Pool Sizes

[Hilton]

I've been trying to determine the sizes of a large kidney shaped pool (pool "A"). I have a similar pool at the site next door for which I do have data, so I thought I'd test calculation methods based on it. Here's what I got on that pool (pool "B"), and I can't figure it out.

According to the original engineer spec sheet from 1977, pool "B" is 37,480 I.Gallons (approximately 170,000L or 45,000 US Gal). [extra info - it specifies 1,020 sq ft surface area, 3 feet min to 9 feet max depth (although the depth at the main drains actually measures 9'8").] Measuring the pool and using typical calculations for kidney shaped pools, I came up with 160,000L. Reasonably close, but pool "A" has a different side wall profile which would add a lot more water despite a roughly similar surface area, and a different shape, having one flat side where pool "B"'s outer side is very round. The same calculations come out at about 180,000L.

So I thought I'd try the chemical method described on this site. I waited until my TA was low, tested with 10x accuracy and got 85. I added exactly 5kg of sodium bicarbonate (weighed with a kitchen scale), re-tested about an hour later and got 95. This gives me a result over 300,000L...? Curiously, I tested again the next day (using normal accuracy) and got 110. So perhaps the bicarb wasn't dispersed enough in the pool after one hour?

I'm confused at this point and considering completely draining the pools for next spring and filling with a water meter.

 

[y_not]

Hilton,

I know the test method for volume calculation using TA, says to add PRECISELY 10lbs of sodium bi-carb for pools over 30k gal.

You state you added 5 kilos, which converts to just over 11 lbs (11.0231131), not "precisely 10 lbs". Is this number and measurement of your chemical addition correct?
This would be an error of approx 10.23% in your dosing.

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[Hilton]

Yes, but the calculation is 71400 * (pounds added) / (alk increase) - the emphasis is in adding an exact known amount, hence why I used a scale instead of the typical weight to volume conversion.

 

[y_not]

If that were the case, then why would OnBalance not have stated that in his original thread? As I didn't glean that from his post. He uses the word "precisely", and even states a different weight for different pool volume ranges. He doesn't say, pick an amount desired to raise your TA.

I would think that would work, it's just he didn't say so, so that's the only major discrepancy I could see in your execution of his method.

It is also more possible that your TA didn't raise overnight, after your final 10x accuracy test, but that in actuality, you're seeing the accuracy factor come into play that JasonLion talks about in the thread, here: http://www.troublefreepool.com/threads/64047-A-Chemical-Way-of-Calculating-Pool-Volume

I would suggest re-testing using the 10x method, and using that as your final calculation. Blows a lot of reagent, but hey, they're pretty cheap if you know where to look.
At least here in the USA.

There's also a method of measuring depth and corner/bottom radius at various points across a pole/board, then plugging that into Google Sketch Up. Search for it and I'm sure you'll find it easily enough.

Edit: Here it is - http://www.troublefreepool.com/thre...k-way-of-measuring-your-free-form-pool-volume

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[onBalance]

Hilton,
Yes, re-testing the pool water within one hour is probably not enough time, especially with a larger pool, where the Bicarb hadn't completely diluted throughout the water. Also, you should have used the 10X program the next morning, instead of the normal test. My guess is that the TA would have increased to around 105 ppm.

If the TA increased to 105 ppm, the formula would be 71,400 (formula number for Bicarb) multiplied by 11 lbs. (Bicarb added), divided by 20 (ppm increase) which would equal about 39,270 gals. or 148,640 L. But that is about a plus or minus 5% accuracy.

 

[Hilton]

I'll run the test again shortly - due to high CYA, after this long weekend I have to dump half the water in both of the pools. One I have a 'defined' size for (as specified by the original engineer, I'm sure he must be close), so it's a good one to test the calculation against. Both of the pools are in an area where the source water typically has a TA of 20-40, at least in the spring when we first fill the pools, so I should need to raise TA once I'm done!

 

[Hilton]

Hilton,
Yes, re-testing the pool water within one hour is probably not enough time, especially with a larger pool, where the Bicarb hadn't completely diluted throughout the water. Also, you should have used the 10X program the next morning, instead of the normal test. My guess is that the TA would have increased to around 105 ppm.

If the TA increased to 105 ppm, the formula would be 71,400 (formula number for Bicarb) multiplied by 11 lbs. (Bicarb added), divided by 20 (ppm increase) which would equal about 39,270 gals. or 148,640 L. But that is about a plus or minus 5% accuracy.

Done again today (had to drain the pool halfway to drop CYA, and the incoming water has very low TA). This is the same pool, where I have a (hopefully accurate) known size and am "testing" this method to make sure I'm doing it right and getting a good result before using it to determine the size of another pool. I had a brain fart and added 10kg instead of 10lb, but based on your previous response it appears any amount works as long as it is precise. I weighed out 10kg using a kitchen scale (4 + 4 + 2kg), with the weight of the measuring cup TAREd.

Pre-application, I had a TA of 56; Tomorrow I'm hoping to get a result of around 91 (a 35ppm increase), which would be exactly the increase expected for 10kg bicarb in 170,000L, and evaluates backwards with your formula to 44909 gallons (170,000L). (This pool has a known size of 170,000 L based on the original engineering data sheet - hopefully the engineer got it right back in 1977!)

Tomorrow I add bicarb to the OTHER pool - the one I don't know the size of and need to figure out. I think it is bigger (180,000-200,000 L), and the TA will likely be in the similar 50-60 range, so I will likely add 10kg to that pool as well.