Estimating size of pool

I recently added salt to our pool to soften the water because my kids liked the feel of our friends saltwater pool. Based on several online pool volume calculators, which returned 17000 to 23000 gallons, I estimated 5 bags would raise the salt level to 1800. I did not measure the salt level before adding it because I assumed it would be 0 since we have never added salt, but when I measured it a few days after adding the salt it was 3000ppm. So either there was already salt in the pool, which I doubt since it was drained and refilled 5 years ago, or my test kit is off.

So to get to my question, what is the best method of calculating the volume of water that my pool holds? The pool is 30' x 18' grecian shaped with a hot tub.

Thanks,
Zach

I ran your figures several ways, and no matter how I did it, your numbers didn't sync up. So, it's time for questions:

1. What is the deepest WATER depth in your pool?
2. What is the shallowest (not steps!) water depth in your pool?

( The depth of the water, not the distance from pool edge to the bottom!)

3. What was the weight of the salt bags you added?
4. What test kit / strips did you use?

I believe that both salt strips and the salt drop kit actually measure the chloride content of the water so adding chlorine of any form will increase the measured salt level over time unless you backwash or refill often.

Ben, thanks for your reply. This is my first time maintaining a pool, up until 3 weeks ago we used a pool service to maintain the chemicals.

1 & 2: The depth ranges from 3ft to just over 7ft.
3: According to the mfg each 40lb bag adds 240ppm to a 10,000 gallon pool. I used Diamond brand pool salt from Lowes.
4: The test kit I used was a Taylor K-1766 that I just purchased from Amazon.

Today is the first day that I had enough time to get up to speed on testing & adjusting the water. So using a Taylor K-2006C and the salt kit mentioned above, I got the following results:
FC: 0 (the water did not turn pink when adding the powder so I stopped there)
CC: 0
pH: 8
TA: 80
CH: 280
CA: 90
Salt: 3000

Then using the pool calculator I plugged the numbers in and entered 10,000 gallons for the pool (to be on the safe side). It recommended adding 83oz of bleach to got from 0 to 4 which brought the FC up to 2.8 an hour later. So if my assumptions are correct and I did the math right I should have a 16,000 gallon pool. As for the salt, the only thing I can think of is the pool company dumped some in at some point of time.

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Originally Posted by mas985

so adding chlorine of any form will increase the measured salt level over time unless you backwash or refill often.

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Yes, I read that too but I didn't think it would raise it that much over a 5 year period. We backwash at least 2-3 times a year.

The amount of salt added by chlorine addition depends on the type of chlorine that is used. For sodium hypochlorite and lithium hypochlorite, every 1 ppm FC increases salt by 1.65 ppm. For Trichlor and Dichlor, every 1 ppm FC increases salt by 0.82 ppm. The extra salt in the hypochlorite sources comes from the way it is made:

Cl2 + 2NaOH ---> NaOCl + NaCl + H2O

Cal-Hypo is in between since it forms a solid where it is apparently more easily separable from the salt so it has around 1.2 ppm salt. I'm not sure why lithium hypochlorite, which is also a solid, is similar to sodium hypochlorite, but it is (I know the above equation still applies, but not sure why Cal-Hypo is more easily separated than lithium hypochlorite.

The 1:1 or 1:2 molar relationships have a molecular weight factor of sodium chloride weight to chlorine gas weight so 58.44/(2*35.453) = 0.824

The salt tests measure chloride, but report their results in ppm sodium chloride units.

So yes, salt builds up over time if there is no water dilution. At 2 ppm FC per day using bleach or chlorinating liquid, then after 6 months this is almost 600 ppm salt added. This is one reason why I dilute my pool water with winter rains every season since I have no backwashing due to my over-sized cartridge filter I only need to clean once a year. A backwash of only 2 or 3 times a year would barely dent the salt level in a 20,000 gallon pool -- even a 500 gallon backwash 3 times a year would lower salt by only 7.5%

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Originally Posted by zacht

what is the best method of calculating the volume of water that my pool holds? The pool is 30' x 18' grecian shaped with a hot tub.

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Quote:

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The depth ranges from 3ft to just over 7ft

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OK.

