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Thread: TA and PH drift

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    TA and PH drift

    I use 15% chlorine and also find the PH constantly rises and have to use a lot of acid to counter this rise. Keep an eye on your ALK though because high acid usage will lower your ALK leading to PH bounce, so have plenty of baking soda available.

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    Re: pH creeping up

    Quote Originally Posted by Freelancer
    I use 15% chlorine and also find the PH constantly rises and have to use a lot of acid to counter this rise. Keep an eye on your ALK though because high acid usage will lower your ALK leading to PH bounce, so have plenty of baking soda available.
    If you just let your TA get low, then the rate of pH rise and especially the amount of acid needed should drop. Even at a low TA of 40-50 ppm, you should not get "pH bounce" when using a hypochlorite source of chlorine unless you are referring to the rise and fall in pH when chlorine is added and later consumed. There won't be much getting into the water that is acidic so the tendency will be for the pH to rise, though not as quickly. If you want to have additional pH buffering, then you can use 50 ppm Borates. You will also need to raise your CH and/or pH target levels if you have a plaster pool (i.e. to make the saturation index closer to 0).

    Now, some amount of acid will be needed to counter the "excess lye" in the chlorinating liquid you are using, but that shouldn't be very much unless your chlorinating liquid is very high in pH. The best 12.5% chlorinating liquid has a pH of around 12.5 which is roughly 0.25% excess lye. For every 10 ppm FC, it only needs 0.7 fluid ounces of Muriatic Acid per 10,000 gallons to compensate. Some have a pH of 13 for 0.8% excess lye which for every 10 ppm FC requires 2.3 fluid ounces per 10,000 gallons. The worst have a pH of 13.5 for 2.5% excess lye which for every 10 ppm FC requires 7.2 fluid ounces per 10,000 gallons.
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    Re: pH creeping up

    Richard, I run all my pools at 40-50 PPM Alkalinity and you are absolutely correct I do not get PH bounce, my highest CYA level is 50 with most pools being 40. I'd love to run the pools at higher ALK levels but find it practically impossible due to the amount of Bicarb I have to use and acid to counter to PH rises. My PH in the summer time are constantly at the 7.8 mark purely due to two reasons, one a higher PH aids my water balance given the low ALK levels I have and I'm pretty sure without a shadow of a doubt that the 15% chlorine I use is at the top end of the PH scale and has a lot of lye in it. The second reason is that I have to use so much acid to get the PH down it drastically reduces the ALK. I also find that I require a lot more acid to get my PH down than the calculator suggests.. In a 100,000 litre pool 5 Litres of 20% acid will only drop the PH 0.1.. The pool calculator suggests if I was using 15% acid and put 5 litres into a body of water in order for it to drop just 0.1 PH then the volume of the water would have to be 950,000 litres so given that the pool is kidney shaped and I may have a 10% margin of error when calculating the volume no way am I 850,000 litres out. All my pools are automatically dosed, all the returns are turned right down so little if any aeration.

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    Re: pH creeping up

    Thanks for the info. As you noted, targeting a somewhat higher pH helps compensate the saturation index, but also is more stable against rising pH. As shown in this table pools are intentionally over-carbonated and a lower TA and higher pH reduces that amount of over-carbonation. At 40-50 ppm TA and a pH of 7.8, your pools have about twice as much carbon dioxide in them as they would if in equilibrium with air so would have very little outgassing compared to most pools that have 8x or more.
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    Re: pH creeping up

    That is a very informative table thanks. Also I seem to remember you suggesting in one of your posts that it might be better for public pools to be run at 10% - 20% of the CYA. I run mine in the closed season 7% - 10% and as you suggested 10% - 20% during the swim season. I find that this works great and is very easily achievable given that I have automatic dosing systems fitted to all my pools allowing me to put in little and often.

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    Re: pH creeping up

    Richard you wrote that the CH were going to have to be kept higher in order to help the balance. I fully understand this but I was reading this and wondered if you have any comments on it as it will have an effect on CH and one which I have not seen mentioned on these forums.

    "The alkalinity of the sodium hypochlorite solution also causes the precipitation of minerals such as calcium carbonate"

    Unfortunately it does not state by how much..

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    Re: TA and PH drift

    Sodium hypochlorite will raise the PH temporarily, it then goes down again as the chlorine gets used up. If your CSI is too high that temporary PH boost might be enough to precipitate CH. If your CSI is anywhere reasonable it won't be an issue.
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    Re: TA and PH drift

    Jason, so what you are saying is that as a stand alone statement it is not true but with mitigating circumstances it could be true.. All good stuff, thanks.

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    Re: TA and PH drift

    Actually you have just explained the reason why I have constantly high PH's when everybody else seems to not suffer this problem. It's pretty clear that constantly dosing throughout the day causes PH rises and before the effects of the chlorine being used can have an effect on the PH the pools are being re-dosed with chlorine which in turn will prevent the PH from dropping.. Not to mention on days where I have a light bather load the problems will be further compounded, but you have to dose for the worse case scenario..

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    Re: TA and PH drift

    It is something like that, but I don't think you understand it exactly right just yet. If you are maintaining a more or less constant FC level by doing through the day, the PH will remain more or less constant, since chlorine is getting used up as quickly as it is being added.

    There are two effects during shocking. PH goes up a little while the FC level is elevated, and in most cases the FC level also gets high enough to start throwing the PH test off. The PH test will read just a little higher than actual when FC is between 10 and 15, noticeably higher than actual from 15 to around 20, and then way wildly higher than actual when FC is higher than that. Those two effects combine to make PH readings during shocking problematic.
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    Re: TA and PH drift

    Thanks for the clarification..

