Estimating a free form pool volume

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Water_man

Water_man

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Jun 7, 2008
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Brookline, MA
My friend has a kidney-shaped pool with variable depth and he's not a good records keeper, so he doesn't know what's his pool volume. I'm trying to help with water testing and balancing, but obviously without knowing the volume that wouldn't be easy. I can trace the circumference on graph paper with some degree of accuracy, but then the depth variance is hard to measure.
I thought about adding a known amount of chemical and then compare the “before” and “after” test results. Then the pool calculator or another simple calculating method can give the estimated volume. The question is which chemical. Chlorine won't stay unchanged after 8 hours of circulation, and pH and alkalinity won't be accurate because they affect each other. What's left is Ca Hardness. Just add some Ca in order to raise the CH by an estimated 10 ppm and then check the actual change. This should indicate the true volume.
Can anyone comment on this method, and is there a better way?
 
If you want to do it non-chemically, you just need to do a lot of perpendicular measurements. We do the same thing when we average out a rectangular pool's depth (8 deep, 3 shallow = 5.5 avg for the pool). The only difference is it's not as simple as 8 to 3, so you need a lot more samples.

Start at one end of the pool and take as many measurements of the width as is practical as you work your way down the side. Then repeat with the length, being sure that you're working perpendicular to your width measurements. And finally, same with the depth. Average all your measurements for each dimension, multiply the three averages together to get ft^3, multiply by 7.48 to get gallons. The closer together you take your measurements, the more accurate it will be.
 
CH isn't good to use because the CH test measures by 10s. Chlorine is better, because the FC test can be done by 0.2s. Still, any measurement you get chemically isn't likely to be any more accurate than estimating the volume the way spishex suggested, there are simply too many different ways the result can be inaccurate.
 
spishex said:
If you want to do it non-chemically, you just need to do a lot of perpendicular measurements. We do the same thing when we average out a rectangular pool's depth (8 deep, 3 shallow = 5.5 avg for the pool). The only difference is it's not as simple as 8 to 3, so you need a lot more samples.

Start at one end of the pool and take as many measurements of the width as is practical as you work your way down the side. Then repeat with the length, being sure that you're working perpendicular to your width measurements. And finally, same with the depth. Average all your measurements for each dimension, multiply the three averages together to get ft^3, multiply by 7.48 to get gallons. The closer together you take your measurements, the more accurate it will be.
Click to expand...

How to average all depths for a kidney shaped pool? It's a nightmare.
 
It isn't all that difficult, and you don't need to be exact. Get out a tape measure and measure the depth in a dozen places and average all the numbers. If you make a few mistakes it shouldn't make any significant difference.
 
Jason, the result would depend on how I choose the 12 points, and since I have no idea what is the depth distribution there's a large error margin. Taking the worst case scenario in order to make a point:
Suppose I choose the 12 depth points mostly at the 8 ft depth part of the pool I'll get a depth average of 8 feet. Since the depth is variable and the distribution is unknown I can suspect a variance of 25- 50%. I wonder if there's another chemical testing method not mentioned so far.
 
I promise you, if you make even a casual attempt to spread the depth test points out across the entire surface of the pool you will get a reasonably close number.

The best chemical approach is shocking the pool and using the FAS-DPD chlorine test setup to measure by 0.2. However, you can't use bleach or liquid chlorine because it's potency varies quite a bit. Your best bet would probably be cal-hypo purchased recently at a store with good stock turn over. If you use a high quality digital scale to measure the quantity of cal-hypo applied, you could calculate the pool size to within about 5%.

But you really can be just about as precise if you measure length, width, and depth in a dozen spots each. Measuring will take less time and not cost anything.

It really isn't important to know the volume exactly. All of the procedures we recommend depend on adding chemicals and then testing to see if you got the desired result. Even fairly large errors in the pool volume estimate are easily corrected for with subsequent adjustments. And if you find that you are consistently off in the same direction you can adjust your estimated pool volume.
 
Thank you, Jason, now you convinced me. I sketched the kidney shaped pool on a graph paper and calculated the total area by drawing sub-cells of rectangles, trapezoids and triangles. Then I multiplied the surface by an estimated 6 feet average depth. If future water adjustments would be off in the same direction I'd know how to compensate for water volume. After all the entire concept of water adjustment isn't based on precise dosage. It's trial and error, with an effort not to off -shoot in the first adjustment.
 