18 x 30 x 3 = 1620 cft (top 3' of water across the entire pool)
18 x 15 x ((7-3)/2) = 540 cft (hopper at 1/2 of pool, and average ADDITIONAL depth of 2')
1620 + 540 = 2160 cft, or 16,160 gallons => I'd estimate your pool, with the Grecian cutouts, at 15,500 - 16,000 gallons. That gives you a pool factor of 7.5, which means that 1# of stabilizer OR chlorine gas adds 7.5 ppm of CYA or chlorine. Likewise, 1# of salt adds 7.5 ppm of salt.

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According to the mfg each 40lb bag adds 240ppm to a 10,000 gallon pool. I used Diamond brand pool salt from Lowes.

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PF for a 10K gallon pool is 120,500/10,000 or 12.1 . 40 x 12.1 is 480 ppm. Somebody got off by a factor of 2 in their calculations. I ran into something like this, years and years ago, on dosing charts that came with a LaMotte acid demand test. Somebody forgot that, although sulfuric acid is a diprotic acid, sodium bisulfate is half-neutralized and is (sorta) a mono-protic acid. I spoke to Chuck Dulin at LaMatte, and he looked, acknowledged the problem and promised to get it fixed in the next printing.

I don't think Richard "Chem_Geek" makes errors like that, but the rest of us do. I had somebody like Richard in my high school Calculus class who could just glance over a long page of calculations, that I'd gone over again and again, and go, "Well, why'd you do that? it's wrong?".

My sister could find 4-leaf clovers; my high school friend could spot math errors in a glance; I can't do either one. But, you might want to call the mfg and suggest they fix the label.


You added 5 x 40 or 200# to a pool with a PF of 7.5, which gives an increase of 1500 ppm. So you're missing 1500 ppm. You comment

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Yes, I read that too but I didn't think it would raise it that much over a 5 year period.

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Actually, as Chem_Geek showed, it could raise it MORE than that -- I'm surprised that, in 5 years, you ONLY added 1500 ppm. Anyhow, I think we can say "the mystery is solved!".


BUT . . . you report a "CA" = 90 ppm. I'm assuming you mean "CYA", the abbreviation we use here for cyanuric acid, since "CA" can also refer to "carbonate alkalinity". A CYA level of 90 is wonderful by SWCG standards, but it does mean you'll have to run higher chlorine levels than you would otherwise.

THIS IS NOT A BAD THING! It is NOT something that has to be fixed or changed. If you want to change you can; tell us how much you want to change it, and we can tell you how far to drain. If you drain 2' of water, and replace it, you'll lower your CYA from 90 ppm to 45 ppm . . . and you'll pay the water company for 8,000 gallons of water.

But, alternatively, you can just follow the Best Guess chart, adjust your chlorine levels accordingly, and let the CYA go down over time. Chlorine does break it down slowly, and of course, you lose some with backwashing. So, as long as you don't add more, by using dichlor for shock, it will go down. Use bleach for shock, instead.

On your pool, 2 gallons of plain 6% bleach will add 7.5 ppm of chlorine (a gallon of 6% bleach is roughly equivalent to 1/2 lb. of chlorine gas)

Now, go enjoy your pool! ;)

Are there any of the pool water tests that are accurate enough to calculate the volume of water based on the change in test results after adding a prescribed treatment? In other words, let's say I measure the salt with the Taylor K-1766 drop test method. Then I add a 40lb bag of salt, wait a day or two, and test the salt again. Using something like the pool calculator site, I should be able to figure out the volume of the water based on the change in salt PPM after adding a 40lb bag of salt.

Are any of the other tests and treatment methods any more accurate to be able to do this? I would think it would have to be a test like Salt or CYA that doesn't evaporate or change rapidly, or an application of something maybe like bleach or acid that would have an almost immediate effect so you can measure right away without other factors affecting the results.

Just wondering.

Robert

Some years ago Que Hales of Pool Chlor and the JSPSI (Journal), published a method of doing just that, using alkalinity tests.

You could do the same thing, using a K-1766 and salt additions. Your accuracy would be limited by the accuracy of the tests AND the salt addition. In other words, you'd better weigh the salt. You'd need to test salt levels before and after the addition. I doubt that you'd be more accurate than direct calculation, when pool layout is simple (rectangle, etc.). But, with free form pools in irregular shapes, you very well might get superior results.

Thank you for a very detailed answer!

So based on the best guess chart I should keep the chlorine between 5 & 10. I guess I need to hit the store, I dumped in the last of my bleach today and FC is at 2 right now. It drops about 1 ppm a day.