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Couple Of Chem Questions


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1. If aerating and adding acid lowers TA, won't TA creep down over time with use and adding acid to balance pH?

2. Is there something wrong with keeping FC up near 10 all the time? Most of the advice I've read suggest maintaining it at 2-6 and shocking to 10 once a week. What's wrong with maintaining a higher level of FC, to allow more room for error?

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You are correct that if you are adding acid to lower pH (which also lowers the TA) and the reason for doing this is that there is a rise in pH from carbon dioxide outgassing, then over time the TA will drop and you'll have to eventually add baking soda to bring it back up. In fact, one can precisely calculate how much the TA will drop from acid addition -- you can use The Pool Calculator to figure that out (in the effects of chemicals section). This is why you want to lower the TA level to try and get more pH stability -- otherwise, with a higher TA you just end up in a more vicious cycle adding more acid to keep the pH down and baking soda to keep the TA up.

An FC of 10 ppm all the time, I assume you mean with a CYA level of 30 ppm, would probably be a high enough level of chlorine to be annoying in a spa. You'd likely smell the chlorine fairly strongly -- most people can smell it when it gets above 2 ppm FC or so (at 30 ppm CYA in a hot spa). It's roughly equivalent to 1.6 ppm FC with no CYA so still somewhat lower than typically found in commercial spas, but at such higher active chlorine levels it will oxidize swimsuits, skin, hair, etc. that much faster. It's not necessary to be that high and if you are using the Dichlor-then-bleach method then you may not need to shock weekly anyway as you are oxidizing bather waste continually each day (if you are using enough chlorine to always have some residual).

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