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Thread: Air needed for outgassing CO2

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    Air needed for outgassing CO2

    Is there any way to calculate how much air needs to be introduced to water to get the desired outgassing effect when trying to lower alkalinity?

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    JasonLion's Avatar
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    It isn't really a function of air volume but rather a function of the surface area of the bubles times the contact time. Smaller bubbles have more surface per unit volume than large bubbles.

    A couple of spa jets work very well. You could look up the specs on some average spa jets.
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  3. Back To Top    #3
    I don't have spa jets, only one return with no eyeball. I've been contemplating a few things though, like:

    1- Using a combination of elbows and fittings to make an angled 1.5" pvc attachment that would have an end cap drilled with small holes that breaks the surface of the water, so the smaller streams arc out and rain down creating smaller bubbles. My biggest concern with using my only return for this approach is I don't want to diminish circulation.

    2- Using a tank of compressed air (like an LP tank or something) that has a nozzle at the end of a length of hose that can be weighted underwater in the deep end (to maximize air bubble exposure?) that will emit small bubbles in a continuous stream when the valve is cracked open. This is why I asked the original question, I was wondering if it was possible to calculate how many tanks it may take to do the job. Feel free to laugh at me

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    Your method #2 will be VERY effective. There were some people on another forum who used an air compressor with a nozzle that produced small bubbles at the end and found that is aerated and had the pH rise VERY quickly. The best nozzle was one that produced tiny bubbles but was not so restrictive that it prevented a large volume of air to be introduced. So a nozzle that had LOTS of very tiny nozzles would be best since the tiny nozzles would produce tiny bubbles (with lots of surface area per volume) and it would put a lot of such bubbles (air) into the pool.

    Though technically using tanks of a gas that did not contain carbon dioxide in it would be better, the difference isn't worth it as an air compressor is very convenient (compared to tanks of nitrogen, oxygen, hydrogen or some combination without carbon dioxide). Scuba tanks typically have a mixture of nitrogen and oxygen without the carbon dioxide, but I think that's much more expensive than using a simple air compressor.

    I could calculate the theoretical "best case" for perfect aeration where a full equilibrium was achieved between the bubbles and the pool, but I don't know how close to this best case actual aeration will be. Henry's Law constant (using the combination of carbonic acid and dissolved carbon dioxide for Molarity) is around 0.034 M/atm and 100 ppm TA with 30 ppm CYA (so 90 ppm carbonate alkalinity) is 9.7x10-5 moles/liter of H2CO3 and CO2(aq) so this is a partial pressure of 2.8x10-3 atmospheres compared to a normal atmospheric partial pressure of carbon dioxide of 3.2x10-4. At the deep end of the pool, the pressure is higher (18% higher under 6 feet of water) but ignoring that if you were to aerate with the equivalent at one atmosphere of your pool's water volume, then this would remove 0.0025 of that volume (0.25%) as carbon dioxide gas. n/V=P/RT=1/(0.082*300) = 0.04 moles/liter so multiplying by 0.0025 gives us 0.0001 moles/liter of carbon dioxide which is 5 ppm CaCO3 equivalent (each TA represents two moles of CO2). So, lowering a pool's TA by 5% (5 ppm out of 100 ppm in this case) would require the pool water volume's amount of air at 1 atmosphere. That gives you an idea of the absolute minimum amount of air required assuming that all outgassing comes from that aeration (some will also come from the pool surface, but that's a lot slower than the tiny air bubbles). That seems high for the required volume so maybe I did something wrong with the calculation. On the other hand, an air compressor with a 10 CFM (cubic feet per minute) rating is 600 cubic feet per hour. A 15,000 gallon pool is about 2,000 cubic feet so one pool volume would be 3.3 hours. It does seem to me that the TA drops faster than that, but over 24 hours that would be a 5% reduction in TA every 3.3 hours which translates to a 31% reduction of TA in 24 hours so perhaps this is indeed a reasonably correct calculation.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Richard, after I typed that down I started thinking about an air compressor, using a long hose and keeping it plugged in in my garage so it could replenish it's air when needed.

    Hmmm...

    It should be easy enough to fashion a nozzle of some sort . Could possible use a peice of PVC with an end cap drilled with several tiny holes.

    I've notice less PH drift this year, my alk is hovering around 140 and I've only had to hit my pool with acid twice so far this season, but I do have high alk fill water. I'd like to get my alk down to 80 to give me some breathing room, but it doesn't seem to want to drop more than 10 ppm or so, even when I lowered my PH from 7.8 to 7.0.

