TA high, crazy scaling in just 1 day

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
fatybabe
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Postby fatybabe » Wed 29 Aug, 2007 01:21

one a day introduce once the stuff gets settled in you can bee too ahsty with these things.


justjabbin
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Postby justjabbin » Wed 29 Aug, 2007 16:00

Any idea of how much TA will be reduced after each round of acid/aerate?

Also I have 2 tester kits. They are giving different TA measurements from the same water sample.... After first round of acid/aerate my first kit read 230ppm. My second kit read 270ppm. I am just going to take the average of the two. Does this make sense or should I do something different?

Is it normal for a discrepancy of this magnitude between 2 kits?
Best,
Justjabbin
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Postby chem geek » Wed 29 Aug, 2007 18:56

I just noticed you said (in the first post in this thread) that you had a salt water pool. So you have a salt water chlorine generator (SWG) and you have a salt level of around 3000 ppm? That should actually have somewhat less scaling (the higher salt level allows for more calcium carbonate to be in the water), though would have an even greater tendency for the pH to rise. Essentially, an SWG provides lots of aeration due to the hydrogen gas bubbles (you can see them readily at night if the pump and SWG are on and you turn on the interior pool lights).

When I last answered this sort of question, I was told (legitimately) that I was scaring pool users. The alternative of doing a partial drain/refill was suggested instead, though that only works if the TA of the fill water is low.

As for your inconsistent results, no that is not normal. Are these both drop tests, or are they test strips, or a mixture? The drop test should be more accurate than a test strip. I strongly suggest you get a Taylor K-2006 test kit from Taylor here or from Leslie's here or from poolcenter(dot)com here or the even better TF100 test kit from tftestkits(dot)com here .

Starting with a TA of 250, pH of 7.8, I calculate it should take 162 fluid ounces (20.3 cups or 1-1/4 gallons) to get to a pH of 7.0

I calculate that after the above, it will take a further 566 fluid ounces of acid (70.8 cups or about 4-1/2 gallons) to get the TA lowered to 100, but it will take quite a few cycles of aeration and acid addition to do so. It will go fastest if you add acid every time the pH rises to 7.2. You do NOT just add acid every so often -- you don't want to go below 7.0. You would add around 59 fluid ounces (7.4 cups or about 1/2 gallon) when the TA is high and around 32 fluid ounces (4 cups) when the TA gets closer to 100. As for how many cycles, this depends on how high you let the pH go. It's a tradeoff between making the whole process go more quickly but adding acid more frequently vs. letting the pH go higher which makes the process go more slowly but you don't add acid as often (but add more acid each time since there's a larger pH more to make).

Regardless of how you do this -- whether quick or slower or even not at all and just adding acid every week over a year -- it's the same total amount of acid to lower the TA from 250 to 100 in 19,000 gallons of 728 fluid ounces (91 cups or 5.7 gallons). Of course, if you've got lots of evaporation and your fill water is high in TA, then that will tend to increase TA over time and will be counter to your TA lowering efforts, but there's not much you can do about that (unfortunately) except getting a pool cover to reduce evaporation.

Richard
justjabbin
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Postby justjabbin » Wed 29 Aug, 2007 21:13

I have a test kit from Taylor and a smaller drop test kit from Pentair that came with my brush. I like the Pentair because it has PH down to 6.8 and Taylor only down to 7.0. However Pentair has no distication from the 6.8 to 7.2 range so using both helps me really dial it in. The pentair TA test seems a little inconsistent but my first taylor TA this morning was 270. this evening was 230. I started at Ph 7.8...lowered it to 7.0 with approx 2 gallons of acid then aerated for a few hours and was at 7.2 So I added more acid (1/2 gal) and am now back to 7.0 I am aerating again right now. I am assuming that as my TA comes down it will take less acid to lower PH or will it take more?
Best,

Justjabbin
fatybabe
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Postby fatybabe » Wed 29 Aug, 2007 21:59

As stated already by Richard both kits should give the same results, might i suggest you use Palintest i tend to find them being the most reliable.

