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Thread: A Chemical Way of Calculating Pool Volume

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    A Chemical Way of Calculating Pool Volume

    There is a way to use pool water chemistry to calculate pool water volume without using the traditional geometric calculations.

    The geometric formulas are fine for pools that are traditionally shaped. However, when pools do not fall into these convenient shapes, a chemical method can be used.

    What is required? An alkalinity test kit, sodium bicarbonate or acid, and the formula.

    This method to calculate pool volumes, then, is to add a specific amount of acid or base to the pool water, observe the results, and “plug” the dose and result into a formula, so you can calculate the gallons of water in the pool.

    Here is the method:
    1. Take a water sample from the pool, test and determine the total alkalinity content. However, for sufficient accuracy, it is important to use a test method which is accurate to within 1 or 2 ppm. Most test kits have each drop of titrant equal 10 ppm. To accomplish this, add 10 times the amount of pool water as the normal test kit sample amount into a small cup. Then each drop of titrant becomes 1 ppm instead of 10 ppm. If the alkalinity is on the low side and could be raised, then use sodium bicarbonate to raise the alkalinity.
    2. For pools between 15,000 and 30,000 gallons, add precisely 5 pounds of sodium bicarbonate. For larger pools, add 10 pounds.
    3. With the circulation system running and proper dilution, wait for the chemical to completely blend throughout the water, which will probably require waiting at least several hours.
    4. Take a second water sample from the pool and test the alkalinity again using the same 10 times of pool water volume.
    5. Apply the following formula: 71,400 (formula number for Sodium Bicarbonate) times (multiply) by 5 (amount of pounds of sodium bicarbonate added) divided by the ppm change in alkalinity increase equals the pool volume.
    (You may add a different amount of sodium bicarbonate, but be precise in weighing, and change the formula accordingly).

    Example – The starting alkalinity was 100 ppm, 5 pounds of Bicarb is added, and the ending TA (alkalinity) is 116 ppm. (116 – 100, equals 16 ppm change)

    71,400 X 5 (pounds added) divided by 16 (ppm ALK increase) equals 22,312 gallons.

    Then you can round off to the nearest 1000 gallons, or however you prefer. The margin of error should be less than 5%.

    If the Total Alkalinity of the pool is high (over 160 ppm) and you want to lower the TA instead of raising it, then you can use muriatic acid (31.45%) to lower the alkalinity to determine the pool volume. The formula for adding acid is as follows:

    125,000 (formula number for Muriatic Acid) times (X) the quarts of acid added, divided by the ppm reduction in alkalinity. Of course, be careful to not add too much acid at one time. Generally, it is safe to add two quarts of acid per 20,000 gallons of pool water when the TA is over 160 ppm, and the pH is above 7.7. Two quarts of acid will lower the TA by 12.5 ppm per 20,000 gallons.

    The accuracy of this method is directly dependent on the accuracy of the chemical dose and the alkalinity test method. You will notice that using 10 times more test water sample means that if the alkalinity level is 100, it will use 100 drops of titrant. That may seem a lot, but you get good accuracy and results.

    Do not go overboard on volume precision. Since a one-inch change in a pool’s water level can result in hundreds of gallons of water volume difference, round off the calculated volume to the nearest 1000 and call it good.

    The method of addition of the acid dose does not affect the amount that the alkalinity changes, so it is not necessary to worry about “columns” or “slugs” or any other methods of adding liquid pool acid. However, it is best to dilute the acid and trickle it around the pool or over the return lines when the system is on.

    What is at issue is that the acid or sodium bicarbonate added is allowed to fully blend throughout the pool before the second sample is taken and tested. Wait at least 3 hours with equipment running for good accuracy.
    Last edited by onBalance; 07-27-2014 at 07:45 AM. Reason: Correction on waiting time & clarification

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    Mod Squad jblizzle's Avatar
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    Re: A Chemical Way of Calculating Pool Volume

    There should be a "Useful Posts of onBalance" thread
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    Re: A Chemical Way of Calculating Pool Volume

    Nice write up.

