Using FC to estimate pool size

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Vandergraff

Well-known member
Jun 22, 2010
118
Coastal Mountains, Northern CA
I have never really known our pool size in gallons - the pool was there when we bought the house 14 years ago.

It is an 30 x 15 ft kidney shape - with round 8 ft deep bottom - small shallow end which quickly gets deep.

Recently I have been guessing it is about 15000 gallons using various pool size calculators.

Anyway I thought I might try using the 'Pool Calculator' to check if my guess was close. So tonight the FC was 2.5 - I added 60 oz of 12% chlorine and after mixing/circulating the water for an hour the FC was 6. Using the Pool Calculator this gives a pool size of 16500 gallons. Understanding there is some +/- 0.5 error in my FC testing (TF100) - this still gives me confidence that I am in the 15000 - 17500 gallon range.

Edit - it is kidney shaped not oval....
 
The error in that pool size measuring approach is +-35% or worse. 12% chlorine is never exactly 12%. It can easily vary from 15% down to 8%. Once you add in the error in the FC test, all you can say with confidence is that the pool is between 12,000 and 22,000 gallons.

16,000 gallons is certainly a workable guess based on those results. Just keep in mind that you will need to continue adjusting that estimate based on averaging a number of different test results from a number of different chemical additions before you can have any real confidence in the results.
 
When I bought this house I was told that the pool was 15k per the last owner but that volume wasn't working: I was consistently overshooting my target FC. I started playing with the volume sizes and honed in on 16K. I confirmed this when I found the installer who had replaced the liner and per his records the pool is 16K. I wouldn't go off one measurement but I think that with careful testing and enough sampling you can hone in closer than +/- 35%.

And you wouldn't want to do this with an algae bloom. I don't see sunlight as a factor if you do this after the sun is off the pool. Consistency is key for this type of thing, which by the way is true for the BBB method too.

Using pH I think will have a wide variance because you don't have as accurate a measurement as you do with the FAS-DPD test. Color matching is very subjective and the resolution is limited at best. I have two pH test - one measures in increments of 3 the other in 2...and I often don't feel confidnet that I've honed in on an exact # even with both tests. At best I'm at +/-1.

YMMV, yada yada yada.
 
Yes I understand it is not precise - although I am surprised chlorine could vary from 8% - 15%

It is really another way to give me confidence that I am in the right range.

Luckily everything seems to working well - with noticeably clearer water (and I didn't think it was bad before) and no algae at all yet this year
 
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