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Thread: The 1/3 Rule

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    AnnaK's Avatar
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    The 1/3 Rule

    If I understand this correctly, we are to subtract 1/3 of the CYA reading in ppm from the TA reading to get 'actual' or 'functional' or adjusted TA.

    I had never heard of this rule but now it seems it's everywhere I look. That explains the "adjusted alkalinity" I've seen on water test printouts from the pool stores.

    In one of the CYA threads one of the forum gurus (sorry, I cannot find the thread I'm referring to) said that we already take the CYA/TA relationship into account when we make TA recommendations. Am I correct in my understanding then that, when we shoot for a TA of between 80 and 100 in a vinyl pool in which CYA is between 30 and 50, those TA value recommendations are based on the CYA levels?

    I understand that alkalinity acts as a buffer for pH, keeping it from fluctuating. The pH readings of my water have been 7.5 all season. I started with a TA = 100 and CYA = ~30 back in May. I'm now at TA = ~85 and CYA = ~50, pH at 7.5. Since there has been no change in the pH in many weeks I had no plans of increasing the alkalinity - until I read about this 1/3 rule. Should I?

    Sometimes, I think I'm way better off the less I know
    AnnaK

    12,500 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    frustratedpoolmom's Avatar
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    Re: The 1/2 Rule

    Quote Originally Posted by AnnaK
    Sometimes, I think I'm way better off the less I know
    Amen sister, I know what you mean!
    Helpful links: Pool School; CYA/Chlorine Chart
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    meatloaf's Avatar
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    Re: The 1/3 Rule

    I think the post you are referring to is this,
    alkalinity-test-question-t8441.html
    The 3rd thread should provide you with the correct answer you are looking for.
    My pool is a 10,500 gallon (420 Sq Ft ) kidney shape. A hand made sheer decent waterfeature. STA-RITE HRPB30, Sand filter 30" 98 GPM,
    1 HP Maxi-Glas 2 pump, BBB Method, Taylor K-2005 test kit, 50 Watt 12V low-voltage pond light illuminates the pool at night for a nice soft glow.
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    AnnaK's Avatar
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    Re: The 1/3 Rule

    Thanks for the pointer.
    AnnaK

    12,500 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    Re: The 1/3 Rule

    Total Alkalinity is a measure of all forms of alkalinity including the carbonate alkalinity and the cyanurate alkalinity. You should keep your “total” alkalinity between 80 and 120. When calculating the saturation index you want to use just the carbonate alkalinity in the calculations.

    With a ph of 7.5 and a cyanuric acid of 50 you would have a cyanurate alkalinity of about 16 and a carbonate alkalinity of about 85-16=69. The saturation index is calculated using the temperature, ph, calcium hardness and the carbonate alkalinity.

    The Taylor test kit comes with a slide rule to calculate the Saturation Index of the water. For your reading of ph=7.5, carbonate alkalinity of 69, (assuming a temp of 80F) your calcium should be about 340ppm. If you raised your total alkalinity to 120 your carbonate alkalinity would be 120-16=104 then your calcium should be about 240. Here is a calculator that can help: http://www.poolcalculator.com/. What you should do depends on your calcium level and what equipment you have. If you can provide a calcium level, temp., and a description of all equipment, especially heater or SWCG, I could make a better recommendation.

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    Re: The 1/3 Rule

    Subtracting 1/3 of the cya reading from the TA is actually an estimation. there is an actual mathematical equatiron to determine the correction and it varies with pH.

    Also be aware that many in the industry question the usefulness of 'corrected alkalinity' and even Taylor (who is a big proponant of it) admits to this.

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    AnnaK's Avatar
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    Re: The 1/3 Rule

    Quote Originally Posted by PoolOwnerNumber9
    What you should do depends on your calcium level and what equipment you have. If you can provide a calcium level, temp., and a description of all equipment, especially heater or SWCG, I could make a better recommendation.

    Thank you for the information.

    My pool specs are in my sig line. Vinyl AGP, sand filter, no heater, no SWG. Water temp this morning is 82°F. I only ever tested for CH once and it was 80 ppm then, in June this year. This coincides with the CH of my fill water. No foaming, no scaling. I do have 120# of solar salt in there, corresponding to ~1250 ppm of salt.

    What is the Saturation Index and how does it affect an AGP without any additional peripherals?
    AnnaK

    12,500 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    JasonLion's Avatar
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    Re: The 1/3 Rule

    Total alkalinity, TA, is the total buffering against PH change from acid. Most of the TA in a swimming pool will be from carbonate alkalinity (from baking soda), with additional contributions from cyanurate alkalinity (from CYA), and borate alkalinity (from borax).

    Carbonate alkalinity, CA, is a measure of alkalinity contributed by carbonate and bicarbonate (essentially baking soda) in the water. CA is important for figuring out the calcite saturation index, ie the tendency for calcium to be dissolved out of the paster or deposited as scale.

    Adjusted total alkalinity, Adj TA, is an estimate of CA, typically made by subtracting 1/3 of the CYA level from TA. The usual rule for figuring out the Langelier Saturation Index, LSI, uses Adj TA.

    Any time we talk about TA at TFP we are talking about total alkalinity, as measured directly by the test. All of the recommendations here and articles in Pool School are based on TA. Likewise, my Pool Calculator always uses TA directly from the test. When my Pool Calculator needs to know the CA level it will calculate it from the various levels you have entered.

    There is really no reason to know what the adjusted total alkalinity is unless you are trying to calculate your LSI. If you have access to a computer and need to know your calcium saturation index, you should be using my Pool Calculator to calculate CSI, which is more accurate than LSI. The CSI calculation does not require knowing the adjusted total alkalinity.

    Since TA shows the total amount of buffering against PH changes caused by acid, that is the number you almost always want to know. There really is no reason to try and figure out your CA unless you are trying to figure out something to do with calcium saturation. Even when trying to figure out calcium saturation, using TA directly is frequently good enough. Only if your CYA level is quite high does your CYA level make a significant difference. If you are using my Pool Calculator, you don't even need to think about it, since my Pool Calculator will do all of the math for you.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: The 1/3 Rule

    That was a wonderful short seminar. Thank you much!
    AnnaK

    12,500 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    Re: The 1/3 Rule

    Discussion of negative CSI and metal corrosion was moved to it's own topic. JasonLion
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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