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    Re: Post-bathing shock

    The absolute quantity of oxidizer is per person-hour independent of the size of the tub. It's only if you use a rule-of-thumb based on ppm FC that the size of the tub needs to be factored in.

    For every person-hour of soaking in a tub with no ozonator, it takes roughly 3-1/2 teaspoons of Dichlor, 5 fluid ounces of 6% bleach, 3-1/2 fluid ounces of 8.25% bleach, or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize the bather waste.

    However, the REAL rule is to add whatever amount of oxidizer that is needed after a soak so that you have a small residual of chlorine 24 hours later. You don't want all the chlorine to get used up and you don't want its level to be high the next day.
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    Mod Squad jblizzle's Avatar
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    Re: Post-bathing shock

    Ah, I think I see that now. Thanks for the clarification.
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    Re: Post-bathing shock

    Quote Originally Posted by chem geek View Post
    The absolute quantity of oxidizer is per person-hour independent of the size of the tub. It's only if you use a rule-of-thumb based on ppm FC that the size of the tub needs to be factored in.

    For every person-hour of soaking in a tub with no ozonator, it takes roughly 3-1/2 teaspoons of Dichlor, 5 fluid ounces of 6% bleach, 3-1/2 fluid ounces of 8.25% bleach, or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize the bather waste.

    However, the REAL rule is to add whatever amount of oxidizer that is needed after a soak so that you have a small residual of chlorine 24 hours later. You don't want all the chlorine to get used up and you don't want its level to be high the next day.
    Thanks, so how do you deal with really high FC in small tubs? If I have a 1-hour party with 4 total people, I add 20 oz 6% bleach and hit 33+ FC at 30 CYA. With 6 people, that's 42+ FC at 30 CYA. This seems like a bad idea for the tub, but it sounds like that's par for the course.

    Maybe the answer is to do it in stages before, after, and an hour after again?
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    Re: Post-bathing shock

    Adding that much bleach at one time isn't a problem for the spa, given that there is CYA in the water. You wouldn't want to go in yourself until the level comes down, which it will do fairly quickly (hours).
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    Re: Post-bathing shock

    Does this same bather-hour formula apply to pools, too?

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    Re: Post-bathing shock

    Technically it does except scaled down for lower temperatures, but it's irrelevant because the quantity is so small compared to the amount of chlorine loss from sunlight and other losses. While for a spa the rough rule-of-thumb per person-hour is 7 grams of chlorine (in Cl2 units) and 4 grams for swimming pool temperatures (one sweats less), in your 26,000 gallon pool which is 98,400 liters this is only (1000 mg/g)*(4 grams)/(98,400 liters) = 0.04 mg/L or ppm chlorine. Daily FC usage is usually around 2 ppm per day so you can see how this bather load calculation isn't relevant to most residential pools. Now in a smaller pool with a pool party then that's different so 10 people in a 7500 gallon pool for 2 hours would be 2.8 ppm while in a high bather-load commercial/public pool it can be even higher.

    If one expects to have long soaks or lots of people in a residential hot tub especially if used every day or two, then that's when an ozonator makes the most sense since it can oxidize a lot of the bather waste cutting the chlorine demand at least in half. The downside is that ozone also oxidizes chlorine so if you don't use the spa every day or two then it ends up doubling (or more) the chlorine demand in between soaks (i.e. the background chlorine usage).
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