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Ta Rise - Why?


robquick
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Hi,

I use the Dichlor/bleach method and have done for some time, my last water change was in December (results below).

TA - 40ppm

PH - 7.7

CH - 110

CYA - 35 (when I switched to bleach)

Borates - 50 ppm

I do find that my PH rises and needs to be pulled down from about 7.9 every couple of weeks. If I'm honest I'm not great at testing my TA frequently however, I just checked both my PH and TA with my Lovibond photometer !

TA 95

PH 8.15

Whats causing my TA to rise so significantly and whats the best way of reducing ? Am I right in thinking I should bring the TA/PH down bit by bit with dry acid and then get the blowers on to increase the PH back to the required level without impacting the TA ???

Thanks.

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There is a small amount of lye in the bleach so that raises the pH and TA some, but adding acid to lower the pH should lower both back to where they started. If you've got carbon dioxide outgassing, then the net result would be that the TA would get lower, not higher.

Note that the pH and TA move together somewhat so measuring TA when the pH is high can be confusing. Nevertheless, if I use your numbers and assume 50 ppm Borates, then a pH of 8.15 with TA of 95 would go down after adding acid to 7.5 and a TA of 74. So yes, the TA is somewhat high.

If your water level was dropping from evaporation and you were topping off with fill water then anything in the fill water gets added to the spa water including TA. Maybe that is what was happening. What's the TA of your fill water?

You lower the TA by following the procedure in Lowering Total Alkalinity which is a process of aeration and acid addition done at low pH.

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Chem,

Ok thanks for update. I will check the TA again once I've got the PH back in line. Interestingly my fresh water TA is very low about 25ppm and I haven't topped the tub up for some time so I guess I need about 100 litres (1500 litre tub) to fill up. Am I right in thinking this would help bring the TA down too if I do a top up ? What would you aim for as a target PH ?

Thanks

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Yes, with a low fill water TA you would be diluting your water to a lower TA level than where it is now though not by much since it's not a lot of water (probably barely measurable since it will be < 10 ppm change). Strange though how the TA rose in your situation. Is this a wooden tub or is it plaster (say, attached to a pool)? I'm just wondering if there's any other chemistry going on that could raise the TA.

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Your pH might have been even higher so yes you probably just need to keep your pH in check more frequently. I also suspect that the photometer isn't measuring TA or pH accurately when the pH gets high.

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