# Thread: How many gallons really?

1. ## How many gallons really?

The literature for my pool lists it as 18K gallons, but we were told by the builder that that measurement is when the pool is filled to the very top (which it never is). Do I use 18K gallons for the purposes of pool chemistry? If not, how do I know how many gallons to use? If it helps, when the pool was filled for the first time, they said it took exactly two 7K gallon truckfuls, but I don't know how accurate that is.

Thank you!!
tdp

2. ## Re: How many gallons really?

Start with a number you feel is close and add chemicals based on that number. If you find that the chemical additions are NOT turning out to be what you thought, adjust your volume of water up or down when you do Pool Math.

Let's say you use 15k gallons and your pH is at 8.0 when you measured with your TF-100 or Taylor K-2006 test kit. Now you want to take it to 7.2. According to Pool Math, you would need to add 38 ounces of 31.45% muriatic acid to go from 8.0 to 7.2. Let's say you do that and you measure 7.0 with your kit an hour later. Well now you know right away that 15k gallons was too high. Adjust Pool Math to 10k gallons and you'll see that it takes 38 ounces of MA to take a pH of 8.0 down to 7.0.

You can do this with your FC measurement using the FAS/DPD test as it is quite a bit more accurate than measuring the pH. The above was just an example.

3. ## Re: How many gallons really?

Yep. Chris gave a good example of how the Poolmath calculator can work for you. Also, another Viking owner. 2nd one I've seen today. Welcome to TFP!.

4. ## Re: How many gallons really?

Originally Posted by Agent99
Start with a number you feel is close and add chemicals based on that number. If you find that the chemical additions are NOT turning out to be what you thought, adjust your volume of water up or down when you do Pool Math.

Let's say you use 15k gallons and your pH is at 8.0 when you measured with your TF-100 or Taylor K-2006 test kit. Now you want to take it to 7.2. According to Pool Math, you would need to add 38 ounces of 31.45% muriatic acid to go from 8.0 to 7.2. Let's say you do that and you measure 7.0 with your kit an hour later. Well now you know right away that 15k gallons was too high. Adjust Pool Math to 10k gallons and you'll see that it takes 38 ounces of MA to take a pH of 8.0 down to 7.0.

You can do this with your FC measurement using the FAS/DPD test as it is quite a bit more accurate than measuring the pH. The above was just an example.
that is the right idea, but don't use pH for confirming, use FC. pH calculations are tough to use since its hard for pool math to calculate/determine exact pH changes, AND the pH scale is tough to determine between levels since its color based, AND pH is not linear in nature. FC is a good way to judge, plus you are doing additions regularly

5. ## Re: How many gallons really?

I have a SWG, so don't do regular chlorine additions...but I agree about the pH being hard to judge.

6. ## Re: How many gallons really?

Originally Posted by tdp
I have a SWG, so don't do regular chlorine additions...but I agree about the pH being hard to judge.
Having an SWCG doesn't mean that you can't add chlorine.

7. ## Re: How many gallons really?

I meant I don't normally add chlorine except to SLAM.

8. ## Re: How many gallons really?

Originally Posted by tdp
I meant I don't normally add chlorine except to SLAM.

Sent from my iPhone using Tapatalk

Make sure your SWG is off (and will be off for an hour or two) - towards the evening or on a day with little sun would be good too. Measure your FC, add a measured amount of bleach (say 4 cups - that should be roughly 1 ppm of FC), circulate for an hour or two, and then measure your FC again. Use the calculator to calculate your volume. (you know two of three variables - amount of bleach added and your FC rise, you can now calculate your volume).

The added 1 ppm or so of FC is not going to hurt anything, and your pool will quickly settle back to normal day to day levels.

-dave

9. ## Re: How many gallons really?

Another way to do it is to use TA. TA changes very slowly in water and is only consumed if you add acid. Although, in the case of TA, you need to add enough baking soda to raise it by 20ppm and you probably want to use a larger test sample size to get more precision (+/- 5ppm/drop). Here's what I would use to do that -

50mL water sample size
3 drops of R-0007 (4 drops if the colors don't come out right)
8-10 drops of R-0008 Indicator (exact indicator amount is not critical it just depends on how much color intensity you need)
Add R-0009 for a precision of 5ppm/drop

You can then put your pool volume guess into PoolMath, figure out how much baking soda you need to get an increase of 20ppm (I'd pre-dilute it in a bucket of water and add it to the skimmer so that it circulates quickly, and then wait an hour or two (spend the time brushing your pool for extra cleanliness). Finally measure your TA rise and, if it's off, adjust the pool volume number until you get the TA rise you measured. It's a bit crude because the TA test is not very precise, but it avoids the problems with chlorine getting consumed and having to do the test at night. You can add a greater amount of TA if you have the room to do it and you want to count more drops, but it just means adding more acid later on to lower it back down.

10. ## Re: How many gallons really?

Originally Posted by phonedave

Make sure your SWG is off (and will be off for an hour or two) - towards the evening or on a day with little sun would be good too. Measure your FC, add a measured amount of bleach (say 4 cups - that should be roughly 1 ppm of FC), circulate for an hour or two, and then measure your FC again. Use the calculator to calculate your volume. (you know two of three variables - amount of bleach added and your FC rise, you can now calculate your volume).

The added 1 ppm or so of FC is not going to hurt anything, and your pool will quickly settle back to normal day to day levels.

-dave
Beware though that chlorine/bleach may not be exactly at the percentage on the bottle -- I'm pretty sure some of the 12.5% jugs I get at the pool store are closer to 15% when really fresh, and after sitting in a store or at home for a while could be significantly lower than 12.5% also. So you'd have to measure the chlorine concentration in a known volume of water (which is hard to do accurately) to be sure.

But as others like to point out, we aren't making rocket fuel here, so treating it as 15k vs 18k gallons isn't going to make a big difference day to day. I suspect most of us don't really know our pool sizes closer than ~15% (I know I don't). In your case, since the builder indicated that it definitely isn't 18k to the normal fill level, I'd just use 16k as a "close enough" number. You could measure the surface area, multiply by the depth from the deck to the surface of the water currently, and see how many gallons that comes to (7.48 gallons per cubic foot).

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