Another easy and very accurate method of determining surface area is to measure the gallons of water it takes to add 1" to the pool. A hose measuring device should be sufficient although I have used the curb water meter as well (just make sure nothing else is on). The sq-ft of the surface is then:

Surface area (sq-ft) = Gallons (1" rise) / 7.48 * 12

Average depth is another issue and little more difficult to measure.
 

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mas985 said:
Another easy and very accurate method of determining surface area is to measure the gallons of water it takes to add 1" to the pool. A hose measuring device should be sufficient although I have used the curb water meter as well (just make sure nothing else is on). The sq-ft of the surface is then:

Surface area (sq-ft) = Gallons (1" rise) / 7.48 * 12
Click to expand...
This is a great idea. It would be particularly useful for those who are "geometry challenged". :lol:
 
waterbear said:
If you want to determine the volume chemically here is a method that works well:
http://www.poolhelp.com/ChemicallyDeriv ... olumes.pdf
Click to expand...
This is very interesting and easy to perform, waterbear, thank you.
I wonder if you've ever used it.
They mention "Simplified Formula Numbers chart" which is missing, but fortunately they gave an example of sodium bicarbonate and they give the formula number for it. I wonder if people here agree that this is a good method. However, if the pool's TA is too high it won't make sense to increase it.
 
The TA measurement method they suggest is not especially precise. If you do the TA test in the standard way, where one drop counts as 10, it is completely useless (the error is larger than the signal). You can do the TA test with ten times the pool water, and one drop then counts as 1 ppm, but doing the test this way is quite tedious, requires a 250 ml sample vial that can be stirred easily, and still only gets you to +-20% overall for the small TA changes they are talking about. It does have the advantages that the strength of baking soda doesn't degrade over time and TA testing reagent is less expensive than FC testing reagent.

You want to use a level that you are willing to change significantly relative to the precision of the test. That is why I suggested using FC. Most people can raise FC by at least 10 without problems, plus doing the test by steps of 0.2 is well documented and can be done with standard test equipment. That gets you a change of 50 test steps or better, which gives you a reasonable accuracy. Also, done with FC you can start at a fairly low starting FC value, so the really huge numbers of titration drops only needs to be done once. The disadvantage of using FC is that the testing reagent is more expensive than the other testing reagents and some sources of chlorine have rather variable potencies, which can limit the precision of the results in some situations.
 
Water_man said:
waterbear said:
If you want to determine the volume chemically here is a method that works well:
http://www.poolhelp.com/ChemicallyDeriv ... olumes.pdf
Click to expand...
This is very interesting and easy to perform, waterbear, thank you.
I wonder if you've ever used it.
They mention "Simplified Formula Numbers chart" which is missing, but fortunately they gave an example of sodium bicarbonate and they give the formula number for it. I wonder if people here agree that this is a good method. However, if the pool's TA is too high it won't make sense to increase it.
Click to expand...
Yes, I have used it along with their charts which are here:
http://www.poolhelp.com/SimplifiedDosageFormulas.pdf
http://www.poolhelp.com/SimplifiedFormulaNumbers.pdf

They use TA since the test for that is one that is easy to determine with high accuracty They are only suggesting cvhanigning it 10-20 ppm so if it's high you are going to have to lower it anyway afterards, right?
 
waterbear said:
Yes, I have used it along with their charts which are here:
They use TA since the test for that is one that is easy to determine with high accuracty
Click to expand...
Waterbear: Jason brought up a valid point. Have you used 250ml sample flasks? Otherwise, if each drop counts as 10 ppm in the standard test of 25 ml sample, how can it be accurate?

Another question to both you and Jason:
How does their chart for chemicals additions compare to the Pool Calculator?
 
They include some chemicals I don't and I include some chemicals that they don't. For the chemicals we both include we agree fairly well once you do all the units conversions. I saw one or two that we disagreed on by a couple of percent one way or the other.
 
I have used their charts extensively at work since I did not have live internet access and could not use the Pool calculator and found that their dosing was pretty much 'on the money' (certainly close enough for our purposes) and I used a graduated cylinder to measure my 250 ml sample, whoch is what they say to do, use 10 times as large a sample,.
Their CYA numbers and alkalinity numbers are just about perfect, IMHO.
 
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