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    Re: TA and PH drift

    Freelancer, in your case with pools that have high bather loads, there is another factor to pH rise and that is the "excess lye" that is in chlorinating liquid. While in a low bather load residential pool this effect is usually negligible, in a high bather load pool such as a commercial/public pool, the effect can be significant. For example, if you are using high quality 12.5% chlorinating liquid with a pH of 12.5, then every gallon requires 0.84 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) to compensate. If you are using medium quality 12.5% chlorinating liquid with a pH of 13.0, then every gallon requires 2.65 fluid ounces of acid. If you are using poor quality 12.5% chlorinating liquid with a pH of 13.5, then every gallon requires 8.4 fluid ounces of acid. Even if you had ZERO carbon dioxide outgassing, you would still need to add these amounts of acid.
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    Re: TA and PH drift

    Richard, I absolutely concur with what you say because I use a lot of acid, I use the 15% acid for two reasons, one per acid percentage it is cheaper and second it is far friendlier to work with. The full strength acid is pretty nasty stuff requiring full protection when using including a mask for the fumes.

    I am currently experimenting with the pools to find the ideal amount of acid/sodium bicarb to firstly lower the PH and raise the alkalinity. So far I have found that in 240,000 litres of water, 10 litres of 15% acid and 10Kg of Sodium bicarb will lower my PH by 2.5 and raise my Alkalinity by 25. I did this to a pool whose PH was 7.85 and ALK 45.

    Only problem is after one week the PH was right back up at 7.85.. So today I just put acid in. I sometimes feel as though I'm banging my head against a brick wall. I think my only option is to maintain correct CYA/chlorine levels and just monitor the PH and let the ALK go because any adjustment with Sodium Bicarb has a large effect on the PH. If I get PH bounce then of course I will have to deal with it but if not I'll just have to make up the water balance with low end CYA 30 -40, the high end Calcium and luckily the water is warming up nicely now so that is going to help with the water balance now as well.

    I'm assured by my chlorine supplier that his is the highest grade chlorine but what can you say when the same said person told me it was 90% chlorine he was selling me.. When I pointed out to him that on the label it stated 15ml of chlorine per litre which equates to a 15% chlorine solution he was not convinced because him being the only person selling 90% chlorine was his big selling point.

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    Re: TA and PH drift

    Quote Originally Posted by Freelancer
    I am currently experimenting with the pools to find the ideal amount of acid/sodium bicarb to firstly lower the PH and raise the alkalinity. So far I have found that in 240,000 litres of water, 10 litres of 15% acid and 10Kg of Sodium bicarb will lower my PH by 2.5 and raise my Alkalinity by 25. I did this to a pool whose PH was 7.85 and ALK 45.
    If the amount of acid per time (per week, for example) is the same at TA 45 ppm as it is at, say, TA 65 ppm, then the higher TA would be fine, but if the lower TA uses less acid then just leave it there. At some low TA you should find that adding acid to maintain pH doesn't lower the TA anymore. At this point, your acid addition is exactly compensating for the "excess lye" in the chlorinating liquid and the carbon dioxide outgassing is minimal. I find it hard to believe that at a TA of 45 ppm that you've got much carbon dioxide outgassing which is why I think your chlorinating liquid may have more excess lye in it than most.

    The acid and sodium bicarb amounts with the numbers you gave should have had the pH go to 7.1 and the TA to 61 ppm (I think you meant that the pH would lower by 0.25, not 2.5). So in the intervening time before you measured again you may have added the equivalent of 1.4 kg of "excess lye" since that would result in a pH of 7.6 and a TA of 68 ppm which is close to what you are seeing. However, even if your chlorinating liquid had a pH of 13.5 and 2.5% excess lye, that implies 56 kilograms of chlorine which if 12.5% chlorinating liquid would be around 52 liters or around 27 ppm FC which seems too high for your chlorine demand.
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    Re: TA and PH drift

    Richard that really is great information.. Again you are correct, I did not use 52 litres of chlorine during the PH rise, currently that particular pool is using 4 litres a day and that rise was over seven days so only 28 litres went into the water.. The pool during the height of the season will be using 10 litres per day.

    In light of what you have said I shall let the Alkalinity go down to around 40 PPM and provided I see no PH bounce will just adjust the PH to be maintained at 7.7 - 7.8.

    Also you are correct I did mean a 0.25 PH drop.

    Thanks once again for all your help.

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    Re: TA and PH drift

    If you do end up at the lower TA level and find it more stable, then you can not only have the higher target pH, but can raise the CH level as needed to get the saturation index closer to zero if this is a plaster pool (I assume it is given its size).
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    Re: TA and PH drift

    >>.. pointed out to him that on the label it stated 15ml of chlorine per litre which equates to a 15% chlorine solution he was not convinced because him being the only person selling 90% chlorine was his big selling point

    1 Liter = 1000 Milliliters

    1 Milliliter = 0.001 Liters

    15 Millileters = 1.5 %

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    Re: TA and PH drift

    Quote Originally Posted by susa
    >>.. pointed out to him that on the label it stated 15ml of chlorine per litre which equates to a 15% chlorine solution he was not convinced because him being the only person selling 90% chlorine was his big selling point

    1 Liter = 1000 Milliliters

    1 Milliliter = 0.001 Liters

    15 Millileters = 1.5 %
    You are of course quite correct, I meant 150 grams per litre.

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