Pool Volume Calculation - Alternate Method

[JPhresh]

I have been trying to more accurately calculate the volume of my pool, which is tough because it has an unusual shape... I had a rough estimate from using the dimensions and various online calculators, but was suspicious of the result. Then it occurred to me that I could use chemistry to figure it out... I'd like to run this by y'all to see if you think it holds water (pun intended).

Recently I added two jugs of Clorox to my pool, which amounts to 1.89 gallons of a solution containing 7.85% available chlorine. The next day (after 24-hours circulation) I retested my pool chemistry and observed a 5.9 ppm increase in free chlorine. Then I plugged this data into the dilution equation:

(C1)x(V1) = (C2)x(V2)

where C = concentration and V = volume

Therefore:

78,500 ppm x 1.89 gallons = 5.9 ppm x [unknown volume]

Divide both sides of the equation by 5.9 and then solve for X, which yields a volume of 25,146 gallons. This is pretty close to the 26,000 gallons I have been previously assuming based on various estimates from width and depth...

So have I missed anything here, or is this a correct methodology and result?

 

[bmoreswim]

Analyzing chemistry additions is definitely a good way to estimate volume. I posted the following the other day elsewhere regarding your question...it's a bit less mathematical than your solution, instead relying on the Pool Math algorithm. Yours may work as well but I honestly didn't try to verify it.

"Yes, using chlorine or MA to zero in on pool volume are the easiest I believe. Put it what you think you need. Test before and 30 minutes after addition then go to Pool Math and use the bottom "effects of adding chemicals" plugging in the amount and strength of additions, then vary the pool volume at the top until it tells you the change that equals what actually happened. If you do this for both FC and pH you should be able to prove your answer. Then just pick the 1000 gallon number closest to it and that's your number. No need to be more exact than that. And of course you can do this a few times to eliminate any testing inconsistencies."

 

[JasonLion]

Using this approach results in larger errors, usually worse than what you would get from estimating based on measuring the pool. Each of the two FC measurements will have at least +-10%, resulting in a chemical level change measurement that is off by +-20% at best (possibly much worse if the FC change is noticeably smaller than the FC level). It is also impossible to know the bleach concentration better than +-10%, and usually no where near that precise. There will also be measurement errors in the amount of bleach added, though presumably small. And the FC level can change significantly from when the chlorine is added until when you take the second FC measurement, especially if you do any of this during the day, when FC levels are certain to be falling due to sunlight. Using PH changes is worse, as reading the PH color scale is typically a larger percentage error than measuring FC levels. Both FC and PH measurements can easily be +-40% or worse by the time you are done.

The most reliable single chemical measurement is to test TA before and after a large baking soda addition using a kitchen scale to weigh the baking soda, and adjusting the TA test procedure to measure by 1s or 2s. Because baking soda is fairly consistent in purity and does not lose strength, this can get the error down below +-25%, still far from perfect but usable. However, this does require making a large TA increase, which isn't always such a good idea.

Repeating this procedure using a single chemical level, say FC, over and over does not cancel out much of the error, as several of the errors will bias in the same direction on each test (i.e. bleach strength will remain the same, but not necessarily what you think it is). However, using this basic idea and averaging across a large number of different chemical additions of various chemicals can potentially cancel out the errors as both the test kit errors and the chemical strength variation will have errors that are not correlated when adding different chemicals and using different tests.

 

[Divin Dave]

Here is a bit of an in-depth (pun intended) post which explains more of what Jason is saying.
http://www.troublefreepool.com/threads/64047-A-Chemical-Way-of-Calculating-Pool-Volume

 

[JPhresh]

Thanks for the responses! I had not found the previous thread about calculating pool volume from chemistry, so that was some good info. In the back of my mind I had been wondering about the accuracy and precision of the tests, and how that would affect the volume calculation... Sounds like I can take the 25K gallon volume from my FC analysis and consider it rough confirmation of the 26K gallon volume I got from using the pool's physical dimensions, but that I shouldn't hang my hat on it. At least the numbers are reasonably close to each other! Some day I might try the TA method of volume estimation, although for now my most immediate issue is to do a water replacement to address my high CYA issue. Going to work on that this weekend.

Happy New Year!