    The TA test is +-10%, doing it twice means you are nominally +-20%. In practice you are going to get results a little better than that because the error in the two tests is correlated and partially cancels out, but by the time you add in the other sources of error you are back to about +-20%. That isn't so bad, but it does argue against using this approach if it is practical to measure in a more conventional way. Also, I wouldn't measure baking soda by volume, that is going to add more error as baking soda can be packed to a range of densities.
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    Re: A Chemical Way of Calculating Pool Volume

    Jason, you are correct regarding the possible "range of densities" of various Bicarb packaging. I am been personally weighing the sodium bicarbonate that I purchase in bulk, but I will edit the volume suggestion out of my post above.

    Is your estimate of +/- 10% based on one drop of titrant for every 1 ppm of alkalinity, as oppose to 10 ppm for every drop?
    And I agree that rectangle pools is easily calculated the conventional way. So this chemical way is only meant to help confirm the pool volume calculation of all pools.

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    JasonLion's Avatar
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    Re: A Chemical Way of Calculating Pool Volume

    All of the Taylor titrations are +-one drop for every 10 drops, and +-another drop for the entire test. The extra +-drop for the entire test isn't significant when the drop count is high so roughly +-10% overall in this context. That comes from things like the drop size not being completely uniform, the reagent not being formulated exactly right, etc.
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    Re: A Chemical Way of Calculating Pool Volume

    Nice write up onbalance. Bookmarking for future use.
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    Re: A Chemical Way of Calculating Pool Volume

    Thank you and glad you liked it.

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    Re: A Chemical Way of Calculating Pool Volume

    I tried this test and even though it didn't come out exactly how I expected it would, I'd like to share how I removed at least some variability from this test.

    With TA tests it's hard to know exactly when color changed from green to red since there are some many shades in between. However, what it's important for this test is to catch the same shade of red before and after test.

    This could be easily done by using the same vessels for both tests and keeping the first around. I used two identical glass jars and it was easy to spot the difference between the shades. I'm pretty confident that I could tell the difference in 0.5PPM. The shade went from not being red enough compared to another jar to being a bit too red after another reagent drop. And I could even use the speed stir with it!

    Here are some pics to make it simple to demonstrate:







    I hope the fact that I kept the first jar for few hours didn't affect the color and my test was still somewhat valid.
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    Re: A Chemical Way of Calculating Pool Volume

    Excellent idea for comparing the color! And thank you for demonstrating that.

    But there is one issue with this. The final "red" color is based on what the pH is (which is about 4.5).
    After the test is finished (and the TA has been determined by the amount of drops added), the pH will begin to rise (due to carbon dioxide off-gassing over time), especially if the test water sample is continually stirred.
    That means that the Red color will begin to revert back to towards a pale red and then to a light green as the pH rises over time.

    However, if the color comparison (between to two samples) is performed within a few minutes of each other (and no continued stirring), then I agree that you would obtained a more accurate result.
    Good job!

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    Re: A Chemical Way of Calculating Pool Volume

    I have a quick question on running this test. Because of the dilution to get to 1ppm drops instead of 10ppm drops, the sample volume is increased for the titration - so instead of using 25ml of pool water, i now use 250ml of pool water. Am I also multiplying the other chemicals by 10? E.g. , instead of adding 2 drops of R-0007 and 5 drops of R-0008, i use 20 and 50 drops respectively?


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    Re: A Chemical Way of Calculating Pool Volume

    Yes, that also means to add 10 times more drops of the initial test chemicals.

    Thank you for raising that question.

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    Re: A Chemical Way of Calculating Pool Volume

    I would say you might want to tinker with this a bit. Two drops of R-0007 is typically capable of neutralizing well over 10ppm FC so there may not be a need to add as much R-0007. Even using less than is necessary for complete FC elimination, the only effect will be color differences in the indicator dye's start and endpoints. As for the indicator dye (R-0008), it's only contribution to the test is to act as an indicator. So, if 25 drops of dye is enough to add an amount of green color to the solution that is visible to your eyes, then you don't need to go overboard and use anymore than is necessary.

    I do this a lot with the CH test and the mannitol borate test that I use. I often add an extra drop or two of the R-0011L CH indicator because I can get a more visible color intensity to the water sample. Also, my CH is rather high so I want to ensure there is plenty of dye to react with the calcium ions. As for the mannitol test, it uses an indicator called bromothymol blue which is a yellow/blue transition at low pH. It only forms very weakly concentrated solutions with water (I think it's only got a 0.4% solubility) and so I have to add enough of that indicator to get a good color. The indicator is not part of the reaction (other than a color change) and so I can add as much as I need to see the colors.
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