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    Quote Originally Posted by chem geek
    Your method #2 will be VERY effective. There were some people on another forum who used an air compressor with a nozzle that produced small bubbles at the end and found that is aerated and had the pH rise VERY quickly. The best nozzle was one that produced tiny bubbles but was not so restrictive that it prevented a large volume of air to be introduced.
    LOL! Anyone who has ever kept an aquarium could tell you this! Once again I say that pools and aquariums are NOT that different!

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    MikeInTN's Avatar
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    Quote Originally Posted by Rangeball
    Richard, after I typed that down I started thinking about an air compressor, using a long hose and keeping it plugged in in my garage so it could replenish it's air when needed.

    Hmmm...

    It should be easy enough to fashion a nozzle of some sort . Could possible use a peice of PVC with an end cap drilled with several tiny holes.

    I've notice less PH drift this year, my alk is hovering around 140 and I've only had to hit my pool with acid twice so far this season, but I do have high alk fill water. I'd like to get my alk down to 80 to give me some breathing room, but it doesn't seem to want to drop more than 10 ppm or so, even when I lowered my PH from 7.8 to 7.0.
    No need to just drill an end cap - drill tiny holes up and down a length of PVC pipe, and put a solid endcap on both ends, and rig a fitting on one end to go to your compressor hose.
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    Richard, thanks a bunch for those calculations. I'm not going to pretend I understand them, but seeing you work through the process is amazing You're right, at first I thought "Man, that's a bunch of air" but when you quantified it to run time with an air compressor, that made me feel better

    Next time I need to lower Alk I plan to do this. However my PH seems to have settled in around 7.4, alk at 130. I suspected faster PH rise at this ratio, but so far so good. I'll keep an eye on it.


    Quote Originally Posted by MikeInTN
    No need to just drill an end cap - drill tiny holes up and down a length of PVC pipe, and put a solid endcap on both ends, and rig a fitting on one end to go to your compressor hose.
    Excellent

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    I have the same issues. Being new at maintaining both a pool and spa I am trying to become a fast learner.

    Both my pool and spa have the same issue. Both are running with a PH = 7.2 and a TA = 140.

    After reading about the BBB, was thinking of adding Borax to raise PH without raising TA, then lowering both with muratic acid. But now I read this tread and was wondering if I just experiment with bubbles? Does anyone know if both methods will work?
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    Borax raises both pH and TA just as any base does (and any acid lowers both the pH and TA), but it raises TA less than if you use sodium carbonate / soda ash / washing soda / pH Up. These latter products (they are all the same chemical) are exactly like adding a pure base (such as sodium hydroxide / lye / caustic soda) PLUS sodium bicarbonate / baking soda / Alkalinity Up. If you add Borax and then add acid, the pH and TA will go up and then down and you'll be back to where you started (almost -- you'll have some borates in the water). Your TA will NOT get lowered.

    Aeration plus acid addition is the ONLY way to get TA to drop. You don't actually have to aerate explicitly -- if you lower the pH you will raise the rate of outgassing, but if you aerate as well then you speed up the process. If you do nothing at all except add acid over time whenever the pH rises, then eventually you will end up lowering your TA, but it will take a long time. All you are doing with the aeration at low pH plus acid addition is to accelerate the process. You will use exactly the same amount of acid as you would have added over months or years to get the TA down -- you just get it over with more quickly with the result that going forward the amount of acid you need to add (if any) is far less and the rate of pH rise is somewhat less.

    To lower your pool's TA level, follow the procedure as described in this post. On the other hand, why are you running at a low pH of 7.2? You should find less of a rise in pH if you tried to maintain a pH of 7.5 or even 7.7 instead. If you still find a strong pH rise, then you can certainly lower the TA to see if that helps.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    I have always had trouble keeping my spa PH in the normal range. Seems I went through a bunch of bottles of PH up over the last 6 months. I just refilled the spa and added a metal gone product and a bromine concentrate and the ph measured 7.2 and ta=140.

    The pool was running around 8.0 and I added a ph down but seems I went way to far to get to 7.2. It was the hth ph down dry powder, not muriatic.

    I'm new to pools and water chemistry so I am trying to learn fast. I just built my own pool and spent all my research time trying to learn how to do it. Now it is done and it turned out awesome. Inground 18x36 vinyl liner pool. Now it's time to learn BBB method and chemistry on keeping my pool nice and clean.

    Here are my test results from about 5 days ago. The only thing I test yesterday was ph and TA and it's still at 7.2 and 140 in both the pool and spa.

    I used the Taylor K-2005 kit
    FC=~5 (highest red on chart)
    TC=~5 (almost looks same color)
    PH=7.2
    TA=140
    CH= 120
    CYA=35
    __________________________________________
    18x36 inground, sand filter, sta-rite sr400 ng heater

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