In addition there are some cheap photometers availble in the market that might give you a better reading, check out Lamotte for those.
chem geek
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Postby chem geek » Wed 29 Aug, 2007 22:14

I personally like the Taylor kits (and the tftestkits TF100 is a more logical repackaging with better reagent sizes), but regardless I think something else is going on with regard to your varying testing, even over time. I suspect that your varying results are due to the amount of shaking of the test vial between drops, though technically speaking the outgassing of carbon dioxide raises the pH and there should be no change in TA from that (later adding acid is what causes both pH and TA to drop). So though it's a bit of a mystery, you might try adding, say, 20 drops before doing the mixing and swirling.

Also, read this link from Taylor regarding a buildup of static electricity that can affect drop size. This would apply to all brands of tests, not just Taylor's. I wouldn't worry too much about the variation at this point. If it's due to the number of drops (either from extra mixing or from static electricity), this error will reduce as you get to lower TA levels (and therefore use fewer drops in the test).

As for the amount of acid to lower pH, as the TA goes down it will take less acid to lower the pH by the same amount. That's why I gave a range from 59 fluid ounces of acid at high TA to 32 fluid ounces of acid at low TA. This is the sort of lowering to be expected -- each to drop the pH from 7.2 to 7.0. Obviously, what you read on your pH test trumps whatever I tell you (i.e. you don't want to go too low). Fortunately, there is a bit of a spring effect in lowering pH in a carbonate buffer system (that the pool has) -- it takes more acid to go from 7.2 to 7.0 than it does to go from 7.6 to 7.4, for example. So, fortunately, it gets harder to overshoot as you go lower in pH (though it can still be done -- many Trichlor users that don't watch their pH levels and do not compensate accordingly can end up with acidic pools at a pH near 4 and zero measured TA -- an obviously very, very bad thing).

Richard
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Postby justjabbin not logged in. » Thu 30 Aug, 2007 22:32

How much is too much acid to add to a 19000 gal pool in one day? I seem to get answers all over the place from a few cups to a few gallons? How much at each time?
fatybabe
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Postby fatybabe » Fri 31 Aug, 2007 00:58

10 ounces at most.
chem geek
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Postby chem geek » Fri 31 Aug, 2007 09:26

10 ounces of Muriatic Acid (31.45% Hydrochloric Acid) in 19000 gallons with a TA of 100 would lower the pH from 7.5 to 7.38 (a drop of 0.12) so is a decent limited adjustment to do at one time. But this is more for safety since the calculations are complex for arbitrary TA and starting/ending pH. For example, with a TA of 200, the pH would drop from 7.5 to 7.43 (a drop of 0.07) while at a TA of 100 but starting at a pH of 7.2 the pH would only drop down to 7.13 (a drop of 0.07).

I've safely added more as have others when using my spreadsheet to calculate the actual amount needed (for the TA lowering procedure, for example), but one needs to also consider measurement errors. Fortunately, as the pool gets lower in pH, it resists further lowering that much more so this prevents overshooting by too much unless one really mis-estimates by a large amount.

Regardless of how much acid is added, it should be added very slowly over the flow from a return (with the pump running, obviously). Do not ever pour it into the skimmer -- not even slowly. I've calculated that the acid is so strong that it completely overwhelms the carbonate pH buffer in the pool water where it is poured (even with skimmer flow rates) so even a slow pouring in the skimmer results in a pH hitting the pump, filter and heater of around 1-2 which is very, very acidic. Pouring slowly over a return flow at the deep end of the pool has the largest volume of water dilute the acid and minimize the low pH exposure.

Richard
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Postby Guest » Thu 20 Sep, 2007 22:23

WOW...you do not just add a gallon of acid in the pool all in one day if you are a 20k pool! i would add like 4pt(half a gallon) max!!! and when you add the acid you add it all in one spot away from jets that brings the alk down if you boardcast it then it bring down your ph more than the alk...You want to do acid treatments ONLY when you ph is 7.6 or higher...sometimes you can put in so much acid your ph will NEVER come back up, plus if the water is really low in ph that is bad on the salt generator!!! for the scaling you can put in some scale inhibitor or metal control same thing and that should stay in your pool for a couple week then at like a cup or so a week to keep it from scaling again

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