1. Determining Gallons Size

I am starting to question how many gallons our new IG pool is. It is an 18' x 38' free form with a surface area of 640.1 square foot. The shallow end is 3' and the deep end is 8'. The engineer's drawings indicated that the volume is approximately 23,000 gallons but I'm not sure that is accurate. We just added 182 oz of 6% bleach and it raised the FC from 0 to 5. Using the pool calulator, that increase corresponds to roughly an 18,000 gallon pool. We also just added 8 pounds of 100% CYA. I will test the CYA levels a week from now using the TF Test kit and see what the level is. If the CYA level turns out to be around 50, is it safe to assume that my pool is around 18,000 gallons instead of the 23,000 gallons that I though it was? I am trying to back into the pool size by knowing what effect a given amount of chemicals has on the water. Will this work?

I looked at this a different way by calculating the volume of the 3' deep shallow end across the entire surface area of the pool (640.1SF x 3FT x 7.48 GAL/CF = 14,364 gallons). Is it reasonable to assume that the deep end adds another 3,636 gallons to bring the total up to 18,000 gallons?

2. Re: Determining Gallons Size

I get 17,907 when I calculate it so I'd bet that 18000 is pretty darn close. Figuring the volume by backing out of the chem changes is not a bad way to double check the volume either. The engineer most likely included the size all the way to the top of the coping in their calcs.

3. Re: Determining Gallons Size

Thanks, I just wanted to see if anyone else has done it this way and how accurate of a method it is. The CYA test next week should confirm my assumption about the volume.

4. Re: Determining Gallons Size

there are 3 or 4 different methods here that may be better than the cya test:
estimating-a-free-form-pool-volume-t11772.html

5. Re: Determining Gallons Size

! would be concerned the FC would not be precise....it degrades too rapidly in the day.

CYA test is pretty subjective and not very accurate.

A combination of calculations would probably end up being better than either of those tests unless you did the FC test at night within 30 min of each other.

6. Re: Determining Gallons Size

I think 23,000 gallons is right. If your area is 640, and your average depth is 5.5 feet [(3+8)/2], then that gives you 640 x 5.5 x 7.48 = 26,330 gallons. However, because you have a freeform shape and we don't know the actual slope of the bottom, it could vary quite a bit from this number.

7. Re: Determining Gallons Size

Originally Posted by poolneophyte
Thanks, I just wanted to see if anyone else has done it this way and how accurate of a method it is. The CYA test next week should confirm my assumption about the volume.
I did adjust my pool size by using the Pool Calculator backwards. After some observations following adding muriatic acid to my pool, I decided that it was about 1000 gallons more than the pool builder stated. Most likely that is the 700 gallons of the spa, plus some piping runs. I think the only way to be sure is to read the water meter as it is being filled up.

8. Re: Determining Gallons Size

If you want to determine the volume chemically here is a method that works well:
http://www.poolhelp.com/ChemicallyDeriv ... olumes.pdf
Here is the dosage chart that they talk about in the procedure above:
http://www.poolhelp.com/SimplifiedDosageFormulas.pdf
I've used it several times on different pools and it gets you in the ballpark and then some.

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